Jurassic Trigoniida (Bivalvia) from Gebel Maghara, northern Sinai, Egypt

Abstract

The Jurassic trigoniid bivalves of Gebel Maghara, northern Sinai, Egypt are described and figured. They belong to 14 species, nine genera, and two families. The identified taxa occur in rocks ranging in age from the Toarcian (Lower Jurassic) to the lower Kimmeridgian (Upper Jurassic). Five genera and one species are new: Magharitrigonia asymmetrica new genus new species; Cotswoldella aff. C. hemisphaerica (Lycett, 1853); Cornbrashella pullus (J. de C. Sowerby, 1826); Parorthotrigonia lepidomorpha (Abdallah and Fahmy, 1969); and Retetrigonia imbricata (J. de C. Sowerby, 1826). Five species, Trigonia reticulata Agassiz, 1840, T. cf. T. castor d'Orbigny, 1849, Promyophorella tuberculata (Agassiz, 1840), Orthotrigonia exortiva (Kitchin, 1903), and O. gracilis (Kitchin, 1903) are recorded from the Jurassic rocks of Gebel Maghara for the first time.

UUID: http://zoobank.org/4dc3cc67-6bc3-406f-a022-6a05ed97efc6

Non-technical Summary

On the Sinai Peninsula, a thick pile of terrestrial to shallow-marine sediments accumulated during the Jurassic period. The sea floor at that time was colonized chiefly by brachiopods and bivalves. Among the latter, trigoniids were a comparatively rare but diverse faunal element, represented by very small to small individuals. They belong to 14 species and nine genera. One of these species is new to science, but five genera with species occurring also elsewhere in Jurassic seas were recognized as new, all of them characterized by a very small to small size.

Introduction

Although trigoniid bivalves reached their greatest diversity during the Cretaceous period, they steadily increased in species richness during the Jurassic (Stanley, 1977; Kelly, 1995). Trigoniids became widely dispersed during the late Early Jurassic to early Middle Jurassic (Echevarría et al., 2021), reaching the European Tethys (Francis, 2000; Francis and Hallam, 2003) and the southeast Asian Tethys (Wandel, 1936; Hayami, 1972) by the Toarcian, the high latitudes of Antarctica by the Pliensbachian–Toarcian (Kelly, 1995), New Zealand by the Aalenian (Fleming, 1987), and eastern Africa by the Bajocian (Cox, 1965).

The Jurassic succession of Gebel Maghara is the thickest and most complete Jurassic succession not only of northern Sinai but also of the African–Arabian Plate (1800 m thick; Al Far, 1966; Hirsch, 1980; Keeley, 1994). Bivalves are the most abundant group in the Jurassic strata of Gebel Maghara, followed by gastropods, ammonites, brachiopods, corals, and echinoids. Ayoub-Hannaa et al. (2023) pointed out that the bivalves and gastropods of Gebel Maghara have been neglected from a taxonomic point of view since the pioneering study of Douvillé in the early twentieth century (Douvillé, 1916). Hirsch (1980), Abdelhamid (2002), and Khalil (2003) illustrated some bivalve and gastropod taxa from the area, but without a comprehensive taxonomic study. Abdelhady (2014) and Abdelhady and Fürsich (2014, 2015a–c) listed 232 benthic and nektonic macrofaunal taxa from Gebel Maghara but did not perform a taxonomic analysis. The faunas are dominated by bivalves (60 species) and gastropods (52 species) (Abdelhady, 2014, appendix B). Therefore, Ayoub-Hannaa et al. (2017, 2023) began a detailed taxonomic revision of the bivalve fauna, dealing with protobranchs. The present study on the trigoniids is the second part of the comprehensive taxonomic study on the Jurassic bivalves of Gebel Maghara. In addition, the revision of the trigoniid fauna from the Jurassic of Egypt fills a gap in our knowledge of the Jurassic paleobiogeography of the group.

Geologic setting

The Levant region was strongly affected by Neo-Tethys rifting that resulted in the formation of intracratonic graben and horst systems, possibly starting as early as late Permian (e.g., Keeley, 1994). In northern Sinai, Jurassic strata crop out at Gebel Maghara, situated 50 km south of the Mediterranean shoreline, in an anticlinal structure that covers an area of about 400 km², and represents the fill of a half-graben (Al Far, 1966; Keeley, 1994; Fig. 1.1–1.3). The oldest Jurassic rocks (Mashabba Formation) at Gebel Maghara are ?Pliensbachian to Toarcian in age and represent fluvial, marginal-marine, and shallow-marine environments (e.g., Ghandour and Fürsich, 2022). Subsequently, shallow-marine and carbonate sediments deposited on a homoclinal ramp prevailed up to the Kimmeridgian, except during the global early Bathonian sea-level fall (Haq et al., 1987) during which deltaic sandstones and coals of the Safa Formation (Al Far, 1966; Abdelhady and Fürsich, 2015a) accumulated. The basin was closed by compressional forces due to convergence between Eurasia and Africa at the Jurassic-Cretaceous transition (Abd El-Motaal and Kusky, 2003).

Figure 1. (1) Locality map of Gebel Maghara. (2) Landsat image of the Gebel Maghara area showing positions of the three sections (numbered 1–3). (3) Geologic map (modified after Al Far, 1966; Hirsch, 1980) with positions of the investigated sections. (4) Satellite image of the Middle–Upper Jurassic succession; width of photograph corresponds to 1600 m. (5) Field photograph of the Middle to Upper Jurassic formations, exposed in the eastern saddle of the anticline of Gebel Maghara; thickness of Kehailia Formation is 250 m. (6) Jurassic subdivision and equivalent formations from older to younger (after Abdelhady and Fürsich, 2015a) with stratigraphic ranges of the trigoniid taxa. New genera are Magharitrigonia, Cotswoldella, Cornbrashella, Parorthotrigonia, and Retetrigonia.

Bivalve specimens have been collected from four sections, from west to east: Gebel Homayir, Gebel Arousiah, western Bir Maghara, and Gebel Mowerib (Fig. 1.2, 1.3). The first three sections belong to the western flank of the anticline with the strata dipping 15–30°, while the Gebel Mowerib section is exposed on the eastern flank dipping at angles exceeding 60° in some parts. Al Far (1966) subdivided the succession into seven formations, which were revised in detail by Abdelhady (2014). These formations are, in stratigraphic order, the Mahl, Bir Maghara, Safa, Kehailia, Arousiah, Tauriat, and Masajid formations (Fig. 1.5, 1.6). For more details concerning the litho- and biostratigraphy, faunal content, and lateral facies changes see Abdelhady (2014) and Abdelhady and Fürsich (2015a–c).

Material and methods

The studied trigonioid bivalve fauna consists of specimens collected by F.T. Fürsich in 1993 with complementary sampling by A.A. Abdelhady and F.T. Fürsich in 2012 from the Jurassic succession of Gebel Maghara, northern Sinai. Specimens were recovered from three sections: Gebel Homayir, Gebel Arousiah, and Gebel Mowerib. F.T. Fürsich collected additional specimens from the Toarcian Rajabiah and Shusha formations and the lower Kimmeridgian Masajid Formation of western Bir Maghara (section 4; Fig. 1.2). Abdelhady (2014) and Abdelhady and Fürsich (2015a–c) studied the litho- and biostratigraphy of the four sections, made taphonomic observations, and collected macrofossils bed by bed. Primary identification of the collected fossils took place during 2012–2014 as a part of a Ph.D. project by A.A. Abdelhady (2014). Faunal identification was greatly aided by the Jurassic bivalve catalogue at the GeoZentrum Nordbayern of Friedrich-Alexander-Universität, Erlangen. This catalogue contains photocopies of about 95% of all figured Jurassic bivalves worldwide. The trigonioid bivalves are mostly preserved with shell, are disarticulated, incomplete, and are small (H = 10 to ˂ 20 mm) to very small (H ˂ 10 mm) in size. Few specimens are preserved as composite molds. The hinge and other internal structures are usually covered with poorly indurated matrix, which was removed using diluted hydrogen peroxide (H₂O₂). Specimens were cleaned in the laboratory and prepared mechanically using steel needles under a binocular microscope. In addition, an air abrasive was used to clean the flank costae, area, and teeth. The specimens were photographed after having been coated with ammonium chloride to enhance details of ornamental features.

The systematic classification of the bivalves is that of Carter et al. (2011). The morphological terminology follows Carter et al. (2012). As in Ayoub-Hannaa et al. (2023), the synonymy lists contain only references, which have been carefully checked by the authors, beginning with the original description of the particular taxon, and followed by literature records from the Jurassic of Egypt. More comprehensive synonymies can be found in the references cited. For abbreviations in synonymy lists and open nomenclature to indicate the degree of confidence in allocation of each entry see Matthews (1973) and Bengtson (1988). Using “cf.” for some specimens refers to a provisional identification due to poor preservation, whereas a question mark indicates uncertain identification at the species level.

Linear measurements (taken with a Vernier caliper, accurate to 0.1 mm) are in millimeters. Measured dimensions, general morphologic terminology, teeth, and orientation are given in Figure 2. Abbreviations for dimensions are: L = length, H = height, I = inflation of articulated valves, and D = distance between umbo and anterior margin with the posterodorsal margin horizontal.

Figure 2. (1–3) Descriptive terminology and orientation of trigoniid shell, (4, 5) hinge structure of left and right trigoniid valves. RV = right valve; LV = left valve; m.b. = myophoric buttress; c.s. = cardinal socket; am = anterior adductor muscle scar; pm = posterior adductor muscle scar; Pl = pallial line; ppr = posterior pedal retractor muscle scar; L = length; D = length between umbo and anterior end; H = height; I = inflation of articulated valves; 2, 4 = cardinal teeth of left valve; 3, 5 = cardinal teeth of right valve; a, b = anterior and cardinal teeth, respectively.

Repository and institutional abbreviation

All specimens are housed in the collections of the Bayerische Staatssammlung für Paläontologie und Geologie in Munich under the prefix BSPG 2014 V.

Systematic paleontology

Infraclass Heteroconchia Hertwig, 1895
Cohort Uniomorphi J. Gray, 1854
Megaorder Unionata J. Gray, 1854
Order Trigoniida Dall, 1889
Superfamily Trigonioidea Lamarck, 1819
Family Trigoniidae Lamarck, 1819
Subfamily Trigoniinae Lamarck, 1819
Genus Magharitrigonia new genus

Type species

Magharitrigonia asymmetrica n. gen. n. sp., by original designation herein. Lower Toarcian Rajabiah Formation, western Bir Maghara, Sinai, Egypt.

Referred species

Magharitrigonia senex (Kobayashi and Mori, 1954) n. gen. n. comb. (Kobayashi and Mori, 1954, p. 167, pl. 16, fig. 8) from the Hettangian of Japan; Magharitrigonia brevicostata (Kitchin, 1903) n. gen. n. comb. (Kitchin, 1903, p. 23, pl. 2, figs. 4, 5) from the Middle Jurassic of Kachchh, western India; Magharitrigonia cristagalli (Bigot, 1893) n. gen. n. comb. (Bigot, 1893, p. 290, pl. 2, fig. 3) from the Bathonian of France.

Diagnosis

Very small to small, subtrigonal, moderately to strongly inflated with orthogyrate to slightly opisthogyrate beak, a wide and smooth antecarinal sulcus, coarsely nodate escutcheon and marginal carinae; escutcheon small, cordate, and smooth; right and left valves of some species differently ornamented on areas; LV with narrow and shallow area, bipartite, divided into two unequal halves by a distinctly crenulated median carina, the dorsal half with tuberculated radial costellae in later growth, its ventral half with reticulate ornament; RV with narrow, deep area, tripartite, divided into three unequal parts by well-developed tuberculated median and additional carinae, all parts covered with widely spaced sharp commarginal costellae; few and widely spaced commarginal flank costae (7–10), terminating in nodes close to the antecarinal sulcus, and separated by smooth and wide intercostal spaces (1.5–3.2 mm).

Occurrence

Upper Lower Jurassic (lower Toarcian) of Sinai, Egypt.

Etymology

Combination of the type locality Gebel Maghara (Sinai, Egypt) and the genus Trigonia.

Remarks

Magharitrigonia n. gen. is established largely on the basis of the different ornamentation of the areas of the two valves but, where only one valve is available, the genus can be easily distinguished from other Jurassic trigoniid genera by its small to very small size, low number of widely spaced commarginal costae terminating in nodes close to the antecarinal sulcus, orthogyrate to slightly opisthogyrate beak, smooth escutcheon and cordate escutcheon carina, and wide and smooth antecarinal sulcus. Trigonia Bruguière, 1789 (type species: Venus sulcata Hermann, 1781, pl. 4, figs. 2–4, 9, 10) resembles Magharitrigonia n. gen. in having a trigonal outline and commarginal flank costae, but differs in being much larger (L = 75 mm), with reticulate ornament to the entire area that has a broader lower area with at least 12 radial costellae, and a higher number of commarginal flank costae that curve downwards posteriorly and do not terminate in nodes.

Magharitrigonia asymmetrica new species
 Figures 3, 4.1, 4.2

Figure 3. Magharitrigonia asymmetrica n. gen. n. sp. from the lower Toarcian Rajabiah Formation, western Bir Maghara. (1–6) Holotype, articulated specimen BSPG 2014V 317/4; (1) left valve view showing smooth and wide intercostal sulcus, (2) dorsal view, (3) right valve view, (4) close-up showing bipartite area of left valve differing in ornamentation, (5) posterior view showing deep area of left valve, (6) close-up showing tripartite area of right valve with widely spaced commarginal costellae. (7, 8) Paratype, articulated specimen BSPG2014V 317/5, showing the differently ornamented area of left (7) and right (8) valve. (9, 10) Paratype, left valve, BSPG2014V 317/6; (9) exterior view with tuberculated escutcheon carina, (10) interior view showing hinge. (11) Paratype, left valve, BSPG2014V 317/7, with few and widely spaced commarginal costae of area. (12) Paratype, left valve, BSPG2014V 317/8, dorsal view. Scale bars = 2 mm.

Figure 4. (1, 2) Magharitrigonia asymmetrica n. gen. n. sp. from the lower Toarcian Rajabiah Formation, western Bir Maghara, paratype, articulated specimen, BSPG 2014V 317/9; (1) dorsal view showing an additional carina in the dorsal half of the area of right valve, (2) right lateral view. (3) Magharitrigonia n. gen. sp. indet. from the lower Kimmeridgian Masajid Formation, western Bir Maghara, right valve, BSPG 2014V 312/23, exterior view showing widely spaced commarginal costae terminating in nodes close to antecarinal sulcus. (4–7) Cotswoldella aff. C. hemisphaerica (Lycett, 1853) n. gen. n. comb., articulated specimen, BSPG 2014V 316/28, from the lower Kimmeridgian Masajid Formation, western Bir Maghara; (4) left lateral view, showing regular, equally spaced commarginal costae, (5) anterior view, (6) right lateral view showing the small area, (7) dorsal view showing faint radial costellae of area. (8) Cotswoldella hemisphaerica (Lycett, 1853) n. gen. n. comb. right valve, BGS GSM 113248, Inferior Oolite, Leckhampton, Gloucestershire, England (figured by Francis, 2000, pl. 8, fig. j). (9–12) Cornbrashella pullus (J. de C. Sowerby, 1826) n. gen. n. comb., from the middle−upper Bathonian Kehailia Formation of Gebel Maghara; (9–11) right valve, BSPG 2014V 325/1, Kehailia Formation, Gebel Mowerib, (9) right lateral view showing the smooth antecarinal sulcus, (10) posterior view showing the wide area with well-developed radial costellae, (11) dorsal view with ornamented escutcheon (arrowed); (12) right valve, BSPG2014V 305/7, Kehailia Formation, Gebel Homayir, posterior view showing ornamented area and escutcheon. (13) Cornbrashella pullus (J. de C. Sowerby, 1826) n. gen. n. comb., right valve, OMU J21371, upper Bajocian, Hook Norton, Oxfordshire, England (figured by Francis, 2000, pl. 4, fig. a). (14–17) Cornbrashella n. gen. sp. indet., right valve, BSPG 2014V 308/54 from the middle–upper Bathonian Kehailia Formation, Gebel Homayir; (14) interior view, (15) dorsal view showing an ornamented area with radial costellae, (16) lateral view showing very wide antecarinal sulcus, (17) close-up showing widely spaced, sharp, sub-straight costae crossed by faint radial riblets (indicated by white arrows), forming a reticulated pattern. Scale bars = 4 mm.

Type material

Holotype: BSPG 2014V 317/4, Lower Jurassic (Lower Toarcian), Rajabiah Formation, western Bir Maghara. Paratypes: eight specimens, BSPG 2014V 317/5–317/12, with shell preservation, mostly articulated, from the same formation and locality.

Diagnosis

Small to very small, outline trigonal; beak orthogyrate to slightly opisthogyrate; commarginal flank costae widely spaced, terminating in nodes close to antecarinal sulcus, and separated by smooth and wide intercostal spaces; escutcheon small, smooth; area of left valve bipartite with different ornamentation patterns; area of right valve tripartite with widely spaced commarginal costellae; antecarinal sulcus wide, smooth.

Description

Shell small to very small, outline trigonal, inequilateral, equivalved, slightly longer than high (Table 1), and moderately inflated. Maximum inflation slightly below umbo. Anterior margin slightly convex, meeting ventral margin in rounded angle. Posterior margin oblique, subtruncated, narrow, forming angle with ventral margin. Posterodorsal margin slightly concave, meeting posterior margin at obtuse angle. Ventral margin irregularly convex. Umbo triangular, inflated, located at about one-third of total valve length from anterior end. Beak orthogyrate to slightly opisthogyrate. Marginal carina distinct, sharp in early growth, thicker and tuberculated towards posteroventral corner (Fig. 3.1). Escutcheon carina slightly tuberculated in early growth, later with prominent oblique nodes. Escutcheon small, elongated-cordate in outline, smooth (Fig. 3.2). Antecarinal sulcus of left valve wide, smooth, broadening gradually towards posteroventral corner, with faint intercalatory radial riblet (Fig. 3.1). Area of left valve narrow, shallow, divided into two unequal parts by distinctly tuberculate median carina; dorsal half ornamented with commarginal costae in early growth followed by faint tuberculated radial costellae running parallel to posterodorsal margin; ventral half with reticulate ornament (Fig. 3.4, 3.5). Area of right valve wider and deeper than that of left valve, tripartite with well-developed tuberculated median carina and additional carina in dorsal half (Fig. 4.1, 4.2), covered with widely spaced commarginal costellae (Fig. 3.6). Flank inflated, triangular, occupying about two-thirds of shell surface, ornamented with 8–10 widely spaced commarginal costae (0.5–0.8 mm), asymmetrical in profile (steepest dorsally), terminating in node close to antecarinal sulcus and separated by very wide and smooth intercostal spaces (1.5–3.2 mm). Terminal nodes with distinct ventral extensions that are connected by thin radial riblet. Hinge of left valve wide, with large triangular cardinal tooth (2) separated from anterior and posterior teeth (4a, 4b) by anterior and posterior sockets, respectively (Fig. 3.10). Posterior tooth (4b) narrower than anterior one and running parallel to posterodorsal margin.

Table 1. Measurements (in mm) of Magharitrigonia asymmetrica n. gen. n. sp. *Holotype; all other specimens are paratypes; see Figure 2 for measurement key.

Etymology

Asymmetricus (Latin) = asymmetric; after the differently ornamented areas of the right and left valves.

Measurements

See Table 1.

Remarks

Magharitrigonia asymmetrica n. gen. n. sp. is easily distinguished from other trigoniid species identified herein by its small to very small size (Table 1), a smooth and elongated-cordate escutcheon, a flank with few and widely spaced commarginal costae terminating in nodes close to antecarinal sulcus, a bipartite area to the left valve differing in ornamentation, and a tripartite area to the right valve with widely spaced commarginal costellae.

The most closely related species is Magharitrigonia senex (Kobayashi and Mori, 1954) n. gen. n. comb. (Kobayashi and Mori, 1954, p. 167, pl. 16, fig. 8) from the Hettangian of Japan in having widely spaced commarginal costae with small spines close to the antecarinal sulcus, and a bipartite area of the left valve with reticulate ornamentation but differs in being larger and in having strongly convex anterior and ventral margins, a narrower antecarinal sulcus and a rounder valve.

Like M. asymmetrica n. gen. n. sp., Magharitrigonia brevicostata (Kitchin, 1903) n. gen. n. comb. (Kitchin, 1903, p. 23, pl. 2, figs. 4, 5) from the Middle Jurassic of Kachchh, western India, has widely spaced commarginal costae, a tuberculated marginal carina, and a wide antecarinal sulcus, but differs in being larger (L = 23–28 mm, H = 20–23 mm as opposed to L = 6.5–15.2 mm, H =6.1–13.5 mm), less inflated, and in having a higher number of commarginal flank costae, which do not terminate in nodes close to the marginal carina. In addition, the ornamentation of the areas (right and left valves) of M. brevicostata n. gen. n. comb. are similar (reticulated).

Magharitrigonia cristagalli (Bigot, 1893) n. gen. n. comb. (Bigot, 1893, p. 290, pl. 2, fig. 3) from the Bathonian of France differs in being more elongated (higher than long) and in having a narrower antecarinal sulcus, and its commarginal costae are straight.

In Trigonia stelzneri Gottsche (1878, p. 24, pl. 6, fig. 1a–d) from the Bajocian and Callovian of Argentina (Weaver, 1931, p. 240, pl. 20, figs. 103, 104), the ornamentation of the area of the left valve resembles that of the present species. However, that species differs from M. asymmetrica n. gen. n. sp. in having a wider area, a narrow antecarinal sulcus, a blunt and tuberculated marginal carina, a higher number of commarginal flank costae, and in being larger (L = 58 mm, H = 52 mm; Weaver, 1931, p. 241, as opposed to L = 6.5–15.2 mm, H = 6.1–13.5 mm). Similarly, T. (Lyriodon) sumiyagura Kobayashi and Kaseno (1947, p. 42, pl. 10, figs. 1a–2b) from the Lower Jurassic of Japan has a higher number of commarginal flank costae and is larger than the present species.

Trigonia (Trigonia) patchamensis Fürsich and Heinze (1998, p. 155, pl. 2, figs. 6–10) from the Middle Jurassic strata of Kachchh, India, has an ornamented escutcheon, strongly rounded anterior and ventral margins, a smaller area that is divided into two parts by a median groove, and is less inflated.

With respect to shell outline and size, T. costata J. Sowerby (1815), which was described and figured by Pugaczewska (1976, p. 83, pl. 17, figs. 1–3, pl. 20, figs. 3, 4, pl. 21, figs. 2, 3) from the Middle Jurassic of Poland and by Francis (2000, p. 62, pl. 1, figs. a–f, pl. 2, figs. a–f, pl. 3, figs. a–e) from the Middle Jurassic of Europe, differs in having a strongly rounded posterior margin, a tripartite area in each valve, a very narrow antecarinal sulcus, and a greater number of commarginal flank costae.

Trigonia triangularis (Goldfuss, 1837), figured by the same authors (Pugaczewska, 1976, p. 85, pl. 26, figs. 1–5, pl. 20, figs. 1–2, 6–7, pl. 21, figs. 1, 4, 10) from the Middle Jurassic of Poland, and by Francis (2000, p. 84, pl. 7, figs. e–h) from the Bajocian of Germany is similar to the present species with respect to shell outline. Like M. asymmetrica n. gen. n. sp., it also has a wide and smooth antecarinal sulcus, widely spaced commarginal flank costae, and a small cordate and smooth escutcheon, but differs in having a blunt and strongly tuberculated marginal carina, the areas of the two valves having the same ornamentation (reticulated pattern), and in being larger than the present species. In addition, the ventral margin of T. triangularis is concave below the antecarinal sulcus, and strongly convex anteroventrally.

Trigonia interlaevigata Quenstedt (1857, p. 503, pl. 67, figs. 7–8) from the Middle Jurassic of Germany differs in having strongly convex anterior and ventral margins, a truncated anterodorsal margin, a wider antecarinal sulcus, and in being larger.

A higher number of commarginal flank costae and a wide area distinguish T. castor d'Orbigny, 1850, figured by Thevenin (1913, p. 74, pl. 27, fig. 19) from the Bathonian of France from the present species.

Another related species is T. similis Agassiz (1840, p. 36, pl. 2, figs. 18–21, pl. 3, fig. 7) from the Lower Jurassic of France. However, that species is larger and has a higher number of commarginal flank costae.

Magharitrigonia n. gen. sp. indet.
Figure 4.3

Description

Shell very small, sub-triangular in outline, strongly inequilateral, slightly longer than high (Table 2). Posterodorsal margin slightly concave. Anterior margin strongly convex, meeting ventral margin in rounded angle. Umbo inflated and located one-third of total valve length from anterior end. Beak sharply pointed and orthogyrate. Marginal carina distinct, blunt, smooth. Escutcheon carina slightly tuberculated in early growth, later with strong nodes. Antecarinal sulcus smooth, widening gradually towards posteroventral corner. Area wide, deep, tripartite with well-developed median and additional carina, and covered by faint commarginal costellae. Flank inflated, triangular, ornamented with nine widely spaced commarginal costae terminating in nodes close to antecarinal sulcus (Fig. 4.3), and separated by very wide, smooth intercostal spaces.

Table 2. Measurements (in mm) of Magharitrigonia n. gen. sp. indet. See Figure 2 for measurement key.

Materials

A single right valve, with shell preserved, from the lower marl beds of the lower Kimmeridgian Masajid Formation, western Bir Maghara (BSPG 2014V 312/23).

Measurements

See Table 2.

Remarks

The present material is somewhat similar to Magharitrigonia asymmetrica n. gen. n. sp. from the Lower Jurassic of Gebel Maghara in having a small triangular shell, tripartite area with faint commarginal costellae, widely spaced commarginal flank costae, terminating in nodes close to antecarinal sulcus, and on tuberculated escutcheon carina, but differs in having a blunt, smooth marginal carina and in being more elongated (H/L = 0.78). Moreover, Magharitrigonia n. gen. sp. indet. is stratigraphically distinctly younger (lower Kimmeridgian) than M. asymmetrica n. gen. n. sp., which is from the lower Toarcian.

Genus Cotswoldella new genus

Type species

Trigonia hemisphaerica Lycett, 1853 (p. 231, pl. 11, fig. 2); by original designation herein. Inferior Oolite (Bajocian) of England.

Other species

Cotswoldella culleni (Lycett, 1877) n. gen. n. comb. (Lycett, 1877, p. 173, pl. 31, fig. 9), C. elegantissima n. gen. n. comb. (Meek, 1873, p. 474; Imlay, 1964, p. C29, pl. 3, figs. 1–7), C. kidugalloensis (Cox, 1965) n. gen. n. comb. (Cox 1965, p. 75, pl. 2, fig. 3a–c), C. densestriata (Behrendsen, 1892) n. gen. n. comb. (Behrendsen, 1892, p. 12, pl. 1, fig. 8), C. gadoisi (Cossmann, 1911) n. gen. n. comb., (Cossmann, 1911, p. 8, pl. 1, figs. 6–8), C. langrunensis (Bigot, 1893) n. gen. n. comb. (Bigot, 1893, p. 287, pl. 1, figs. 10–12, pl. 2, fig. 10), C. parva (Kitchin, 1903) n. gen. n. comb. (Kitchin, 1903, p. 37, pl. 3, figs. 7, 8), C. ranvilliana (Bigot, 1893) n. gen. n. comb. (Bigot, 1893, p. 290, pl. 9, fig. 8), C. hemisphaerica, race asiatica (Douvillé, 1916) n. gen. n. comb. (Douvillé, 1916, p. 29, pl. 4, fig. 9), and C. asiatica (Douvillé, 1916) n. gen. n. comb. of Hirsch (1980, pl. 4, figs. 13, 14) from the upper Bajocian of Gebel Maghara.

Diagnosis

Small, moderately inflated, trigonal-ovate to subquadratic, with opisthogyrate beak, low inconspicuous umbo, slightly depressed escutcheon, fine escutcheon carina, and sharp marginal carina; area with feeble to obsolete median carina, with or without longitudinal furrow; ventral part of area with reticulate ornament, dorsal part with radial costellae predominant; antecarinal sulcus narrow; commarginal flank costae numerous (50 or more in large specimens), fine, as narrow as or narrower than interspaces, coarsening ventrally. Upper Sinemurian-lower Kimmeridgian.

Etymology

From the Cotswolds, the rolling hills of central-southwest England, mainly in Gloustershire and Oxfordshire, from where the type species was collected.

Remarks

In the past, species of Cotswoldella n. gen. have been assigned to the genus Trigonia but differ from it in being smaller, possessing an ovate shape and an almost straight posterodorsal margin, lacking a sinus at the posteroventral margin, having fewer (5–9) radial costellae in the ventral part of the area as opposed to 14 in the type species of Trigonia, which are reticulate only in the lower part, and in having a narrower antecarinal sulcus and denser, more numerous flank costellae that barely curve downwards posteriorly, if at all. Except for the areal ornament, which is rarely described in the older literature, these are the characters of the referred species.

Cotswoldella aff. C. hemisphaerica (Lycett, 1853) n. gen. n. comb.
Figure 4.4–4.8

aff. *1853

Trigonia hemisphaerica Lycett, p. 231, pl. 11, fig. 2.

aff. 1877

Trigonia hemisphaerica; Lycett, p. 174, pl. 31, figs. 4–8.

aff. 1877

Trigonia hemisphaerica var. gregaria Lycett, p. 174, pl. 33, figs. 4–6.

aff. 1877

Trigonia Culleni Lycett, p. 173, pl. 31, fig. 9.

aff. 1904

Trigonia hemisphaerica; Riche, p. 154, pl. 5, fig. 3a–c.

aff. 1916

Trigonia hemisphaerica, race Asiatica Douvillé, p. 29, pl. 4, fig. 9.

aff. 1932

Trigonia hemisphaerica; Fabiani and Ruiz, p. 29, pl. 2, figs. 11, 12a–c.

aff. 1979

Trigonia sp. A aff. T. hemisphaerica; Poulton, p. 15, pl. 1, figs. 1, 2.

aff. 1979

Trigonia sp. B aff. T. hemisphaerica; Poulton, p. 15, pl. 1, fig. 3.

aff. 1980

Trigonia asiatica; Hirsch, pl. 4, figs. 13, 14.

aff. 1981

Trigonia hemisphaerica; Parnes, p. 35, pl. 4, figs. 36–39.

aff. 1981

Trigonia hemisphaerica lunulata Parnes, p. 35, pl. 4, figs. 34, 35.

aff. 2000

Trigonia hemisphaerica; Francis, p. 81, pl. 8, figs. i, j (with additional synonyms).

aff. 2003

Trigonia costulata; Khalil, p. 308, pl. 1, figs. 3–5.

Occurrence

Bajocian of England (Lycett, 1877; Francis, 2000), Negev, Israel (Parnes, 1981), upper Sinemurian–Bajocian of California (Poulton, 1979), Bajocian of France (Riche, 1904), Middle Jurassic of south Italy (Fabiani and Ruiz, 1932), and Bajocian–lower Kimmeridgian of Egypt (Douvillé, 1916; Khalil, 2003; present study).

Description

Shell small, rounded-subquadratic, as long as high, inequilateral, equivalved, moderately inflated (Table 3). Maximum inflation one-third of total shell height from the umbo, gradually fading towards posterior margin. Subvertical anterior margin high, slightly convex, meeting ventral margin in rounded corner. Ventral margin convex, anterodorsal margin slightly convex. Umbo small, moderately inflated, located one-third of total valve length from anterior end. Beak small, sharp, and slightly opisthogyrate. Marginal carina indistinct, slightly concave dorsally. Area slightly concave, divided by submedian groove into two unequal parts, ventral part narrower than dorsal, and ornamented with ~5 faint radial costellae that dominate the dorsal half (Fig. 4.7). Antecarinal sulcus of left valve very narrow, smooth. Escutcheon carina moderately developed. Escutcheon excavated, small, slightly concave, cordate, smooth. Flank moderately inflated, ornamented with regular, equally spaced commarginal costae, narrower than interspaces, coarsening ventrally and fading anteriorly (Fig. 4.4, 4.6).

Table 3. Measurements (in mm) of Cotswoldella aff. C. hemisphaerica (Lycett, 1853) n. gen. n. comb. See Figure 2 for measurement key.

Material

A single specimen, with shell preserved, from the lower part of the Masajid Formation (lower Kimmeridgian), western Bir Maghara (BSPG 2014V 316/28).

Measurements

See Table 3.

Remarks

Cotswoldella aff. C. hemisphaerica n. gen. n. comb. can be easily separated from Jurassic Trigonia by its regular commarginal flank costae (Fig. 4.4, 4.6), very small escutcheon, faint marginal carina, and small area. The shape of the shell and the ornamentation of the flank of the present specimen resemble C. hemisphaerica (Lycett, 1853) n. gen. n. comb. figured by Lycett (1877) and Francis (2000) from the Middle Jurassic of England, but differs in having a narrower area with far fewer radial costellae that are not reticulated in the ventral part, a finer marginal carina, and flank costae that are narrower than the interspaces. For this reason, the specimen is referred to Lycett's species with qualification. Hirsch (1980) used the “race Asiatica” of Douvillé, 1916, as a separate species, Trigonia asiatica, on the basis of numerous commarginal flank costae, strong radial costellae of the area, a well-defined marginal carina, and its small size.

Cotswoldella ranvilliana (Bigot, 1893) n. gen. n. comb. (Bigot, 1893, p. 290, pl. 9, fig. 8) from the Bathonian of France resembles Cotswoldella aff. C. hemisphaerica n. gen. n. comb. in having a narrow area, strongly convex anterior and ventral margins, and relatively widely spaced flank costae, but differs in having a well-developed marginal carina and in being larger and more elongated (higher than long).

Cotswoldella langrunensis (Bigot, 1893) n. gen. n. comb. (Bigot, 1893, p. 287, pl. 1, figs. 10–12, pl. 2, fig. 10) from the upper Bathonian of France resembles Cotswoldella aff. C. hemisphaerica n. gen. n. comb. in having a small area and a fine marginal carina, but differs in being larger, more elongated and in having more numerous fine commarginal flank costae.

Cotswoldella parva (Kitchin, 1903) n. gen. n. comb. (Kitchin, 1903, p. 37, pl. 3, figs. 7, 8) from the Jurassic of Kachchh, India, resembles Cotswoldella aff. C. hemisphaerica n. gen. n. comb. in having regular commarginal costae, and a small, smooth escutcheon, but differs in having an extended posterior area, a concave posterodorsal margin, and in being smaller (L = 11.0–15.0 mm as opposed to L = ?26.5 mm) and much more elongated. Similarly, C. culleni (Lycett, 1877) n. gen. n. comb. (Lycett, 1877, p. 173, pl. 31, figs. 9, 9a) from the Jurassic of England is posteriorly produced with a deeply concave posterodorsal margin.

Cotswoldella kidugalloensis (Cox, 1965) n. gen. n. comb. (Cox, 1965, p. 75, pl. 2, fig. 3a–c) from the Bathonian of Kenya differs by its smaller size (L ~17 mm), greater inflation, and in having only 6–7 nodose threads on the area, whereas in adult C. hemisphaerica n. gen. n. comb. there are 13–14.

Cotswoldella gadoisi (Cossmann, 1911) n. gen. n. comb. (Cossmann, 1911, p. 8, pl. 1, figs. 6–8) from the Bajocian of France has very fine flank costae.

With respect to general outline and ribbing pattern, C. elegantissima (Meek, 1873) n. gen. n. comb. (Meek, 1873, p. 474; Imlay 1964, p. C29, pl. 3, figs. 1–7) from the lower Callovian of Utah, USA, resembles Cotswoldella aff. C. hemisphaerica n. gen. n. comb., but differs in being smaller (L = 21 mm, H = 14 mm; Imlay, 1964, p. C29, as opposed to L = ?26.5 mm, H = 26.5 mm) and more elongated (H/L = 0.67).

Cotswoldella densestriata (Behrendsen, 1892) n. gen. n. comb. (Behrendsen, 1892, p. 12, pl. 1, fig. 8; Leanza, 1993, p. 21, pl. 3, figs. 7–9) from the lower Bajocian of Argentina has a poorly defined marginal carina, flat dorsal surface, and numerous fine flank costae.

Genus Cornbrashella new genus

Type species

Trigonia pullus J. de. C. Sowerby, 1826, p. 10, pl. 508, figs. 2, 3; by original designation herein. Cornbrash Formation (Bajocian–Bathonian) of England.

Other species

Cornbrashella distincta (Kitchin, 1903) n. gen. n. comb. (Kitchin, 1903, p. 25, pl. 2, figs. 6, 7).

Diagnosis

Small, moderately inflated, trigonally ovate to ovate, with small pointed umbo, opisthogyrate beak, wide, slightly depressed cordate escutcheon with finely crenulated transverse costellae, prominent tuberculate escutcheon and marginal carinae; median carina of left valve evanescing posteriorly, lacking on right valve; area dominated by crenulated radial costellae, lower area with three, upper area with four in the type; antecarinal sulcus narrow; up to 20 commarginal flank costae, narrower than interspaces, not curving downwards posteriorly. Bajocian–Callovian.

Etymology

From the Cornbrash Formation, in which the type species is common.

Remarks

Cornbrashella n. gen. is easily distinguished from Trigonia by its small size, trigonally ovate outline, transverse costellae to the escutcheon, and non-reticulate area with few radial costellae. The genus Neuquenitrigonia Leanza and Garate Zubillaga (1987, p. 209) is very similar, but the type species Trigonia huenickeni Leanza and Garate, 1985 (p. 290, fig. 3; pl. 1, figs. 1, 2) from the Bajocian of Argentina differs in being much larger, more inflated, and in having an acute apical angle, well-separated almost straight flank costae, and a larger escutcheon.

Cornbrashella pullus (J. de C. Sowerby, 1826) n. gen. n. comb.
Figure 4.9–4.13

*1826

Trigonia Pullus J. de. C. Sowerby, p. 10, pl. 508, figs. 2, 3.

.1877

Trigonia pullus; Lycett, p. 164, pl. 34, figs. 7–9.

.1886

Trigonia pullus; Douvillé, p. 226, pl. 12, figs. 13, 14.

.1888

Trigonia pullus; Greppin, p. 107, pl. 7, fig. 1.

.1893

Trigonia pullus; Bigot, p. 283, pl. 2, figs. 4, 5.

.1895

Trigonia pullus; Newton, p. 82, pl. 3. figs. 6, 7.

.1903

Trigonia pullus; Dainelli, p. 298, pl. 11, figs. 4, 7.

.1904

Trigonia pullus; Riche, p. 153, pl. 5, fig. 2.

.1916

Trigonia pullus; Douvillé, p. 53, pl. 5, fig. 42.

.1969

Trigonia pullus; Fischer, p. 97, pl. 11, figs. 1, 2.

.1980

Trigonia sp. aff. T. pullus; Hirsch, p. 136, pl. 4, fig. 26.

.2000

Trigonia pulla; Francis, p. 91, pl. 3, figs. f–h, pl. 4, figs. a, b (with exhaustive synonymy list).

.2013

Trigonia pullus; Néraudeau et al., pl. 25, fig. c.

Lectotype

BMNH 43133, designated by Cox and Arkell (1948, p. 22) from Anc1iff, Wiltshire; figured by J. de. C. Sowerby, 1826, p. 10, pl. 508, figs. 2, 3.

Occurrence

Bajocian–Bathonian of England (J. de C. Sowerby, 1829; Lycett, 1877; Francis, 2000), Bajocian–lower Kimmeridgian of France (Bigot, 1893; Riche, 1904; Fischer, 1969; Néraudeau et al., 2013), Bathonian of Switzerland (Greppin, 1888), Italy (Dainelli, 1903), Ethiopia (Douvillé, 1886), Middle Jurassic of Madagascar (Newton, 1895), and Bathonian–Callovian of Egypt (Douvillé, 1916; Hirsch, 1980; present study).

Description

Shell very small to small, triangular, inequilateral, slightly higher than long (Table 4), moderately inflated. Anterior and ventral margins convex, meeting in rounded curve. Posterodorsal margin slightly concave, meeting posterior margin at obtuse angle. Posterior margin relatively short. Umbo poorly inflated, triangular, located one-third of total valve length from anterior end. Beak sharply pointed, slightly opisthogyrate. Marginal carina well developed. Antecarinal sulcus smooth, narrow dorsally, widening towards posteroventral corner. Area wide, covered with radial, beaded costellae and crossed by fine commarginal growth lines (Fig. 4.9, 4.10). Escutcheon wide, depressed, cordate in outline, and ornamented by transverse costellae (Fig. 4.11, 4.12), bordered by well-developed clavate escutcheon carina. Hinge of right valve with two grooved teeth, running parallel to antero- and posterodorsal margins. Anterior adductor muscle scar subrounded, small, located close to anterodorsal margin. Flank moderately inflated, occupying about two-thirds of shell surface and ornamented with commarginal costae, separated by wide and smooth intercostal spaces (Fig. 4.9, 4.13).

Table 4. Measurements (in mm) of Cornbrashella pullus (J. de C. Sowerby, 1826) n. gen. n. comb. See Figure 2 for measurement key.

Materials

Four specimens, fragmented, from the middle-upper Bathonian Kehailia Formation of Gebel Maghara; one from Gebel Mowerib (BSPG 2014V 325/1) and three from Gebel Homayir (BSPG 2014V 305/7–305/9).

Measurements

See Table 4.

Remarks

With respect to general outline and ornamentation of the area and flank, the Egyptian material corresponds very well to Cornbrashella pullus (J. de C. Sowerby, 1826) n. gen. n. comb. from the Bajocian–Bathonian of southern England. That species can be easily distinguished by its very small shell size and by having a wide area with well-developed radial costellae, a deep, ornamented escutcheon with transverse costellae, a small, smooth antecarinal sulcus, sharp opisthogyrate beaks, and strongly rounded anterior and ventral margins.

Trigonia costata Parkinson, 1811, of Francis (2000, p. 62, pl. 1, figs. a–f; pl. 2, figs. a–f; pl. 3, figs. a–e) from the Middle Jurassic of England and Germany is somewhat similar to Cornbrashella pullus n. gen. n. comb. Although T. costata is a highly variable species, it differs from the present species in having a subtrigonal valve, an acute umbonal angle, a wide antecarinal sulcus, more numerous flank costae, a raised escutcheon (inset and depressed in C. pullus n. gen. n. comb.), and in being larger.

Cornbrashella distincta (Kitchin, 1903) n. gen. n. comb. (Kitchin, 1903, p. 25, pl. 2, figs. 6, 7) from the Middle Jurassic of Kachchh, India, resembles the present species in having an ornamented escutcheon but differs in having an ovate valve, rounded margins, and its radial costellae of the area are fewer and less developed than in C. pullus n. gen. n. comb.

Francis (2000, p. 91) regarded T. langrunensis Bigot, 1893 (p. 287, pl. 2, fig. 10) from the upper Bathonian of France as a synonym of C. pullus n. gen. n. comb. Despite the similarity between the two species, T. langrunensis differs in being larger, more elongated (longer than high) and in having numerous fine flank costae. For more details and comparison with other closely related Jurassic taxa, see Francis (2000, p. 95–98).

Cornbrashella n. gen. sp. indet.
Figure 4.14–4.17

Description

Shell small (L > 16.5 mm), subtriangular, inequilateral, and poorly inflated. Anterodorsal margin slightly convex. Ventral margin missing. Umbo inflated, triangular, located anterior of mid-length of valve. Beak small, sharp, strongly opisthogyrate. Antecarinal sulcus smooth, deep, strongly widening towards posteroventral corner (Fig. 4.16). Area narrow, divided into two unequal parts by median carina and ornamented with radial costellae (Fig. 4.15). Escutcheon carina distinct, elevated. Escutcheon small with transverse riblets. Hinge of right valve large with two striated cardinal teeth (3a, 3b), separated by a wide triangular socket (Fig. 4.14). Anterior muscle scar small, subrounded, located close to anterodorsal margin. Flank weakly convex and ornamented with widely spaced, sharp, sub-straight costae topped with small tubercles (Fig. 4.16). Costae separated by wide intercostal spaces and crossed by faint radial riblets forming a reticulated pattern (Fig. 4.17).

Materials

Two incomplete specimens, with shell preservation, from the middle–upper Bathonian Kehailia Formation, Gebel Homayir (BSPG2014V 308/54, 308/55).

Remarks

The incomplete right valve closely corresponds to similar specimens figured by Cox (1952a, p. 111, pl. 12, figs. 5–9) as Trigonia kheraensis Cox, 1952a, from the Middle Jurassic (Bathonian) of India. That species resembles the present material in having a very wide antecarinal sulcus and widely spaced and sub-straight commarginal flank costae, but differs in having a smooth area. The specimens figured by Mongin (1967, pl. 2, figs. 16–19) as T. kheraensis from the Bathonian of Morocco have an ornamented area and escutcheon and resemble the Egyptian material more closely than the Indian species.

The general outline and ornamentation of the area is close to T. interlaevigata Quenstedt, 1857 (p. 503, pl. 67, figs. 7, 8) from the Middle Jurassic of Germany. That species is, however, larger and has a higher number of commarginal flank costae.

Genus Trigonia Bruguière, 1789

Type species

Venus sulcata Hermann, 1781, by subsequent designation (ICZN, 1955).

Diagnosis

Medium-sized to moderately large, thick-shelled, robust, trigonal to trigonally elongated, strongly inequilateral, moderately inflated, with prominent subterminal umbones, acute umbonal angle and strongly incurved opisthogyrate beaks; anterior margin weakly convex, posteroventral margin with shallow sinus, obliquely truncated respiratory margin, concave posterodorsal margin; escutcheon shallowly sunken with growth rugae and striae; escutcheon carina corded; broad bipartite area with corded median carina and numerous fine radial costellae intersected by strong striae to create a reticulate pattern; prominent marginal carina curved, corded; LV with narrow ante-carinal sulcus, RV with narrow post-carinal groove; flanks with strong, rounded commarginal costellae narrower than interspaces, curving downwards posteriorly.

Trigonia reticulata Agassiz, 1840
Figure 5.1–5.6

*1840

Trigonia reticulata Agassiz, p. 39, pl. 11, fig. 10.

.1840

Trigonia papillata Agassiz, p. 39, 40, pl. 5, figs. 10–14.

.1840

Trigonia monilifera Agassiz, p. 40, pl. 3, figs. 4–6.

.1840

Trigonia parvula Agassiz, p. 41, pl. 11, fig. 8.

.1840

Trigonia meriani Agassiz, p. 41, pl. 11, fig. 9.

.1930

Trigonia reticulata; Arkell, p. 81, pl. 6, figs. 1–4 (with additional synonymies).

.2000

Trigonia reticulata; Francis, p. 116, pl. 6, figs. a–g, pl. 7, figs. a, b (with additional synonymies).

.2001

Trigonia (Trigonia) cf. T. reticulata; Delvene, p. 80, pl. 6, fig. 1.

.2011

Trigonia reticulata; Schneider et al., p. 263, pl. 1, figs. F–L.

v.2015a

Trigonia costata; Abdelhady and Fürsich, p. 41.

v.2015c

Trigonia costata; Abdelhady and Fürsich, p. 267.

Figure 5. (1–6) Trigonia reticulata Agassiz, 1840, from the middle Bathonian–lower Kimmeridgian of Gebel Maghara. (1–3) Articulated specimen, BSPG 2014V 260/1, Kehailia Formation, Gebel Mowerib, (1) left lateral view, (2) dorsal view showing depressed, elongated-cordate escutcheon and tuberculated escutcheon carina, (3) right lateral view with regular, sharp, commarginal costae; (4–6) articulated specimen, BSPG 2014V 432/1, Masajid Formation, western Bir Maghara, (4) left lateral view, (5) dorsal view, (6) close-up showing fine, tuberculated, radial costellae crossed by commarginal costellae, forming a reticulate pattern (arrowed). (7, 8) Trigonia cf. T. castor d'Orbigny, 1849, right valve, BSPG 2014V 323/4, from the Lower Kimmeridgian Masajid Formation, western Bir Maghara, (7) right lateral view showing large area (45% of total valve surface) and strongly elevated escutcheon carina, marked by asterisk (8) dorsal view with strongly opisthogyrate beak; position of escutcheon carina marked by asterisk. (9) Myophorella sp. indet. from the Lower Jurassic (lower Toarcian) Rajabiah Formation, western Bir Maghara, left valve, BSPG 2014V 317/13, left lateral view showing well-developed spinose radial flank costae. (10–23) Parorthotrigonia lepidomorpha (Abdallah and Fahmy, 1969) n. gen. n. comb. from the lower Toarcian Rajabiah Formation (marl unit), western Bir Maghara. (10–12) Articulated specimen, BSPG 2014V 317/14, (10) left lateral view, (11) dorsal view, (12) right lateral view, sub-rounded form; (13, 14) left valve, BSPG 2014V 317/15, (13) lateral view showing smooth radial costae, deep and smooth intercostal spaces, and smooth area, (14) dorsal view showing smooth area with distinct median groove; (15–17) left valve, BSPG 2014V 317/16, (15) lateral view, subtriangular form, (16) interior view showing teeth, muscle scars, and entire pallial line, (17) dorsal view showing sharp orthogyrate beak; (18, 19) incomplete right valve, BSPG 2014V 317/17, (18) lateral view, elongated form with smooth, broadly rounded radial costae, (19) dorsal view with distinct median groove of area; (20) right valve, BSPG 2014V 317/18, lateral view, rounded form; (21) right valve, BSPG 2014V 317/19, interior view with two large cardinal teeth (3a, 3b); (22, 23) left valve, BSPG 2014V 317/20, (22) dorsal view showing large cardinal tooth 2, (23) interior view. Scale bars = 4 mm.

Holotype

Trigonia reticulata Agassiz, 1840, p. 39, pl. 11, fig. 10, from Oxfordian-Kimmeridgian strata near Châtelot, W of Montbéliard, Département Doubs, E France; according to Francis (2000, p. 118), housed in the Agassiz Collection, Neuchâtel Museum, Switzerland.

Occurrence

Upper Oxfordian–lower Kimmeridgian of France (Agassiz, 1840), Spain (Delvene, 2001), central Portugal, and England (Schneider et al., 2011), and middle Bathonian–lower Kimmeridgian of Egypt (first record).

Description

Shell small, subtriangular, inequilateral, slightly longer than high (H/L = 0.87), equivalved, strongly inflated (I/L = 0.62; Table 5). Area of maximum inflation around mid-height of valve. Anterior margin strongly convex, meeting ventral margin in continuous rounded curve. Posterior margin oblique, subtruncated; ventral margin regularly convex. Posteroventral corner angulated. Posterodorsal margin slightly concave, sloping, forming obtuse angle with posterior margin. Umbo weakly inflated, triangular, located one-fourth of total valve length from anterior end. Beak sharply pointed, strongly opisthogyrate. Marginal carina prominent, rounded, and crenulated. Area wide, subdivided into two equal parts by well-developed median carina, and ornamented with fine, tuberculated radial costellae, crossed by commarginal costellae, creating reticulate pattern (Fig. 5.6). Escutcheon carina low, distinct, crenulated. Escutcheon depressed, elongated-cordate in outline, ornamented with faint tuberculated commarginal riblets. Flank strongly inflated and ornamented with prominent, regular, sharp commarginal costae, separated by wide, smooth intercostal spaces (much wider than costae; Fig. 5.3).

Table 5. Measurements (in mm) of Trigonia reticulata Agassiz, 1840. See Figure 2 for measurement key.

Materials

Two specimens with shell preserved from the middle Bathonian–lower Kimmeridgian of Gebel Maghara; one specimen from the middle–upper Bathonian Kehailia Formation, Gebel Mowerib (BSPG 2014V 260/1) and the second one from the upper part of the Masajid Formation, western Bir Maghara (BSPG 2014V 432/1).

Measurements

See Table 5.

Remarks

With respect to shell outline and ornamentation of the area and flank, the present material is very similar to Trigonia monilifera Agassiz, 1840 (p. 40, pl. 3, figs. 4–6) from the Upper Jurassic of France. Some authors such as Arkell (1930, p. 81–82) and Schneider et al. (2011, p. 263) regarded that species, together with T. papillata Agassiz, T. parvula Agassiz, and T. meriani Agassiz, as junior synonyms of T. reticulata Agassiz. This view is followed here. Schneider et al. (2011, p. 264) noted that T. reticulata is a highly variable species, in which the H/L ratio of the shells varies from 0.85 to 0.97, and thus shells are generally slightly longer than high. Although the present specimens are poorly preserved, they exhibit characteristic features of T. reticulata Agassiz, such as the strongly rounded anterior and ventral margins and reticulate area, except that they are smaller.

The most closely related species is T. costata Parkinson, 1811 (p. 175, pl. 12, fig. 7) figured from the Middle Jurassic of Germany (Quenstedt 1856, p. 335, pl. 45, fig. 15, 1857, p. 440, pl. 60, figs. 10–12), the Aalenian–Bathonian of France (Schneider 1927, p. 71, pl. 5, fig. 3) and from the Bajocian of England (Cox et al., 1962, p. 74, pl. 15, fig. 8). Trigonia costata strongly resembles the present species in having the same ornamentation of the flank and area, but differs in having an elevated escutcheon (depressed in T. reticulata). In addition, its anterior and anteroventral margins meet at an obtuse angle (rounded curve in T. reticulata). According to Schneider et al. (2011, p. 264), the two species are also stratigraphically separated, T. costata ranging from the Toarcian to the earliest Callovian, whereas T. reticulata is recorded from lower Oxfordian to upper Kimmeridgian.

Trigonia infracostata Lycett, 1881 (p. 3, text-figs. 3, 4) differs from T. reticulata in being more elongated and the radial costellae of its area are spinose.

Trigonia elongata J. de C. Sowerby, 1823, of Bigot (1893, p. 291, pl. 10, fig. 7) from the Callovian–Oxfordian of France has a reticulated area as in T. reticulata but differs in being much more elongated.

Trigonia cf. T. castor d'Orbigny, 1849
Figure 5.7, 5.8

cf.*1849

Trigonia castor d'Orbigny, no. 321.

cf.1893

Trigonia castor; Bigot, p. 285, pl. 9, figs. 6–7, pl. 11, fig. 7.

cf.1913

Trigonia castor; Thevenin, p. 146 (74), pl. 27, fig. 19.

Holotype

Trigonia castor d'Orbigny, 1849, no. 321 from Bathonian strata of northern France, figured in the “Types du Prodrome” by Thevenin, 1913 (p. 146 [74], pl. 27, fig. 19; No. 321, Coll-no. 2798).

Occurrence

Bathonian of France (d'Orbigny, 1849; Bigot, 1893; Thevenin, 1913) and probably from the lower Kimmeridgian of Egypt (present study, first record).

Description

Shell small (L > 22.0 mm), thick-shelled, subtriangular, inequilateral, moderately inflated. Maximum inflation slightly below umbo. Anterodorsal margin short, slightly convex. Posterodorsal margin long, straight. Ventral margin missing. Umbo narrow, inflated, located anteriorly. Beak small, sharply pointed, strongly opisthogyrate (Fig. 5.8). Marginal carina rounded, strongly tuberculated. Antecarinal sulcus distinct, narrow dorsally, and very widened posteriorly. Escutcheon carina very prominent, strongly elevated (Fig. 5.7). Escutcheon well defined, smooth, deeply excavated. Area well defined, occupying ~45% of total valve surface, bordered by marginal and escutcheon carinae, and ornamented by tuberculated, fine radial costellae running subparallel to marginal carina. Flank moderately inflated and ornamented with strong, tuberculated commarginal costae, separated by wide, deep, smooth intercostal spaces (Fig. 5.7).

Materials

An incomplete specimen, with shell preservation, from the lower Kimmeridgian Masajid Formation, western Bir Maghara (BSPG 2014V 323/4).

Remarks

Trigonia castor d'Orbigny, 1849, can be easily distinguished from other Jurassic Trigonia species by its thick shell, large area, regular flank costae, and thick tuberculated marginal carina. The present specimen is incomplete and is, therefore, placed with reservation in T. castor, a species with similar shell outline and ornamentation of the area and flank.

The most closely related species are T. carinata aliexpandita Leanza and Garate Zubillaga, 1987 (pl. 5, figs. 1–3; pl. 6, fig. 1; Leanza, 1993, p. 24, pl. 10, figs. 3, 4) and T. stelzneri Gottsche, 1878 (p. 24, pl. 6, fig. 1a–d; Weaver, 1931, p. 240, pl. 20, figs. 103, 104) from the Lower–Middle Jurassic of Argentina. These species have a strongly protruding escutcheon, but differ in being much larger than the present material (e.g., L = 78.0 mm [Leanza, 1993, p. 24] as opposed to L > 22.0 mm [present study]) and their flank costae appear to be non-tuberculated.

Trigonia wiedmanni Leanza and Garate Zubillaga, 1987 (p. 209, pl. 6, figs. 3–5; Leanza, 1993, p. 24, pl. 17, figs. 7–10) from the Lower Jurassic of Argentina has fewer flank costae and wider intercostal spaces.

Trigonia similis Agassiz, 1840 (p. 36, pl. 2, figs. 18–21, pl. 3, fig. 7, 7′) from the Lower Jurassic of France and T. patchamensis Fürsich and Heinze, 1998 (p. 155, pl. 2, figs. 6–10) from the Middle Jurassic of Kachchh, India, have a smaller area than T. castor.

Another similar species is T. bella Lycett, 1877 (p. 162, pl. 32, figs. 6, 7, 8, 8a) from the Bajocian of England. That species resembles T. castor in having a large, ornamented area, a blunt, tuberculated marginal carina, and a protruding escutcheon, but differs in having a narrow antecarinal sulcus, less-tuberculated flank costae, and in being larger.

Trigonia americana Meek, 1873 (p. 472; Imlay, 1964, p. C28, pl. 3, figs. 8–13) from the Bajocian–Callovian of Utah, USA, resembles T. castor in having a wide ornamented area, but differs in lacking an antecarinal sulcus and in having an area with regular, smooth, radial costellae.

Superfamily Myophorelloidea T. Kobayashi, 1954
Epifamily Myophorelloidae T. Kobayashi, 1954
Family Myophorellidae T. Kobayashi, 1954
Subfamily Myophorellinae T. Kobayashi, 1954
Tribe Myophorellini T. Kobayashi, 1954
Genus Myophorella Bayle, 1878

Type species

Myophorella nodulosa Bayle, 1878, by subsequent designation (Crickmay, 1932), Oxfordian of France.

Diagnosis

See Duff (1978, p. 85).

Myophorella sp. indet.
Figure 5.9

Description

Shell small, subtrigonal, distinct inequilateral, moderately inflated, length nearly equaling height (H/L = ?0.96; Table 6). Maximum inflation slightly below umbo. Anterodorsal margin slightly convex, higher than posterodorsal one. Anterior margin regularly curved, strongly convex. Ventral margin incomplete. Posterodorsal margin slightly concave, oblique. Umbo triangular, convex, located one-third of total valve length from anterior end. Beak sharply pointed, slightly opisthogyrate. Marginal carina inconspicuous, becoming tuberculated close to posteroventral corner. Escutcheon carina tuberculated, separating ornamented area from moderately wide, steeply inclined, slightly concave, smooth escutcheon. Area wide, slightly concave, and ornamented with fine, transversely and radially aligned spines. Flanks covered with well-developed spinose oblique costae (14), straight, oblique from marginal carina to mid-flank, strongly curved anteriorly, and separated by wide and smooth intercostal spaces (Fig. 5.9).

Table 6. Measurements (in mm) of Myophorella sp. indet. See Figure 2 for measurement key.

Materials

A single incomplete left valve, with shell preserved, from the Lower Jurassic (lower Toarcian) Rajabiah Formation, western Bir Maghara (BSPG 2014V 317/13), Gebel Maghara.

Measurements

See Table 6.

Remarks

The poor preservation of the incomplete left valve precludes identification at the species level, and the specimen is, therefore, kept in open nomenclature. With respect to ornamentation of the flank, Myophorella sp. indet. resembles Myophorella (Promyophorella) spinulosa (Yong and Bird, 1828) figured by Francis (2000, p. 134. pl. 10, figs. a–g) from the Middle Jurassic (Bajocian) of England, but the latter differs in having a trapezoidal shell (rounded in the present specimen) and in being larger (L = 40 mm as opposed to 11.5 mm) and more elongated.

Genus Parorthotrigonia new genus
(= Awadia Abdallah and Fahmy, 1969)

Type species

Trigonia (Awadia) lepidomorpha Abdallah and Fahmy, 1969 (p. 149, figs. 1, 2), from the Middle Jurassic (Bathonian) of Khashm El Galala, Eastern Desert, Egypt.

Other species

Trigonia sp. of Shuwen (1999, pl. 1, fig. 9) and Trigonia (s.l.) sp. of Yin and Grant-Mackie (2005, p. 567, fig. 6.16) from the Middle–Upper Jurassic rocks of Tibet, China.

Diagnosis

Triangularly ovate trigoniid, higher than long, inequilateral, equivalved with sharply pointed orthogyrate beak, an area with distinct median groove, a generally sharp, but occasionally blunt escutcheon carina, an elevated smooth escutcheon, smooth area except for growth lines, 9–13 smooth, rounded sub-radial flank costae meeting the marginal carinae in acute angles, separated by deep and smooth intercostal spaces widening ventrally, and with 3–5 V-shaped early growth costae.

Occurrence

Middle Jurassic (Bathonian) of Khashm El Galala, Eastern Desert, Egypt (Abdallah and Fahmy, 1969), Lower Jurassic (Toarcian) of the Sinai Peninsula, Egypt (Hirsch, 1980; present study), and Middle–Upper Jurassic of Tibet, China (Shuwen, 1999; Yin and Grant-Mackie, 2005).

Etymology

Combination of Para (Greek) = next to, and the genus Orthotrigonia Cox, 1952b.

Remarks

Abdallah and Fahmy (1969, p. 149, pl. 1, figs. 1, 2) erected the subgenus Awadia (erroneously labeled as Galalea on their plate 1) to describe Trigonia (Awadia) lepidomorpha from the Middle Jurassic (Bathonian) of Khashm El Galala, Eastern Desert, Egypt, on the basis of rounded sub-radial flank costae, smooth intercostal spaces, and a terminal beak. Abbass (1962), however, had used the same name to describe his new species Nemocardium (Awadia) magharensis (Abbass, 1962, p. 128, pl. 21, figs. 7, 10, 12) from the Lower Cretaceous (lower Aptian) of Gebel Maghara. Cox et al. (1969, p. N589) regarded Nemocardium (Awadia) of Abbass (1962) as a junior synonym of the genus Nemocardium Meek, 1876. According to the International Commission on Zoological Nomenclature (e.g., Richter 1948, Article 34; ICZN 1999, Article 59.2), the subgenus of Abdallah and Fahmy (1969) is a junior homonym and, therefore, an unavailable name. For this reason, Parorthotrigonia n. gen. is erected to describe T. (A.) lepidomorpha from the Middle Jurassic of the Eastern Desert (Abdallah and Fahmy, 1969) and Lower Jurassic of Sinai (Hirsch, 1980; Abdelhady, 2014; Abdelhady and Fürsich, 2015c). Orthotrigonia Cox, 1952b (type species: Trigonia duplicata J. de C. Sowerby, 1819) resembles Parorthotrigonia n. gen. in general outline, smooth area, and V-shaped costae in early growth, but differs in being larger, more elongated, and its flank costae are sharper, and occasionally bifurcate ventrally. The early commarginal costae and the V-shaped costae that extend across the mid and late growth stages easily distinguish Vaugonia Crickmay, 1930, from Parorthotrigonia n. gen.

Parorthotrigonia lepidomorpha (Abdallah and Fahmy, 1969) n. gen. n. comb.
Figures 5.10–5.23, 6.1–6.4, 7

*1969

Trigonia (Awadia) lepidomorpha Abdallah and Fahmy, p. 149, figs. 1, 2.

.1980

Awadia lepidomorpha; Hirsch, p. 136, pl. 4, figs. 15–19.

v.2014

Awadia lepidomorpha; Abdelhady, p. 68, table 5.1.

v.2015c

Awadia lepidomorpha; Abdelhady and Fürsich, p. 265.

Figure 6. (1–4) Parorthotrigonia lepidomorpha (Abdallah and Fahmy, 1969) n. gen. n. comb. from the lower Toarcian Rajabiah Formation (marl unit), western Bir Maghara. (1, 2) Right valve, BSPG2014V 317/21, (1) dorsal view, (2) close-up showing V-shaped early growth costae; (3, 4) incomplete composite mold of articulated specimen, BSPG2014V 317/22, (3) left lateral view, (4) right lateral view. (5–17) Variation in outline of Retetrigonia imbricata (J. de C. Sowerby, 1826) n. gen. n. comb. from the middle–upper Bathonian and lower Kimmeridgian of Gebel Maghara. (5–7) Left valve, BSPG2014V 305/10, middle–upper Bathonian Kehailia Formation, Gebel Homayir, (5) lateral view showing flank with radial costae crossed by commarginal costae forming a reticulate pattern with large spines at their intersections, (6) dorsal view with very narrow, smooth escutcheon, (7) interior view showing teeth (2, 4a. 4b); (8, 9) right valve, BSPG2014V 305/11, (8) lateral view showing reticulate ornamentation with large spines, (9) interior view with two cardinal teeth (3a, 3b); (10–12) articulated specimen, BSPG2014V 316/29, lower Kimmeridgian Masajid Formation, western Bir Maghara, (10) dorsal view showing the absence of median and escutcheon carinae, (11) right lateral view showing widely spaced commarginal area costellae with numerous fine growth lines in between, (12) left lateral view; (13, 14) left valve, BSPG2014V 305/12, Kehailia Formation, Gebel Homayir, (13) lateral view, subrounded form, (14) interior view showing teeth, muscle scars, and pallial line (arrowed); (15–17) articulated specimen, BSPG2014V 305/13, (15) left lateral view, (16) dorsal view, (17) right lateral view. Scale bars = 2 mm.

Figure 7. Sketches of Awadia lepidomorpha Abdallah and Fahmy, 1969 (= Parorthotrigonia lepidomorpha [Abdallah and Fahmy, 1969] n. gen. n. comb.) from the Toarcian–early Bajocian of Gebel Maghara. (1–3) Rajabiah Formation, Wadi Mashabba, Gebel Maghara, (1) left valve with distinct median groove, (2) right valve showing smooth, rounded radial costae of flank (subrounded form), (3) dorsal view; (4) Shusha Formation, Wadi Mashabba, Gebel Maghara, interior view of right valve showing two cardinal teeth (3a, 3b) and small rounded posterior adductor muscle scar. Scale bars = 10 mm.

Holotype

Trigonia (Awadia) lepidomorpha Abdallah and Fahmy, 1969 (p. 149, figs. 1, 2), from the upper Bathonian of Khashm El Galala, Eastern Desert, Egypt. The holotype is housed in the Geological Museum, Cairo, Egypt (G.M.C.L).

Occurrence

Middle Jurassic (Bathonian) of the Eastern Desert (Abdallah and Fahmy, 1969; Abdel-Gawad and Salama, 2015), Lower–Middle Jurassic (Toarcian–lower Bajocian) of Gebel Maghara, Sinai (Hirsch, 1980; Abdelhady, 2014; Abdelhady and Fürsich, 2015c; present study).

Description

Specimens small to medium-sized, variable in outline, subrounded to subtriangular, occasionally elongated-ovate, distinctly inequilateral, equivalved, slightly higher than long, and little inflated (I/L = 0.39 on average). Anterior margin strongly rounded, meeting convex ventral margin in rounded curve. Posterior margin short, less convex, slightly extended posteriorly, meeting posterodorsal margin in an obtuse angle (~125°). Posterodorsal margin faintly concave. Anterodorsal margin slightly convex. Umbo moderately inflated, located slightly more than one-third of total valve length from anterior end (D/L = 0.38 on average; Table 7). Beak sharply pointed, small, orthogyrate to slightly opisthogyrate. Flank subtriangular, separated from area by a distinct, blunt marginal carina. Area wide, smooth, slightly concave, strongly inclined towards the flank, and subdivided into two equal halves by a median groove (Fig. 5.13, 5.14). Marginal carina well defined, sharp dorsally, becoming blunt towards posteroventral corner. Escutcheon narrow, lanceolate, smooth except for growth lines, delimited by distinct escutcheon carina. Left valve with very large triangular cardinal tooth (2), separated from narrow anterior and posterior teeth (4a, 4b) by anterior and posterior sockets, respectively (Fig. 5.16, 5.22, 5.23). Right valve with large cardinal socket and two narrow, striated anterior and posterior teeth (3a, 3b) (Fig. 5.21). Anterior adductor muscle scar small, subrounded, deep, located close to posterior tooth. Posterior adductor muscle scar subrounded, shallow, and larger than anterior one. Pallial line entire (Fig. 5.16). Flank with 9–13 rounded sub-radial costae, meeting marginal carina at acute angle. Early growth costae (3–5) are fine, spinose, and V-shaped (Fig. 6.1, 6.2), followed by sub-radial costae, separated by wide smooth intercostal spaces centrally and ventrally, rapidly narrowing towards dorsal margin (Fig. 5.13).

Table 7. Measurements (in mm) of Parorthotrigonia lepidomorpha (Abdallah and Fahmy, 1969) n. gen. n. comb. See Figure 2 for measurement key.

Materials

Fourteen specimens, incomplete and disarticulated; few of them are composite molds, from the lower Toarcian Rajabiah Formation (marl unit), western Bir Maghara (BSPG 2014V 317/14–317/27).

Measurements

See Table 7.

Remarks

Parorthotrigonia lepidomorpha (Abdallah and Fahmy, 1969) n. gen. n. comb. Differs distinctly from other Jurassic myophorellid species. It can be easily distinguished by its smooth radial costae (folded in cross-section) with V-shaped early growth costae (3–5), a smooth area, an orthogyrate to slightly opisthogyrate sharp beak, and slightly elevated escutcheon.

Shuwen (1999, pl. 1, fig. 9) and Yin and Grant-Mackie (2005, p. 567, fig. 6.16) described Trigonia sp. and Trigonia (s.l.) sp., respectively, from the Middle–Upper Jurassic rocks of Tibet, China. Their material strongly resembles P. lepidomorpha n. gen. n. comb. in size (e.g., L = 18.0 mm, Shuwen, 1999, fig. 9, as opposed to L = 19.8 mm on average, present study, Table 7), in having subtriangular valves, rounded flank costae, and a smooth area. Because only lateral views of incomplete left valves are available, it is difficult to regard their specimens as belonging to P. lepidomorpha n. gen. n. comb.

Another similar species is Trigonia baylei Philippi, 1899 (p. 86, pl. 36, fig. 9) from the Upper Jurassic (Tithonian) of Chile. Although it also has smooth radial costae, it differs from the present species in being much more elongated (L > H) and in having wider intercostal spaces, a blunt marginal carina, and flank costae meeting the marginal carina almost at right angles (acute angles in the present species).

“Trigonia” catenifera Hupé, 1854 (Philippi 1899, p. 85, pl. 36, fig. 5) from the Upper Jurassic of the same area has, in contrast to P. lepidomorpha n. gen. n. comb., costae that bifurcate towards the anteroventral margin and carry small tubercles on their crests (see Echevarría et al., 2021, fig. 18.3).

Genus Retetrigonia new genus

Type species

Trigonia imbricata J. de C. Sowerby, 1826.

Other species

Retetrigonia parcinoda (Lycett, 1872) n. gen. n. comb. from the Middle Jurassic of England.

Diagnosis

Very small myophorellid, outline pentagonal to subrounded, little inflated, inequilateral, with strongly rounded anterior and ventral margins, broad obliquely subtruncated posterior margin, straight posterodorsal margin, and a convex anterodorsal margin. Beaks very small, orthogyrate to slightly prosogyrate, area wide with widely spaced commarginal costellae passing over marginal carina onto flank. Median and escutcheon carinae absent; escutcheon very narrow and smooth; marginal carina distinct, sharp, and tuberculated; flank with radial costae crossed by commarginal costae forming a reticulate pattern with large spines at their intersections.

Occurrence

Middle–upper Bathonian (ammonite Clydomphalites clydocromphalus Zone) to lower Kimmeridgian of Gebel Maghara, Sinai, Egypt.

Etymology

Combination of Rete (Latin) = net, and the genus Trigonia.

Remarks

Retetrigonia n. gen. can be easily distinguished from other Jurassic genera by its reticulate flank ornamentation, wide area, lack of median and escutcheon carinae, a very narrow, smooth escutcheon, a subtruncated posterior margin, and small orthogyrate to slightly prosogyrate beaks.

With respect to shell outline and size, Retetrigonia n. gen. is somewhat similar to Ibotrigonia Kobayashi in Kobayashi and Tamura 1957, but that genus has a median carina and its flank is covered by widely spaced, tuberculated commarginal costae.

The wide antecarinal sulcus of the genus Frenguelliella Leanza, 1942 (type species: Trigonia inexspectata Jaworski, 1915, from the Pliensbachian of Argentina) distinguishes it from Retetrigonia. In addition, the flank and area of Frenguelliella is differently ornamented.

Retetrigonia imbricata (J. de C. Sowerby, 1826) n. gen. n. comb.
Figures 6.5–6.17, 8.1–8.3

*1826

Trigonia imbricata J. de C. Sowerby, p. 8, pl. 507, figs. 2, 3.

.1853

Trigonia imbricata; Morris and Lycett, p. 63, pl. 6, figs. 8, 8a.

?1872

Trigonia imbricata; Lycett, p. 33, pl. 6, fig. 5a, b.

?1879

Trigonia imbricata; Lycett, p. 209, pl. 36, figs. 9, 10, pl. 41, figs. 10–12.

.1916

Trigonia imbricata; Douvillé, p. 54, pl. 5, figs. 43, 44.

.1936

Trigonia imbricata; Marzloff et al., p. 97, pl. 11, figs. 10–12.

?1980

Ibotrigonia sp. cf. I. imbricata; Hirsch, pl. 4, fig. 23.

Figure 8. (1–3) Retetrigonia imbricata (J. de C. Sowerby, 1826) n. gen. n. comb. from the middle–upper Bathonian Kehailia Formation, Gebel Arousiah, (1) articulated specimen, BSPG 2014V 320/5, left lateral view showing reticulate flank ornamentation, (2) dorsal view with very narrow, smooth escutcheon, (3) right lateral view showing wide area with widely spaced commarginal costellae and absence of median and escutcheon carinae. (4–8) Promyophorella tuberculata (Agassiz, 1840) n. comb. from the Lower Jurassic Toarcian Shusha Formation, .western Bir Maghara; (4) right valve, BSPG2014V 314/15, lateral view; (5–8) articulated specimen, BSPG2014V 317/28, (5) dorsal view, (6) close-up showing thick flank costae, which is coarser close to ventral margin, and carrying well-developed nodes (arrowed), (7) left lateral view showing very widely spaced tuberculated flank costae, (8) right lateral view. (9–11) Orthotrigonia exortiva (Kitchin, 1903) n. comb. from the lower Kimmeridgian Masajid Formation, western Bir Maghara, right valve, BSPG2014V 313/19; (9) lateral view showing tuberculated, oblique radial flank costae, (10) dorsal view, (11) close-up showing arcuate commarginal costae of early growth stages. (12–15) Orthotrigonia gracilis (Kitchin, 1903) n. comb. from the Lower Toarcian Rajabiah Formation, western Bir Maghara, articulated specimen, BSPG2014V 317/29; (12) left lateral view showing tuberculated radial costae, (13) right lateral view, (14) dorsal view, (15) close-up showing the arcuate flank costae of early growth stages extending across the marginal carina to cover the dorsal part of the area (arrowed). (16–18) Trigoniid gen. et sp. indet. from the middle–late Bathonian Kehailia Formation, Gebel Arousiah, left valve, BSPG2014V 258/1; (16) left lateral view showing slightly undulated flank costae, meeting anterior margin at right angle, (17) dorsal view, (18) interior view showing large cardinal tooth 2 and anterior tooth 4a. Scale bars = 2 mm.

Holotype

Trigonia imbricata J. de C. Sowerby, 1826, pl. 507, figs. 2, 3, from the Bajocian of England (Natural History Museum, London).

Occurrence

Upper Jurassic (Bajocian–Bathonian) of England (J. de C. Sowerby, 1829; Morris and Lycett, 1853; Lycett, 1872, 1879), Bajocian of France (Marzloff et al., 1936), Upper Bajocian–Lower Kimmeridgian of Gebel Maghara, Sinai (Douvillé, 1916; Hirsch, 1980; present study).

Description

Shell very small, outline variable, ranging from subrounded to subtriangular, from oval to subpentagonal (Fig. 6.5–6.17), inequilateral, equivalved, slightly longer than high (H/L = 0.84–0.97; Table 8), and weakly inflated. Anterior and ventral margins strongly rounded. Posterior margin broad, obliquely sub-truncated, almost perpendicular to ventral margin. Posterodorsal margin straight, steeply oblique posteriorly, forming an obtuse angle with posterior margin. Anterodorsal margin shorter than posterodorsal one and slightly convex. Umbo weakly inflated, located slightly anterior of mid-length of valve. Beak small, orthogyrate to slightly prosogyrate. Marginal carina narrow, sharp, smooth in early growth, tuberculated in later growth. Area steeply inclined towards flank, wide, covered by widely spaced commarginal costellae with numerous fine growth lines in between. Costellae extending across the marginal carina onto the flank, crossing the radial costae of flank to form a reticulate pattern (Fig. 8.1). No median or escutcheon carina (Fig. 6.10, 6.16). Escutcheon narrow, deep, smooth (Fig. 8.2). Hinge of left valve with a large triangular cardinal tooth (2), a narrow subvertical anterior (4a) one, and very narrow posterior tooth (4b) running parallel to posterodorsal margin (Fig. 6.7). Hinge of right valve with two cardinal teeth (3a, 3b), separated by a deep triangular socket, with narrow anterior tooth (5a) (Fig. 6.9). Pallial line entire. Flank weakly inflated, carrying widely spaced commarginal costae crossed by radial costae and forming a grid-like pattern with spines at intersection points (Figs. 6.5, 6.8, 8.1).

Table 8. Measurements (in mm) of Retetrigonia imbricata (J. de C. Sowerby, 1826) n. gen. n. comb.; *inflation of single valve; see Figure 2 for measurement key.

Materials

Thirteen specimens with shell preserved, mostly disarticulated, from the middle–upper Bathonian and lower Kimmeridgian of Gebel Maghara: ten specimens from the middle–upper Bathonian Kehailia Formation, Gebel Homayir (BSPG 2014V 305/10–305/19); a single specimen from the same formation of Gebel Arousiah (BSPG 2014V 320/5); one specimen from Gebel Mowerib (BSPG 2014V 319/5); and a single specimen from the lower marl beds of the lower Kimmeridgian Masajid Formation, western Bir Maghara (BSPG 2014V 316/29).

Measurements

See Table 8.

Remarks

Characteristic features of Retetrigonia imbricata (J. de C. Sowerby, 1826) n. gen. n. comb. are its very small size, weak inflation, well-developed reticulate pattern of flank costellae with short spines at their intersections, wide and ornamented area, and the lack of median and escutcheon carinae.

The figured specimen of R. parcinoda (Lycett, 1872) n. gen. n. comb. (Lycett, 1872, p. 46, with text-fig.) from the Middle Jurassic of England strongly resembles the present species with respect to shell outline, size, and lack of median and escutcheon carina. Therefore, it probably falls in the variation of R. imbricata n. gen. n. comb. Although R. parcinoda n. gen. n. comb. Also has a reticulate flank ornamentation, it differs by its tiny spines at the intersections of radial and commarginal costae.

Myophorella (Haidaia) pulex Tamura, 1959 (p. 25, pl. 5, figs. 15–18) from the Jurassic of Japan differs from R. imbricata n. gen. n. comb. in having eight or more crenulated flank costae (steep on dorsal area), an ornamented escutcheon, a wide antecarinal sulcus (Tamura, 1959, text-fig. 2.1–2.4), and in being smaller.

Myophorella (H.) gracilenta Kobayashi, 1956 (p. 4, pl. 1, fig. 8; see also Tamura, 1984, p. 25, pl. 2, figs. 3–13, text-fig. 2.1–2.4) from the Upper Jurassic of China agrees with the present material with respect to shell outline and size, but differs in having a median groove, wide and ornamented escutcheon, and an indistinct marginal carina.

Genus Promyophorella Kobayashi and Tamura, 1955

Type species

Myophorella (Promyophorella) sigmoidalis Kobayashi and Tamura, 1955, from the Bajocian of Japan.

Remarks

Kobayashi and Tamura (1955) erected the subgenus Promyophorella based on sharp costae carrying numerous small tubercles. They considered Promyophorella as ancestral to Myophorella s.s., but included it in that genus because they recognized numerous intermediate forms. Echevarría et al. (2021, p. 31) modified the diagnostic features listed by Kobayashi and Tamura (1955) to include a clearly opisthogyrate shell, crescentic outline, well-defined prominent tuberculate marginal carina in early growth becoming conspicuous angular tubercles later, an ornamented area with commarginal costellae that occasionally fade at late growth stages, with median groove; narrow oblique, diverging flank costae carrying numerous small regularly aligned tubercles, a smooth antecarinal space, and an escutcheon that is smooth or covered with transverse pustulose costellae. Myophorella Bayle (1878) differs from Promyophorella in having a triangular valve, a slightly orthogyrate to opisthogyrate beak, a conspicuously nodate median carina, strongly tuberculate flank costae, and by its large size.

Moerickella Echevarría, Damborenea, and Manceñido, 2021 (p. 26, fig. 13) can be easily distinguished by its flank ornamentation. It carries oblique costae, subparallel, anteroventrally sloping, and orthogonal to the antecarinal space, which bear well-developed elongated tubercles on their posterior segments.

Fleming (1964, 1987) and Kelly (1995) considered Promyophorella as a junior synonym of Scaphogonia Crickmay, 1930. According to Francis (2000, p. 133), although Scaphogonia and Promyophorella are closely related, the subgenus Promyophorella should be used to describe those species of Myophorella that have a continuous set of highly aligned tuberculated costae across the flank. Echevarría et al. (2021) regarded Scaphogonia as a separate genus, related to Promyophorella, but differing by its flank ornamentation.

Orthotrigonia has V-shaped or L-shaped flank costae in early growth stages and steep, straight costae bifurcating anteriorly at later growth stages. For more details about the generic history of Promyophorella and comparison with other closely related genera, see Francis (2000, p. 133) and Echevarría et al. (2021, p. 31).

Based on the diagnostic features mentioned above, the present material can be assigned to Promyophorella with certainty. That genus is recorded here from the Lower Jurassic (Toarcian) rocks of Gebel Maghara, Egypt, for the first time.

Promyophorella tuberculata (Agassiz, 1840) new combination
Figure 8.4–8.8

*1840

Trigonia tuberculata Agassiz, p. 20, pl. 2, fig. 17, pl. 9, figs. 6–8.

.1905

Trigonia tuberculata; Benecke, p. 197, pl. 14, figs. 11, 11a.

.1927

Trigonia tuberculata; Schneider, p. 73, pl. 5, figs. 6, 7.

?1993

Myophorella (Myophorella) cf. M. tuberculata; Leanza, p. 29, pl. 1, fig. 9.

Holotype

Trigonia tuberculata Agassiz, 1840, pl. 2, fig. 17 from the Lower Jurassic of Gundershofen, Swabian Alb, housed in the Agassiz Collection, Neuchâtel Museum, Switzerland.

Occurrences

Lower–lower Middle Jurassic of the Swabian Alb, South Germany (Agassiz, 1840; Benecke, 1905), lower Middle Jurassic (Aalenian) of France (Schneider, 1927), and upper Lower Jurassic (Toarcian) of Egypt (present study, first record).

Description

Shell very small, crescentic, longer than high (H/L = ?0.63–0.79; Table 9), strongly inequilateral, equivalved, and strongly inflated. Maximum inflation at mid-height of valve. Posterodorsal margin long, slightly concave. Anterodorsal margin short, slightly convex, meeting anterior margin in rounded curve. Anterior margin convex, ventral margin broken off. Umbo inflated, located one-third of total valve length from anterior end. Beak small, sharply pointed, opisthogyrate. Area widening considerably towards posterior end, divided into two unequal parts by sub-median groove, and ornamented with fine commarginal costellae that fade in later growth (Fig. 8.5). Marginal carina well defined, narrow, slightly curved posterodorsally, with small tubercles, coarsening towards posteroventral corner. Antecarinal sulcus narrow, smooth, widening posteriorly. Escutcheon carina distinct, tuberculated. Escutcheon lanceolate, very shallow, ornamented with short irregular transverse costellae. Flank moderately convex with thick costae, coarser close to ventral margin, carrying well-developed nodes, separated by wide and smooth intercostal spaces (1.5–2.1 mm; Fig. 8.6).

Table 9. Measurements (in mm) of Promyophorella tuberculata (Agassiz, 1840) n. comb. See Figure 2 for measurement key.

Materials

Two incomplete specimens, with shell preservation, from the Lower Jurassic (Toarcian) Rajabiah and Shusha formations, western Bir Maghara (BSPG 2014V 314/15; 317/28).

Measurements

See Table 9.

Remarks

Promyophorella tuberculata (Agassiz, 1840) n. comb. can be easily distinguished by its small size, elongated and inflated valves, very widely spaced tuberculated flank costae, smooth intercostal spaces, and a tuberculated escutcheon carina. Francis (2000, p. 140) mentioned that Agassiz (1840) erected Trigonia tuberculata based on small and broken specimens from the Lower Jurassic of France. Therefore, he regarded T. tuberculata as a junior synonym of Myophorella (Promyophorella) spinulosa (Young and Bird, 1828) from the Middle Jurassic (Bajocian) of England (Francis, 2000, p. 134, pl. 10, figs. a–g). However, T. tuberculata differs from Myophorella (Promyophorella) spinulosa in being smaller, more elongated, and in having a crescentic valve, a tuberculated escutcheon carina, a well-developed marginal carina with small tubercles (coarsening towards the posteroventral corner), and aligned well-developed nodes, separated by wide and smooth intercostal spaces.

Promyophorella tuberculata n. comb. closely matches specimens described and illustrated as Trigonia hispida Kitchin, 1903 (p. 90, pl. 9, figs. 4, 5) from the Bathonian of India, T. aspera Lamarck, 1819, of Hébert (1861, p. 183, pl. 7, fig. 3) from the Oxfordian of France, and Myophorella (M.) bolitoensis Pérez and Reyes in Pérez et al., 2008 (p. 84, pl. 13, figs. 2, 3, 5, 6) from the lower Middle Jurassic (lower Aalenian) of Chile in shell outline and ornamentation of the area and flank. Compared with these species, Promyophorella tuberculata n. comb. differs only in being smaller (L = 11.6–12.2 mm, present material, as opposed to 42.0 mm in Kitchin, 1903; L = 65.0 mm in Pérez et al., 2008).

Trigonia formosa Lycett, 1872 (p. 35, pl. 5, figs. 4–6) from the Middle Jurassic of England (see also Benecke, 1898, p. 28, pl. 5, fig. 1; 1905, p. 193, pl. 14, figs. 7, 8; Bigot, 1893, p. 313, pl. 13, fig. 2, from the Lower and Middle Jurassic rocks of France) resembles the present material in having widely spaced tuberculated flank costae, but differs in being less elongated and larger, and in having convex posterodorsal margins and smooth areas.

Trigonia clavellata J. Sowerby, 1826, of Agassiz (1840, p. 17, pl. 5, figs. 16–18) from the Oxfordian of France is much larger and has tuberculate median and escutcheon carina.

Family Vaugoniidae T. Kobayashi, 1954
Subfamily Vaugoniinae T. Kobayashi, 1954
Tribe Vaugoniini T. Kobayashi, 1954
Genus Orthotrigonia Cox, 1952

Type species

Trigonia duplicata J. Sowerby, 1819, Middle Jurassic (Bathonian–Callovian) rocks of India.

Remarks

Cox (1952b, p. 56) erected Orthotrigonia as a subgenus of Myophorella with the type species Myophorella (Orthotrigonia) duplicata (J. Sowerby, 1819) from the Middle Jurassic (Bathonian–Callovian) strata of England. The subgenus was characterized by relatively straight and steep flank costae, a narrow, prominent umbo, and an ornamented escutcheon (Fürsich and Heinze, 1998). Some authors placed Orthotrigonia Cox, 1952b, as a subgenus of Myophorella Bayle, 1878, in the family Vaugoniidae (e.g., Agrawal, 1956, p. 100; Fischer, 1969, p. 99). Other authors placed it as a genus of the Vaugonia–Myophorella group or as a subgenus of Vaugonia Crickmay, 1930, in the family Trigoniidae (e.g., Poulton, 1979; Fleming, 1987; Kelly, 1995; Fürsich and Heinze, 1998; Fürsich et al., 2019). Kobayashi (in Kobayashi and Mori, 1955) proposed the new subgenus Hijitrigonia from the Jurassic of Japan within Vaugonia. Cox et al. (1969, p. N488) considered V. (Hijitrigonia) as a junior synonym of V. (Vaugonia) and assigned a Jurassic range to the genus, including two subgenera: V. (Vaugonia) and V. (Orthotrigonia). Orthotrigonia resembles Promyophorella in having an ornamented area (commarginal costellae), a median groove, and tuberculate marginal carina, but differs in flank ornamentation. According to Echevarría et al. (2021, p. 32), the flank of Orthotrigonia is ornamented with V-shaped or L-shaped costae at early growth stages and steep, straight costae bifurcating anteriorly at late growth stages. In contrast, Vaugonia has V-shaped tuberculated costae, which cover the entire flank. For more details and comparison with other closely related genera, see Echevarría et al. (2021, p. 31–32).

Orthotrigonia exortiva (Kitchin, 1903) new combination
Figure 8.9–8.11

*1903

Trigonia exortiva Kitchin, p. 88, pl. 9, figs. 2, 3.

.1952a

Trigonia (Scaphotrigonia) exortiva; Cox, p. 117.

.1998

Vaugonia (Orthotrigonia) exortiva; Fürsich and Heinze, p. 166, pl. 4, figs. 8, 9.

.2013

Vaugonia (Orthotrigonia) exortiva; Fürsich et al., fig. 38c.

Holotype

Trigonia exortiva Kitchin, 1903 (p. 88, pl. 9, figs. 2, 2a, 2b) from the lower part of the Jumara Formation (“Chari Group”), Kachchh, western India (collections of the Geological Survey of India, Kolkatta).

Occurrence

Middle Jurassic of India (Kitchin, 1903; Fürsich and Heinze, 1998; Fürsich et al., 2013) and Upper Jurassic (lower Kimmeridgian) of Gebel Maghara (present study, first record).

Description

Shell small, triangular, inequilateral, longer than high (H/L = ?0.87; Table 10), moderately inflated. Posterior margin subtruncated, forming angle with ventral margin. Posterodorsal margin faintly concave. Anterior and anterodorsal margins broken off. Umbo inflated, located anteriorly of mid-length of valve. Beak sharply pointed and opisthogyrate. Flank subtriangular, weakly inflated, separated from area by blunt marginal carina. Area wide, slightly concave, and covered by widely spaced commarginal costellae. Escutcheon carina distinct, slightly incurved, tuberculated. Escutcheon not well preserved. The flank ornament consists of five arcuate costae with numerous radially arranged riblets that extend from the crest of the costae (Fig. 8.11), followed ventrally by ~13 subradial costae, meeting marginal carina at acute angle and carrying small tubercles. Costae separated by narrow, deep, smooth intercostal spaces (Fig. 8.9).

Table 10. Measurements (in mm) of Orthotrigonia exortiva (Kitchin, 1903) n. comb. See Figure 2 for measurement key.

Materials

An incomplete right valve, with shell preserved, from the upper part of the lower Kimmeridgian Masajid Formation, western Bir Maghara (BSPG 2014V 313/19).

Measurements

See Table 10.

Remarks

According to Fürsich and Heinze (1998, p. 166), Orthotrigonia exortiva (Kitchin, 1903) n. comb. can be easily distinguished from other Jurassic species of the genus by having a weakly inflated valve, 13–14 fairly dense, tuberculated, oblique radial costae, an area with widely spaced commarginal costellae, and by its much smaller size. With respect to general shape, outline, and ornamentation, the present material strongly corresponds to the figures given by Kitchin (1903), Fürsich and Heinze (1998), and Fürsich et al. (2013) from the Middle–Upper Jurassic of India, but differs in being smaller (L = ?13.0 mm).

Orthotrigonia duplicata (J. Sowerby, 1819) (Lycett, 1872, p. 14, pl. 1, figs. 8–10) from the Bajocian of England differs from the present species in having bifurcating flank costae close to the anteroventral corner and in being larger and much more inflated than O. exortiva n. comb.

Orthotrigonia gracilis (Kitchin, 1903) n. comb. can be easily distinguished by having seven arcuate costae in early growth stages, which pass over the marginal carina to cover the dorsal part of the area (Fig. 8.12).

Orthotrigonia hispida (Kitchin, 1903, p. 93, pl. 9, fig. 6) from the Jurassic rocks of Kachchh, India (see also Fürsich et al., 2019, p. 170, pl. 7, figs. 4, 5, from the Upper Jurassic of Madagascar) is more inflated and has a shorter escutcheon and fewer subvertical radial flank ribs.

Orthotrigonia kutchensis (Kitchin, 1903) n. comb. (p. 84, pl. 8, figs 7–9) from the Upper Jurassic of Kachchh, India, differs from Orthotrigonia exortiva n. comb. in being larger and in having fewer flank costae (Kitchin, 1903, p. 85).

Orthotrigonia gracilis (Kitchin, 1903) n. comb.
Figure 8.12–8.15

*1903

Trigonia gracilis Kitchin, p. 95, pl. 9, fig. 7, 7a.

.1952a

Trigonia (Scaphotrigonia) gracilis; Cox, p. 117.

.1998

Vaugonia (Orthotrigonia) gracilis; Fürsich and Heinze, p. 164, pl. 3, fig. 10, pl. 4, figs. 1–3.

.2017

Vaugonia (Orthotrigonia) gracilis; Jaitly, p. 49, appendix B.

Holotype

Trigonia gracilis Kitchin, 1903 (p. 95, pl. 9, fig. 7, 7a) from the lower part of the Jumara Formation (“Chari Group”) of Keera Dome, Kachchh, western India (collections of the Geological Survey of India, Kolkatta).

Occurrence

Middle Jurassic of India (Kitchin, 1903; Fürsich and Heinze, 1998; Jaitly, 2017) and Lower Jurassic (Toarcian) of Gebel Maghara (present study, first record).

Description

Shell small, crescentic, distinctly inequilateral, equivalved, longer than high, weakly inflated (Table 11). Anterior and ventral margins strongly convex, meeting in continuous curve. Posterior margin narrowly rounded, meeting ventral margin in rounded angle. Posterodorsal margin long, straight. Anterodorsal margin slightly convex. Umbo inflated, located approximately one-fourth of total valve length from anterior end. Beak small, inconspicuous, slightly opisthogyrate. Flank weakly inflated, separated from wide area by distinct, slightly curved and tuberculated marginal carina. Area wide, flat, divided into two equal parts by median groove, and ornamented by irregular commarginal costellae (Fig. 8.13). Escutcheon excavated, moderately wide, ornamented with small, rounded tubercles, forming transverse costellae. Ornamentation of flank consisting dorsally of seven arcuate costae with numerous fine, radially arranged riblets extending from their crests, where they form short spines. Arcuate costae cross over the marginal carina to cover dorsal part of area (Fig. 8.12, 8.15). Ventrally, flank costae extend radially to ventral margin and carry radially elongated tubercles. Costae separated by wide and smooth intercostal spaces.

Table 11. Measurements (in mm) of Orthotrigonia gracilis (Kitchin, 1903) n. comb. See Figure 2 for measurement key.

Materials

A single articulated specimen, with shell preserved, from the upper part of the lower Toarcian Rajabiah Formation (marl unit), western Bir Maghara (BSPG 2014V 317/29).

Measurements

See Table 11.

Remarks

The present specimen closely corresponds in outline and ornamentation of area and flank to the figures of Trigonia gracilis Kitchin (1903, pl. 9, fig. 7, 7a) from the Middle Jurassic (Callovian) of Kachchh, India. According to Fürsich and Heinze (1998, p. 164), the flank ornamentation of Orthotrigonia gracilis (Kitchin, 1903) n. comb. varies and they regarded these variations to be the result of intraspecific variation. The most diagnostic feature of the species appears to be that the arcuate flank costae of the early growth stages extend across the marginal carina to cover the dorsal part of the area (Fig. 8.12, 8.15). Other characteristic features are the low inflation (I/L = ?0.33) and small size (H = 10.8 mm). The most closely related species is O. jumarensis (Kitchin, 1903) n. comb. (Kitchin, 1903, p. 93, pl. 9, fig. 6) from the Middle Jurassic of Kachchh, India. Fürsich and Heinze (1998, p. 167) noted that the most diagnostic features of that species are the strongly tuberculated flank and a flat area covered with strong, but somewhat irregular commarginal growth rugae. Based on the description of Kitchin (1903, p. 95), the ornamentation of the flank of O. jumarensis n. comb. and O. gracilis n. comb. is very similar. Therefore, they are possibly synonymous.

Orthotrigonia hispida (Kitchin, 1903) (p. 90, pl. 9, figs. 4, 5; see also Fürsich and Heinze, 1998, p. 166, pl. 4, fig. 6) from the Middle Jurassic of Kachchh resembles the present species in having strongly rounded shell margins and in the ornamentation of its flank, but is larger (L = 45.0 mm, Fürsich and Heinze, 1998, as opposed to L = 15.7 mm), more inflated (e.g., Kitchin, 1903, pl. 9, fig. 4a), and has a rounded escutcheon carina. Similarly, O. kutchensis (Kitchin, 1903) n. comb. (Kitchin, 1903, p. 84, pl. 8, figs. 7–9) from the Middle Jurassic of Kachchh differs by being larger and has strong radial flank costae that carry large, rounded tubercles.

Trigoniid gen. et sp. indet.
Figures 8.16–8.18, 9

Description

Shell small, triangular, inequilateral, longer than high (Table 12), and moderately inflated. Maximum inflation just below umbo. Ventral and posterior margins broken off. Umbo triangular, moderately inflated, located near mid-length of valve. Beak sharply pointed, slightly opisthogyrate. Antecarinal sulcus narrow, smooth. Hinge of left valve wide, with triangular cardinal tooth (2) and well-developed anterior tooth (4a; Figs. 8.18, 9). Anterior tooth (4a) striated, running parallel to anterodorsal margin. Flank weakly inflated, triangular, ornamented with commarginal costae separated by narrow and deep intercostal spaces, flexing slightly downwards anteriorly (Fig. 8.16).

Figure 9. Drawing of hinge of left valve of Trigoniid gen. et sp. indet. from the middle–late Bathonian Kehailia Formation, Gebel Arousiah, BSPG2014V 258/1.

Table 12. Measurements (in mm) of Trigoniid gen. et sp. indet. See Figure 2 for measurement key.

Materials

An incomplete left valve, with shell preserved, from the middle–late Bathonian Kehailia Formation, Gebel Arousiah (BSPG 2014V 258/1).

Measurements

See Table 12.

Remarks

Due to the missing area, generic identification is not possible. With respect to shell size, outline, and flank ornamentation, the present specimen is somewhat similar to Trigonia mirandaensis Lambert, 1944 (p. 371, pl. 1, fig. 9) from the Upper Jurassic of Argentina, but the latter differs in having a well-developed marginal carina and a wide and smooth antecarinal sulcus. Trigonia densestriata Behrendsen, 1892 (Leanza and Garate-Zubillaga 1987, p. 207, pl. 1, fig. 4; Leanza, 1993, p. 21, pl. 3, figs. 7–9) from the early Bajocian of the same area has a more elongated valve with a higher number of commarginal flank costae.

Size of trigoniids from Gebel Maghara

In general, trigoniids were very minor elements of Jurassic macrobenthic communities with some notable exceptions such as the Oxfordian “Trigonia clavellata Beds” of Dorset, southern England (Fürsich, 1977) and the Kimmeridgian strata of Portugal where Myophorella lusitanica (Sharpe, 1850) and related species are quite common (e.g., Choffat, 1885-1888; personal observation, F.T.F.). Trigoniids are also comparatively rare in the Jurassic succession of Gebel Maghara, (Abdelhady, 2014).

Nearly all of the taxa from Gebel Maghara are of very small to small size, most specimens not exceeding 1–2 cm in height. This is quite unusual for the group, in which the average size of most taxa is several times larger. There are several potential explanations for the small size. (1) The individuals could be juveniles, stunted, or could represent small species, but there is no indication that the trigoniids are juveniles. In the Kimmeridgian part of the succession, they occur associated with normal-sized brachiopods and other bivalves and there is no indication of size sorting (the fine-grained sediment pointing to low energy conditions). Moreover, the absence of any large adult specimens is difficult to explain. (2) Stunting, which cannot be ruled out with certainty, would imply adverse environmental conditions. However, normal-sized associated faunal elements and the moderate species diversity do not support such conditions. Stunting is, however, commonly known to affect benthic macrofauna in argillaceous substrates that experienced a certain deficiency of oxygen (e.g., Urlichs, 2011). Many of the infaunal trigoniids are found in silty marl and other infaunal bivalves, such as nuculids and the astartid Nicaniella, are commonly also smaller than elsewhere (personal observation, F.T.F.), therefore such substrates might not have been fully oxygenated. (3) With the present evidence we cannot exclude that the trigoniid fauna consists of small-sized taxa. In fact, some trigoniids do exhibit crowding of growth lines towards the ventral margin suggesting that they are adults, but other specimens are not well enough preserved to study this feature in detail. Regardless, whether these trigoniids are stunted to a certain degree or represent small species, their morphology does not just reflect juvenile but also adult features, and thus they should be described as taxa in their own right.

Conclusions

Based on newly collected material from the Jurassic strata of Gebel Maghara, Sinai, 14 species belonging to nine genera and two families of the bivalve order Trigoniida are systematically described, including the five new genera Magharitrigonia, Cotswoldella, Cornbrashella, Parorthotrigonia, Retetrigonia, and the new species Magharitrigonia asymmetrica.

Five species, Trigonia reticulata Agassiz, 1840, T. cf. T. castor d'Orbigny, 1849, Promyophorella tuberculata (Agassiz, 1840), Orthotrigonia exortiva (Kitchin, 1903), and O. gracilis (Kitchin, 1903) have been recorded from the Jurassic rocks of the studied area for the first time.