Cambrian (Stage 4 to Wuliuan) brachiopods from Sonora, Mexico

Abstract

The Cambrian successions at the Chihuarruita Hill outcrop, Sonora, Mexico, have yielded two successive linguliform brachiopod assemblages that are transitional between Cambrian Stage 4 and the newly recognized global Wuliuan Stage. The lowermost assemblage includes Dictyonina sp., Paterina sp., Eothele sp., Hadrotreta rara? (Cooper), and Linnarssonia arellanoi? (Cooper), coming from the upper part of the Buelna Formation. The younger, recently named El Gavilán Formation contains a more diverse linguliform brachiopod assemblage, including Acrothele concava Cooper, Batenevotreta? mexicana n. sp., Dictyonina minutipuncta Cooper, Eothele sp., Eoobolus sp., Hadrotreta rara? (Cooper), Linnarssonia arellanoi? (Cooper), Micromitra sp., Paterina sp., and Prototreta sp. The El Gavilán Formation contains a diverse trilobite fauna suggesting Delamaran age in terms of the Laurentian regional stratigraphical scheme. The base of the global Wuliuan Stage and Miaolingian Series is defined by the first occurrence of Oryctocephalus indicus; in the absence of the index species, the base should be provisionally placed at the base of the El Gavilán Formation. The Wuliuan age of the brachiopod assemblage recovered from the El Gavilán Formation is supported by the occurrence of Acrothele in the Cambrian biostratigraphical succession of Himalaya, where the genus makes its first appearance in the Kaotaia prachina Zone. In addition, the co-occurrences of Acrothele and Eothele can be taken as an indication of the Wuliuan age of the fauna. A new biogeographic analysis confirms that the Eothele Fauna first appeared at the end of Cambrian Stage 4, as a result of increased faunal migration within the southern tropical latitudes directed from Australasian Gondwana to Laurentia.

UUID: http://zoobank.org/6598f51f-cb07-4df3-bd1f-025a13e64edf

Introduction

The Mexican state of Sonora exposes a thick Paleozoic sedimentary basin, mostly composed of carbonate platform deposits that have been studied previously (e.g., Cooper et al., 1952; Stewart et al., 1984, 2002; Cuen-Romero et al., 2016, 2018, 2019, 2020, 2022; Noriega-Ruiz et al., 2020). This study is based on the early–middle Cambrian Chihuarruita Hill section (San José de Gracia, east of Hermosillo area) in the central part of Sonora State (Figs. 1, 2). Nardin et al. (2009, and references therein) studied the fauna and stratigraphy of the Chihuarruita Hill section, which later also was studied by Cuen-Romero et al. (2016, 2018, 2019, 2020, 2022) and Noriega-Ruiz et al. (2020). The main objective of this paper is to provide the first comprehensive description of the poorly known brachiopod fauna recovered from lower–middle Cambrian limestone beds from Sonora State.

Figure 1. Geographic map of the studied area, El Chihuarruita Hill, San José de Gracia, Sonora, Mexico (modified after Cuen-Romero et al., 2022, fig.1).

Figure 2. Stratigraphic framework and section from the El Chihuarruita Hill locality, with sampled levels.

Geological setting and stratigraphy

The Mexican state of Sonora is bordered by the Gulf of California to the west, the state of Chihuahua to the east, and the south border of the United States of America (Arizona and New Mexico) to the north. The studied Chihuarruita Hill outcrop (29°17′2.10″N, 110°35′4.89″W), is located near the town of San José de Gracia, 40 km northeast of Hermosillo, the capital of Sonora (Fig. 1). Cuen-Romero et al. (2016, 2018, 2019, 2020, 2022) and Noriega-Ruiz et al. (2020) provided more detailed comprehensive reports on the geological setting and stratigraphy in the general area.

The stratigraphy described in this report is based on fieldwork on the lower–middle Cambrian, Chihuarruita Hill section in the San José de Gracia area, led by Sébastien Clausen and Juan José Palafox Reyes, Jesus Porifirio Sosa Leon, and Blanca Buitrón-Sánchez. The studied section extends approximately from 29°17′02″N, 110°35′00″W to 29°17′00″N, 110°34′5″W. The same area was studied previously by Nardin et al. (2009), and more recently by Cuen-Romero et al. (2016, 2019, 2020, 2022) and Noriega-Ruiz et al. (2020). Brachiopods were recovered from the Buelna and El Gavilán formations in the Chihuarruita Hill section (Figs. 1, 2; Table 1).

Table 1. Distribution of brachiopods in the sampled levels (see Fig. 2) from the Buelna and El Gavilán formations at the Chihuarruita Hill outcrop, northwest Sonora State, Mexico.

Table 2. Main dimensions and ratios of ventral and dorsal valves of Batenevotreta? mexicana n. sp. from the El Gavilán Formation (sample SJG 2/9). Measurements (in μm) have been made as follows: V = ventral valve, D = dorsal valve, L = sagittal length of shell, W = maximum width of shell, L_a = length of ventral apical process; L_c, W_c = length and width of cardinal muscle scars; L_b = length of dorsal median buttress; L_s = length of dorsal median septum.

The Buelna Formation contains various limestones, dolomites, siliciclastic limestone, siliciclastic dolomite, and a few siltstones and sandstones mostly found in its basal part. The few productive samples (samples SJG2/2 and 2/3; Fig. 2, Table 1) come from limestone beds that are overlaid by alternating sandstones and shales towards the top of the formation. The whole Buelna Formation is ~60 m thick. The overlying Cerro Prieto Formation consists of 22.5 m of oolithic beds, sometimes interrupted by centimetric layers of micritic mudstones, but did not yield any brachiopods.

The richly fossiliferous El Gavilán Formation (samples SJG 2/6, 2/7, 2/9, 2/12, 2/13, 3/7, 3/8, 3/9; Fig. 2; Table 1) contains various lithologies, and consists of thinly bedded limestones, siliciclastic limestones, limy siltstone, and siltstone. Its thickness in Chihuarruita Hill section is ~200 m, and the formation is unconformably overlain by Tertiary volcanic deposits.

Materials and methods

Limestone samples (average weight of 1.2 kg) were collected directly from the lower–middle Cambrian limestone beds from the Chihuarruita Hill section (Figs. 1, 2; Table 1). They were broken into fragments and dissolved, either with ~10% acetic acid when dealing with limestone or with ~8% formic acid for the slightly dolomitic limestone. The acid-resistant residues were sifted (>50 μm), dried, and the microfossils were picked from the residues under a stereomicroscope. The brachiopods were coated with palladium and observed and imaged with a Scanning Electron Microscope at the University of Lille, France, and at the Evolutionary Biology Center, University of Uppsala, Sweden.

Repositories and institutional abbreviations

The described and figured new material is housed in the collections of University of Lille (USTL = Université des Sciences et Technologies de Lille) following the recommendation of the International Commission on Zoological Nomenclature. Other type material referred to is housed in the Smithsonian National Museum of Natural History, Washington DC (USNM).

Systematic paleontology

Subphylum Linguliformea Williams et al., 1996
Class Lingulata Gorjansky and Popov, 1985
Superfamily Linguloidea Menke, 1828
Family Eoobolidae Holmer, Popov, and Wrona, 1996
Genus Eoobolus Matthew, 1902

Type species

Obolus triparilis Matthew, 1902 (subsequent designation by Rowell, 1965, p. H263); Drumian (Eccaparadoxides eteminicus Zone), Cape Breton Island, Canada.

Eoobolus sp.
 Figure 3

Occurrence in Chihuarruita Hill outcrop

Wuliuan El Gavilán Formation (samples SJG 2/7, 3/7, 3/8).

Figure 3. Eoobolus sp. from the El Gavilán Formation (sample SJG 3/7), Cambrian (Wuliuan), Chihuarruita Hill outcrop, Mexico. (1) (USTL4031-1) Dorsal valve exterior; (2, 3, 7) (USTL4031-2) (2) ventral valve interior, (3) detail of ventral pseudointerarea, (7) oblique lateral view; (4, 6) (USTL4031-3) (4) detail of dorsal pseudointerarea, (6) dorsal valve interior; (5, 8–11) (USTL4031-4) (5) ventral valve exterior, (8) detail of ornamentation, (9) detail of weak pustulose ornamentation on juvenile part, (10) detail of umbo, (11) detail of adult pustulose ornamentation.

Description

Shell equibiconvex elongate, subtriangular. Ventral valve acuminate with triangular pseudointerarea bisected by narrow and deep pedicle groove. Propareas elevated above the valve floor, bisected by flexure lines running close to the smooth outer margins of proparea. Dorsal valve elongate, suboval. Dorsal interarea mainly occupied by shallow concave median grove. External surface of post-metamorphic shell finely and densely pustulose. Ventral interior with weakly impressed visceral area. Posterolateral muscle scars well defined, emerging from under raised propareas. Ventral vascula lateralia almost straight, narrowly divergent. Dorsal interior with long median ridge. Posterolateral muscle scars well defined, emerging from under raised propareas.

Figured material

USTL4031-1–4.

Remarks

The scant available material from El Gavilán Formation (Wuliuan) is fragmentary, making detailed description and taxonomic discrimination difficult. The species exhibits characteristic features of Eoobolus, including an elevated ventral pseudointerarea with both a deep pedicle groove and well-developed flexure lines (Fig. 3.2, 3.3, 3.7), as well as a divided and raised dorsal pseudointerarea (Fig. 3.4, 3.6), and, most importantly, a finely pustulose postmetamorphic shell (Fig. 3.1, 3.5, 3.8–3.11). The poorly known Lingulella proveedorensis Cooper in Cooper et al., 1952, and Lingulella sp., reported by McMenamin (1984) from the somewhat older Puerto Blanco Formation (Cambrian Age 3 to 4), appear to be similar in ornamentation and outline and probably represent Eoobolus, but cannot be compared in detail based on existing information. Devaere et al. (2019, figs. 13–16) more recently described material of Eoobolus sp. from the Puerto Blanco Formation; the ventral valve and ornamentation is closely similar to the younger Eoobolus sp. and may be conspecific.

Specimens referred to Lingulella proveedorensis Cooper and Lingulella sp. (probably also representing species of Eoobolus) have been briefly listed and illustrated by Cuen-Romero et al. (2018, 2019, 2020), Ramírez-Valenzuela (2019), and Noriega-Ruiz et al. (2020), but remain poorly understood.

Superfamily Acrotheloidea Walcott and Schuchert in Walcott, 1908
Family Acrothelidae Walcott and Schuchert in Walcott, 1908
Subfamily Acrothelinae Walcott and Schuchert in Walcott, 1908
Genus Acrothele Linnarsson, 1876

Type species

Acrothele coriacea Linnarsson, 1876 (subsequent designation by Oehlert, 1887, p. 1279); Cambrian (Miaolingian), Sweden.

Acrothele concava Cooper in Cooper et al., 1952
Figure 4

1952

Acrothele concava Cooper in Cooper et al., p. 44, pl. 12A, figs. 1–6.

Holotype

USNM116035d, a dorsal valve (Cooper in Cooper et al., 1952, pl. 12A, fig. 6) from the uppermost Arrojos Formation (Wuliuan), Arrojos Hills, Sonora.

Diagnosis

See Cooper in Cooper et al. (1952, p. 44).

Occurrence in Chihuarruita Hill outcrop

Wuliuan El Gavilán Formation (samples SJG 2/9, 2/13, 3/8).

Description

Shell ventribiconvex, transversely oval.

Ventral valve convex; subconical in lateral view with rounded margin, and with umbo at maximum height; well-defined sulcus on adult shell (Fig. 4.5). Ornamentation of fine concentric fila and knob-shaped pustules (Fig. 4.1–4.9). Ventral metamorphic shell (400–535 μm wide), oval with pitted ornamentation, and two low lobes, close to the center and extending to margin (Fig. 4.1–4.4). Oval pedicle foramen (120–170 μm long) not enclosed within metamorphic shell (Fig. 4.1–4.3). Median tubercle in front of pedicle foramen very poorly developed to absent (Fig. 4.1–4.3). Pseudointerarea procline and subtriangular, mostly defined by the absence of pustule-like ornamentation, and lacking well-defined intertrough.

Figure 4. Acrothele concava Cooper in Cooper et al., 1952, from the El Gavilán Formation (sample SJG 2/9), Cambrian (Wuliuan), Chihuarruita Hill outcrop, Mexico. (1–4) (USTL 4032-1) (1) Oblique posterior view of ventral valve exterior, (2) detailed view, (3) ventral metamorphic shell, (4) detail of edge of ventral metamorphic shell; (5–9) (USTL4032-2) (5) oblique lateral view of dorsal valve exterior, (6) detail of dorsal metamorphic shell, (7) detail of dorsal metamorphic shell, (8) pitted dorsal metamorphic shell, (9) pustulose adult ornamentation; (10) (USTL4032-3) dorsal valve interior; (11, 12) (USTL4032-4) (11) oblique lateral view of dorsal valve interior, (12) detail of umbo.

Ventral interior poorly preserved, with somewhat thickened area around foramen, and poorly defined vascula lateralia diverging anteriorly.

Dorsal valve flattened to slightly convex in lateral view, with subcircular margin and very shallow sulcus; fine concentric fila and pustule-like ornamentation (Fig. 4.5). Dorsal metamorphic shell marginal, oval (625–350 μm wide), with pitted ornamentation; single pair of spines at posterior margin of metamorphic shell (Fig. 4.5–4.7, 4.12). Pair of divergent, anteriorly inflated lobes terminated close to anterior margin of metamorphic shell (Fig. 4.5–4.8). Dorsal pseudointerarea poorly developed, orthocline to apsacline; median sulcus sometimes develops at edge of metamorphic shell (Fig. 4.10–4.12).

Dorsal interior poorly preserved, but with low median ridge (Fig. 4.10–4.12).

Figured material

USTL4032-1–4.

Remarks

The most distinguishing character of A. concava from the El Gavilán Formation (Wuliuan) is the unusual morphology of the metamorphic shell morphology (Fig. 4.1–4.8). Most well-known species of Acrothele have a distinctive low median tubercle in front of the pedicle foramen (e.g., Acrothele vertex Reed, 1910; Popov et al., 2015, fig. 13C, D), but here it is very poorly developed to absent (Fig. 4.1–4.3); the ventral metamorphic shell of the Mexican species lacks the characteristic Acrothele spines, and only has two low raised lobes, close to the center and extending to margin (Fig. 4.1–4.3). Moreover, the dorsal metamorphic shell of A. concava only has a single pair of low spines in front of the posterior margin, and anterior to the spines, a pair of divergent raised and inflated lobes are present (Fig. 4.7, 4.12). This type of dorsal metamorphic shell is similar to that described from an unnamed problematic species of Acrothele from the Guzhangian of Novaya Zemlya (Holmer et al., 2020, fig. 6E–H); in both these species, the dorsal lobes are considered to represent impressions from a single pair of metamorphic setal sacks (see also Zhang et al., 2018, fig. 8). In the type species, A. coriacea, the dorsal metamorphic shell has two pairs of well-developed spines (e.g., Rowell, 1980, pl. 2:6; Holmer and Popov, 2000, fig, 46.1). The species from Novaya Zemlya also lacks the paired ventral metamorphic spines, and has two inflated lobes. Most well-known species of Acrothele have a well-developed internal pedicle tube (e.g., Acrothele vertex Reed; Popov et al., 2015, fig. 13G, K), but this is absent in A. concava, where there is only a slightly thickened interior area around the foramen; a very similar morphology also is found in Acrothele? sp. from Novaya Zemlya (Holmer et al., 2020, fig. 5A, C). However, the pustulose adult ornamentation and pitted ornamentation on the metamorphic shell of A. concava is unlike the species from Novaya Zemlya, and closely similar to that of other Acrothele species (Fig. 4.8, 4.9).

Cooper in Cooper et al. (1952, p. 45) provided a short comparative discussion of his species with A. colleni Walcott, 1912, from the middle Cambrian of Montana. Specimens possibly referable to Acrothele concava and Acrothele sp. also have been listed and illustrated from the Cambrian of the region by Cuen-Romero et al. (2018, 2019, 2020) and Noriega-Ruiz et al. (2020).

Genus Eothele Rowell, 1980

Type species

Acrothele spurri Walcott, 1908 (original designation by Rowell, 1980, p. 17); Cambrian (Series 2, Bonnia-Olenellus Zone), Nevada.

Eothele sp.
Figure 5

Occurrence in Chihuarruita Hill outcrop

Cambrian Stage 4 Buelna Formation (sample SJG 2/2), and Wuliuan El Gavilán Formation (samples 2/13, 3/7).

Figure 5. Eothele sp. from the Buelna Formation (Cambrian Stage 4; sample SJG 2/2), Chihuarruita Hill outcrop, Mexico. (1, 4, 5) (USTL4033-1) (1) Oblique posterior view of ventral valve exterior, (4) oblique lateral view of ventral apex, (5) detail of ventral metamorphic shell; (2) (USTL4033-2) oblique posterior view of ventral valve exterior; (3) (USTL4034-1) oblique posterior view of ventral valve exterior; (6) (USTL4034-2) ventral valve interior with pedicle tube; (7–9) (USTL4034-3) (7) ventral pedicle foramen, (8) oblique lateral view, (9) detail of metamorphic shell.

Description

All specimens represent incomplete ventral valves, showing only a part of the shell around the metamorphic shell and the foramen opening.

Ventral valve strongly conical. Pedicle opening (330–400 μm long, 160–240 μm wide) not enclosed within metamorphic shell; well-developed media tubercle bounding anterior margin of oval foramen opening; pitted metamorphic shell with two symmetrical tubercles anterior to apex (Fig. 5). Pedicle foramen opening strongly elongate oval, almost two times longer than wide, and continuing internally as a collar-like tube; pseudointerarea procline, poorly defined, lacking intertrough (Fig. 5). Pustulose adult ornamentation. Ventral interior with short pedicle tube (Fig. 5.8).

Figured material

USTL4033-1–3.

Remarks

Although the material from the Buelna Formation (Stage 4) is fragmented, the large elongate pedicle foramen (Fig. 5.1–5.5, 5.7, 5.8), and the ventral metamorphic shell with the strong median tubercle (Fig. 5.9) and pedicle tube (Fig. 5.6) show some similarities with the type species E. spurri (Walcott, 1908) from Nevada. However, due to the lack of a definite dorsal valve and the poor preservation of the ventral, it is kept under open nomenclature.

Order Acrotretida Kuhn, 1949
Superfamily Acrotretoidea Schuchert, 1893
Family Acrotretidae Schuchert, 1893
Genus Linnarssonia Walcott, 1885

Type species

Obolella transversa Hartt in Dawson, 1868 (original designation by Walcott, 1885, p. 115); middle Cambrian of New Brunswick, Canada.

Linnarssonia arellanoi? (Cooper in Cooper et al., 1952)
Figure 6

?1952

Pegmatreta arellanoi Cooper in Cooper et al., p. 43, pl. 13C, figs. 7–12.

Figure 6. Linnarssonia arellanoi? (Cooper in Cooper et al., 1952) from the El Gavilán Formation: (1) sample SJG 2/9; (2–4), sample SJG 2/13; (5, 6), sample SJG 3/8; (7–9), sample SJG 2/7), Cambrian (Wuliuan), Chihuarruita Hill outcrop, Mexico. (1) (USTL4035-1) Dorsal valve exterior; (2, 4) (USTL4036-1) (2) dorsal valve interior, (4) detail of pseudointerarea and cardinal muscle scars; (3) (USTL4036-2) dorsal valve interior; (5, 6) (USTL4036-3) (5) dorsal view of complete articulated shell, (6) detail of umbo; (7–9) (USTL4037-1) (7) oblique lateral view of ventral valve interior, (8) detail of posterior interior and pedicle foramen, (9) detail of apical process, cardinal muscle scar.

Holotype

USNM116057d, a ventral valve (Cooper in Cooper et al., 1952, pl. 13C, fig. 7) from the El Tren Formation (Wuliuan), Arrojos Hills, Sonora.

Diagnosis

See Cooper in Cooper et al. (1952, p. 44).

Occurrence in Chihuarruita Hill outcrop

Cambrian Stage 4 Buelna Formation (sample SJG 2/2) and Wuliuan El Gavilán Formation (samples SJG 2/7, 2/9, 2/13, 3/7, 3/8, 3/9).

Description

Shell ventribiconvex and elongate oval in adults, ~80–112% as long as wide (Fig. 6.5).

Ventral valve (length 1.2–2.3 mm, width 1.2–2.3 mm) evenly convex in lateral view. Pseudointerarea apsacline, poorly defined. Intertrough short and well defined (Fig. 6.5–6.8). Pedicle opening oval and not enclosed within metamorphic shell.

Ventral interior with prominent boss-like apical process, extending anteriorly for ~36–40% of the total length of valve; process laterally bounded by apical pits and a pair of diverging vascula lateralia. The apical process is triangular in anterior view. Prominent oval cardinal muscle scars placed laterally, located close to the internal foramen (Fig. 6.7–6.9).

Dorsal valve (length 1.7–2.3 mm, width 1.8–2.5 mm) convex. Dorsal metamorphic shell located near posterior edge of shell (Fig. 6.6).

Dorsal valve with short median groove and orthocline propareas. Median groove almost four times wider than long. Well-developed median ridge extending anteriorly for ~64–83% of total valve length; well-developed median buttress fused with median ridge. Prominent cardinal and anterocentral muscle scars (Fig. 6.2–5.4).

Figured material

USTL4035-1, 4036-1–3, 4037-1.

Remarks

Linnarssonia and similar taxa (e.g., Pegmatreta and Hadrotreta) are known to be taxonomically complex genera because of the great morphological variation and general lack of detailed studies (see Holmer et al., 2001); recent discussions on this and related problematic genera can be found in Peel et al. (2016) and Ushatinskaya and Korovnikov (2019). Holmer and Popov (2000) and Ushatinskaya and Korovnikov (2019) included Pegmatreta within Linnarssonia, and this is followed here. Linnarssonia arellanoi (Cooper in Cooper et al., 1952) is a particularly poorly known species that has only ever been recorded from the type horizon in the slightly younger El Tren Formation. It has never been re-studied, but the illustrations and description by Cooper in Cooper et al. (1952, pl. 13C, figs. 7–12) are very similar in all morphological details—including the elongate oval shape of the shell, prominent boss-like apical process, and long dorsal median septum—to the slightly older material described here (Fig. 6). Pending re-study of the type material, they are questionably referred to the same species.

Linnarssonia arellanoi is similar in most characters to the somewhat younger L. ophirensis (Walcott, 1902) from the Drumian of Utah (Rowell, 1966), and pending further studies it is possible that they are conspecific.

Genus Hadrotreta Rowell, 1966

Type species

Acrotreta primaea Walcott, 1902 (original designation by Rowell, 1966, p. 12); lower to middle Cambrian (Stage 4 to Wuliuan) Pioche Shale, Nevada, USA.

Hadrotreta rara? (Cooper in Cooper et al., 1952)
Figure 7

?1952

Hadrotreta rara Cooper in Cooper et al., p. 42, pl. 12C, figs. 10–19.

Figure 7. Hadrotreta rara? (Cooper in Cooper et al., 1952) from the Buelna Formation (Cambrian Stage 4; sample SJG 2/2), Chihuarruita Hill outcrop, Mexico. (1, 10) (USTL4038-1) (1) Dorsal valve exterior, (10) oblique posterior view of dorsal metamorphic shell; (2, 4) (USTL4038-2) (2) dorsal valve interior, (4) oblique lateral view; (3) (USTL4038-3) ventral valve exterior; (5) (USTL4038-4) oblique lateral view of dorsal valve interior; (6–9) (USTL4038-5) (6) oblique lateral view of ventral valve exterior, (7) detail of umbo, (8) detail of ventral metamorphic shell, (9) oblique lateral view of umbo; (11) (USTL4038-6) oblique lateral view of ventral valve interior.

Holotype

USNM116057d, a dorsal valve (Cooper in Cooper et al., 1952, pl. 12C, fig. 18) from the uppermost Arrojos Formation (Wuliuan), Arrojos Hills, Sonora.

Diagnosis

See Cooper in Cooper et al. (1952, p. 42).

Occurrence in Chihuarruita Hill outcrop

Cambrian Stage 4 Buelna Formation (samples SJG 2/2) and Wuliuan El Gavilán Formation (samples SJG 2/6, 2/9, 2/13, 3/7, 3/8, 3/9.).

Description

Shell ventribiconvex, subcircular in outline.

Ventral valve cone-shaped with rounded to sub-rectangular outline (length 863 μm–1.8 mm, width 858 μm–1.9 mm), ~71–102% as long as wide; maximum width about mid-valve (Fig. 7.3, 7.6). Ventral pseudointerarea poorly defined, subtriangular, and procline to apsacline, but with well-developed narrow intertrough that runs from the margin to the pedicle opening. Ventral metamorphic shell convex and circular. Oval pedicle foramen situated outside the metamorphic shell (Fig. 7.7–7.9). Ventral interior poorly preserved in most specimens, with well-defined pedicle tube, a pair of cardinal muscle scars, and well-developed apical process anterior to the foramen. Apical pits placed directly lateral to pedicle foramen (Fig. 7.11).

Dorsal valve convex, subcircular in outline (length 1.1–1.8 mm, width 1.2–2.1 mm), ~83–88% as long as wide, width with flat posterior part. Dorsal metamorphic shell with shallow median sulcus extending from metamorphic shell to anterior margin (Fig. 7.1, 7.10).

Dorsal pseudointerarea anacline, median groove occupying total width of posterior margin, but shortly developed along posterior-anterior axis. Median groove, wide, triangular, supported by median buttress, which is fused with a median septum. Triangular median septum extending for ~65–75% of the total valve length. Dorsal cardinal muscle scars bean-shaped, located directly anterior to propareas (Fig. 7.2, 7.4, 7.5).

Figured material

USTL4038-1–6.

Remarks

As discussed above, Hadrotreta can be difficult to distinguish from Linnarssonia (=Pegmatreta Bell, 1941; Holmer and Popov, 2000). Pegmatreta rara Cooper in Cooper et al., 1952, has never been re-studied, but the illustrations and description (Cooper in Cooper et al., 1952, pl. 12C, figs. 10–19) appear to be very similar in all morphological details—including the shape of the shell, dorsal pseudointerarea, and dorsal median septum (Fig. 7.1–7.6). The new Sonoran material is tentatively considered to be conspecific, pending restudy, and it is placed within Hadrotreta based on the presence of an oval pedicle foramen situated outside the metamorphic shell (Fig. 7.9), widely spaced dorsal cardinal scars (Fig. 7.2, 7.4, 7.5), and deep apical pits directly lateral to the internal pedicle tube (Fig. 7.11), as well as having a low dorsal median ridge (Fig. 7.4, 7.5). However, the ventral interior is comparatively poorly preserved in the available specimens and the characteristic boss-like apical process is not well seen (Fig. 7.11). Hadrotreta rara? appears to be most similar to the type species (see Liang et al., 2022), but has a longer dorsal median ridge, although most dorsal valves are fragmented anteriorly, and moreover, the “forked” septum with the dorsal anterior lateral scars (Fig. 7.2, 7.4, 7.5) are not as well developed as in other species of the genus.

Poorly known specimens referred to Pegmatreta rara also have been listed and illustrated from the Cambrian of the region by Cuen-Romero et al. (2018, 2019, 2020) and Noriega-Ruiz et al. (2020).

Genus Prototreta Bell, 1938

Type species

Prototreta trapeza Bell, 1938 (original designation by Bell, 1938, p. 405); middle Cambrian (Bathyuriscus Zone), Montana, USA.

Prototreta sp.
Figure 8

Occurrence in Chihuarruita Hill outcrop

Wuliuan El Gavilán Formation (samples SJG 2/9, 2/13, 3/7, 3/8, 3/9).

Figure 8. Prototreta sp. from the El Gavilán Formation (sample SJG 2/13), Cambrian (Wuliuan), Chihuarruita Hill outcrop, Mexico. (1–3) (USTL4039-1) (1) ventral valve exterior, (2) oblique posterolateral view, (3) detail of umbo, (4–6) (USTL4039-2) (4) oblique lateral view dorsal valve exterior, (5) oblique lateral view of umbo, (6) oblique lateral view of dorsal metamorphic shell; (7) (USTL4039-3) dorsal valve exterior; (8–10) (USTL4039-4) (8) dorsal valve exterior, (9) oblique lateral view, (10) oblique lateral view of dorsal pseudointerarea, (11, 12) (USTL4039-5) (11) oblique anterolateral view of ventral valve interior, (12) oblique anterolateral view of apical process and muscle scars.

Description

Shell ventribiconvex, with a plano-convex dorsal valve and a highly conical ventral valve, ~25–33% as high as wide. Ornamentation with fine rugae.

Ventral valve conical (length 1.2–2.8 mm, width 1.3–3.2 mm), ~87–97% as long as wide. Pedicle foramen not enclosed within metamorphic shell. Pseudointerarea strongly procline, with well-defined intertrough (Fig. 8.1–8.3).

Ventral interior with apical process forming ridge along posterior wall of shell. Pedicle tube penetrating apical process, which is often broken in the available specimens. Pair of apical pits located postero-laterally to internal pedicle opening. Parallel ventral vascula lateralia extending from apical pits, continuing alongside apical process, before diverging anteriorly. Well-developed oval cardinal muscle scars located postero-laterally to pedicle opening (Fig. 8.11, 8.12).

Dorsal valve circular in outline and almost planar in lateral view (length 1.7–2.9 mm, width 1.6–3.4 mm), ~83–104% as long as wide. Ornamentation of fine concentric growth lines. Metamorphic shell circular, with two parallel small ridges (Fig. 8.4–8.6).

Dorsal pseudointerarea well developed, long, and wide, occupying ~18–20% and 83–100% of total length and width of valve, respectively; anacline to orthocline, with triangular wide dorsal median groove and small propareas. Dorsal cardinal muscle scars located just anterior to propareas and median groove. Moderately high median ridge, extending for ~56–60% of the length of valve; triangular in lateral view with a blade-like structure on the posterior part of the crest (Fig. 8.7–8.10).

Figured material

USTL4039-1–5.

Remarks

Brock and Percival (2006) recently discussed and reviewed Prototreta, which is clearly in need of revision. Pending such revision, it is difficult to make a detailed comparison with other species of the genus, therefore the material is kept under open nomenclature. The dorsal median septum of the Sonoran Prototreta sp. is not very high and digitate, as compared with P. trapeza Bell, 1938, and P. flabellum Bell, 1941, but more similar to P. interrupta Bell, 1941, and P. mimica Bell, 1941, all from the Cambrian of Montana, in having a lower septum with an upper rod (Fig. 8.7–8.9).

Specimens referred to Prototreta sp. also have been listed and illustrated from the Cambrian of the region by Cuen-Romero et al. (2018, 2019, 2020) and Noriega-Ruiz et al. (2020).

?Family Scaphelasmatidae Rowell, 1965
?Genus Batenevotreta Ushatinskaya, 1992

Type species

Batenevotreta formosa Ushatinskaya, 1992 (original designation by Ushatinskaya, 1992, p. 87); middle Cambrian (Amgian), Altay, Russia.

Batenevotreta? mexicana new species
Figure 9

Holotype

USTL4040-6, a ventral valve (Figure 9.8, 9.11, 9.12) from Wuliuan El Gavilán Formation (sample SJG 2/9).

Figure 9. Batenevotreta? mexicana n. sp., from the El Gavilán Formation (sample SJG 2/9), Cambrian (Wuliuan), Chihuarruita Hill outcrop, Mexico. (1, 4) (USTL4040-1) (1) Dorsal valve exterior, (4) oblique posterolateral view; (2, 5) (USTL4040-2) (2) dorsal valve interior; (3) (USTL4040-3) ventral valve exterior, (5), oblique anterolateral view; (6, 9, 10) (USTL4040-4) ventral valve interior, (9) oblique lateral view, (10) oblique posterior view; (7) (USTL4040-5) oblique lateral view of dorsal valve interior; (8, 11, 12) (USTL4040-6) holotype, oblique posterolateral view of ventral valve exterior, (11) oblique posterolateral view of ventral metamorphic shell, (12) detail of umbo.

Diagnosis

Shell transversely oval with strong, irregularly spaced rugae; ventral valve low conical; ventral pseudointerarea procline to catacline, poorly defined laterally, with shallow, poorly defined intertrough; foramen circular, not enclosed within metamorphic shell; apical process high, broad, boss-like, directly anterior to short pedicle tube; dorsal valve moderately convex; dorsal pseudointerarea poorly defined, very short, raised high above valve floor, with shallow very poorly defined median groove; dorsal interior with small, poorly developed median buttress and low median ridge; dorsal cardinal muscle fields large, transversely oval, thickened.

Occurrence in Chihuarruita Hill outcrop

Wuliuan El Gavilán Formation (sample SJG 2/9).

Description

Shell unequally biconvex, rounded, transversely oval, with strong, irregularly spaced rugae (Fig. 9.1, 9.3, 9.4).

Ventral valve low conical (length 1.0–1.2 mm, width 1.3–1.6 mm), ~76–92% as long as wide (Fig. 9.8–9.10). Maximum width in middle of shell length. Ventral pseudointerarea poorly defined laterally, procline to catacline; intertrough, shallow, poorly defined (Fig. 9.8). Ventral metamorphic shell well defined (~150–170 μm wide); pitted ornamentation poorly preserved (Fig. 9.11, 9.12). Foramen circular (~60 μm wide and long), not entirely enclosed within metamorphic shell (Fig. 9.11). Ventral interior with short pedicle tube, directly posterior to well-developed, high, boss-like apical process, extending for around one-third of the total valve length (Fig. 9.6, 9.10). Ventral cardinal muscle scars thickened and raised, short and wide extending for ~20% and 78% of total valve length and width, respectively (Fig. 9.6, 9.10). Apical pits small, located near posterior margin. Ventral vascula lateralia starting lateral to apical pits and diverging widely up to the mid-length of the valve (Fig. 9.6, 9.10).

Dorsal valve moderately convex (length 770 μm–1.66 mm, width 911 μm–1.62 mm), ~82–110% as long as wide (Fig. 9.4). Dorsal pseudointerarea very poorly defined, very short, raised high above valve floor, with shallow, very poorly defined median groove (Fig. 9.2, 9.5, 9.7).

Dorsal interior with small, poorly developed median buttress and low median ridge; dorsal cardinal muscle fields large, transversely oval, thickened, extending for ~25% and 90% of total valve length and width, respectively (Fig. 9.2, 9.5, 9.7). Dorsal median ridge low and long, extending for ~86% of total valve length. Dorsal anterolateral muscle scars located directly lateral to the median ridge (Fig. 9.2, 9.5, 9.7).

Etymology

From the occurrence in Mexico.

Figured material

USTL4040-1–6, from sample SJG 2/9.

Remarks

Batenevotreta is a rare and early member of the Scaphelasmatidae, which was originally described from the middle Cambrian of Altay, Russia, by Ushatinskaya (1992), and later recorded from the late Cambrian of Kazakhstan by Koneva and Ushatinskaya (2010). Unfortunately, the type species, B. formosa Ushatinskaya, 1992, is not well known (Holmer and Popov, 2000). Percival and Kruse (2014) most recently illustrated a questionable record from the mid-Cambrian of central Australia (southern Georgina Basin). In the species from Kazakhstan (Koneva and Ushatinskaya, 2010), B. variabilis Koneva and Ushatinskaya, 2010, and B. ivshini Koneva and Ushatinskaya, 2010, the adults have a very large pedicle foramen, which is oval to slit-like, as in other members of the Scaphelasmatidae, and the metamorphic shells have a characteristic pitting with two distinctive sizes of pits. In contrast, the pedicle foramen of B.? mexicana n. sp. is circular and comparatively small, and moreover, the pitting of the metamorphic shells is not well preserved (Fig. 9.11, 9.12). The new Sonoran material is most similar in most other characters to the species from Kazakhstan and Russia, but assignation to the genus is kept questionable due to the differences in pedicle foramen and lack of information on metamorphic pitting.

Class Paterinata Williams et al., 1996
Order Paterinida Rowell, 1965
Superfamily Paterinoidea Schuchert, 1893
Family Paterinidae Schuchert, 1893

Remarks

The studied material includes very fragmentary specimens of Dictyonina sp., Micromitra sp., and Paterina sp. that are too poorly preserved to allow closer taxonomic discrimination (Fig. 10.10–10.12). A poorly preserved ventral valve, lacking preserved ornamentation, may also represent an unidentified Paterinidae (Fig. 10.13–10.15). Sonoran specimens of Micromitra sp. and Paterina sp. were described by Cooper in Cooper et al. (1952, p. 38–39, pl. 11B, figs. 7, 8, pl. 13A, figs. 1–3), as well as listed and illustrated by Cuen-Romero et al. (2018, 2019, 2020) and Noriega-Ruiz et al. (2020).

Figure 10. Dictyonina minutipuncta (1–9) Cooper in Cooper et al., 1952, from the Cambrian (Wuliuan) El Gavilán Formation: (1–6) sample SJG 2/9, (7–9) sample SJG 2/7); Paterina sp. (10–12) and an unidentified Paterinidae (13–15) from the Cambrian Stage 4 Buelna Formation (sample SJG 2/2), Chihuarruita Hill outcrop, Mexico. (1–6) (USTL4041-1) (1) Ventral valve exterior, (2) oblique posterolateral view, (3) oblique posterolateral view of umbo, (4) detail of umbo, (5) detail of ventral metamorphic shell, (6) detail of adult ornamentation; (7–9) (USTL4042-1) oblique lateral view of ventral valve exterior, (8) detail of umbo, (9) detail of adult ornamentation; (10–12) (USTL4043-1) (10) oblique lateral view of ventral valve exterior, (11) detail of adult ornamentation, (12) detail of adult ornamentation; (13–15) (USTL4043-2) (13) ventral valve exterior, (14) oblique posterolateral view, (15) detail of umbo.

Genus Dictyonina Cooper, 1942

Type species

Trematis pannulus White, 1874 (original designation by Cooper, 1942, p. 228); Cambrian (Stage 4–Wuliuan) Pioche Shale, Nevada, USA.

Dictyonina minutipuncta Cooper in Cooper et al., 1952
Figure 10.1–10.9

1952

Dictyonina minutipuncta Cooper in Cooper et al., p. 40, pl. 11A, figs. 1–6.

Holotype

USNM116054a, a ventral valve (Cooper in Cooper et al., 1952, pl. 11A, fig. 5, 6) from the uppermost Arrojos Formation (Wuliuan), Arrojos Hills, Sonora.

Diagnosis

See Cooper in Cooper et al. (1952, p. 40).

Occurrence in Chihuarruita Hill outcrop

Wuliuan El Gavilán Formation (samples SJG 2/7, 2/9, 2/13).

Description

Ventral valve strongly convex with prominent beak and short homeodeltidium (Fig. 10.1–10.9). Ornamentation of radiating rows of subhexagonal pits, increasing in size distally (Fig. 10.1–10.9). Metamorphic shell bi-lobate, with fine pustulose ornamentation (Fig. 10.5). Ventral pseudointerarea well developed, lacking pitted ornamentation.

Interior ventral valve poorly preserved.

Figured material

USTL4041-1, 4042-1, 4043-1, 2.

Remarks

The pitted ornamentation (Fig. 10.1–10.9) and shape of the ventral valve are closely similar to the material described by Cooper in Cooper et al. (1952, pl. 11A, fig. 5, 6) from the Arrojos Formation, and they are considered to be conspecific. The Sonoran species is also similar to Dictyonina pannula (White, 1874) (see Liang et al., 2022). Specimens also have been listed and illustrated by Cuen-Romero et al. (2018, 2019, 2020) and Noriega-Ruiz et al. (2020).

?Subphylum Rhynchonelliformea Williams et al., 1996
?Indeterminate rhynchonelliform
Figure 11

Remarks

The studied material also includes possible rhynchonelliform brachiopods represented by internal molds that are too poorly preserved for closer taxonomic discrimination (Fig. 11).

Figure 11. ?Indeterminate rhynchonelliform, from the Buelna Formation (Cambrian Stage 4; sample SJG 2/2), Chihuarruita Hill outcrop, Mexico. (1–5) (USTL4043-3) (1) Internal mold of ventral valve, (2) detail of umbo, (3) detail of umbo, (4) oblique lateral view of umbo, (5) posterior view of umbo; (6, 7, 10, 11) (USTL4043-4) (6) internal mold of ventral valve, (7) detail of umbo, (10) oblique posterolateral view, (11), oblique posterior view; (8, 9) (USTL4043-5) (8) detail of umbo of internal mold of ventral valve, (9) posterior view.

Biogeographical affinities

During the Cambrian, the Sonora Region formed a part of the Caborca Terrain, which was an integral part of the southern margin of Laurentia, facing the South American sector of Gondwana (Torsvik and Cocks, 2017; Fig. 12.3).

Figure 12. Results of the pair-group cluster analysis for presence-absence data (Raup-Crick similarity) for the Cambrian linguliform genera (Cambrian Stages 4 and Wuliuan) from 26 localities (for details see Appendix; see also Popov et al., 2015). (1) Cambrian Stage 4 and Eothele Fauna; (2) Cambrian Wuliuan. (3) Paleogeographical reconstruction for Cambrian Stage 4 showing geographical distribution of the localities used in cluster analysis. Relative position of major early Paleozoic continents (Laurentia, Baltica, Gondwana, and Siberia) mainly after Cocks and Torsvik (2002), with significant emendations for Australasian peri-Gondwana.

Two successive linguliform brachiopod assemblages have been recognized in the Sonoran deposits in the time interval between Cambrian Series 4 and the newly recognized Wuliuan Stage. The lowermost assemblage, which includes Dictyonina sp., Paterina sp., Eothele sp., Hadrotreta rara?, Linnarssonia arellanoi?, comes from the upper part of the Buelna Formation (Fig. 2; Table 1). Trilobite-based correlation (e.g., Cuen-Romero et al., 2016, 2018, 2019, 2020, 2022, and references herein) supports assignment of the unit to Cambrian Series 4. The younger El Gavilán Formation contains a more diverse linguliform brachiopod assemblage, including Acrothele concava, Batenevotreta? mexicana n. sp., Dictyonina minutipuncta, Eothele sp., Eoobolus sp., Hadrotreta rara?, Linnarssonia arellanoi?, Micromitra sp., Paterina sp., and Prototreta sp. (Fig. 2; Table 1). The El Gavilán Formation also contains a diverse trilobite fauna that suggests a Delamaran age (in terms of the Laurentian regional stratigraphical scheme). Although, the index fossil for the base of the global Wuliuan Stage and Miaolingian Series–Oryctocephlus indicus–is absent, Cuen-Romero et al. (2018) suggested that it should be provisionally placed in the lower part of the El Gavilán Formation. The Wuliuan or younger age of the linguliform brachiopod assemblage recovered the El Gavilán Formation is also supported by the occurrence of Acrothele. In the Cambrian succession of Himalayas, Acrothele makes it first appearance in the Kaotaia prachina Zone, which is definitely above the estimated position of the Wuliuan Stage base (Popov et al., 2015). The co-occurrence of Acrothele and Eothele also can be taken as indication of the Wuliuan age of the fauna.

To investigate biogeographical relations of the newly recovered Cambrian linguliform brachiopod faunas from the Caborca Terrain, a cluster analysis (Raup-Crick similarity) was performed using the computer program PAST (version 3.06; Hammer et al., 2001; Fig. 12.1, 12.2). The data set (Appendix) used in analysis is modified from Popov et al. (2015), with addition of the newly recovered faunas from Sonora and new data for the Wuliuan faunas from Siberia reported by Ushatinskaya and Korovnikov (2019). In spite of restrictions caused by the low generic richness of the faunas, as well as the heterogenous character of available data (already discussed by Popov et al., 2015), it was possible to recover general patterns of biogeographical differentiation and biotic changes caused by increased migration and major biodiversification of linguliform faunas close to the base of the Wuliuan Stage (Fig. 12).

The first analysis is based on a data matrix including generic composition of 19 individual faunas (A1–A11 and B1–B8) from Cambrian Stage 4 (Fig. 12.1). The core of the analyzed matrix is derived from faunal lists, including 31 total genera, as presented by Popov et al. (2015), with addition of the faunal assemblage from the Caborca Terrane (Sonora, Buelna Formation; Appendix). The resulting cladogram mainly preserves general patterns shown by Popov et al. (2015; Fig. 12.1); however, there are also some important differences, which require consideration.

Excluding the highly endemic fauna of Anti-Atlas, Morocco (B5; located on the margin of the North African sector of Gondwana), two major clusters (Cluster A and B) can be recognized (Fig. 12.1). Cluster A includes two second-order subclusters. The first cluster is formed by the Siberian faunas (B2 and A6) plus the faunal assemblage of the peri-Gondwanan Karatau-Naryn microplate (A2). The second cluster includes most, but not all, of the faunas grouped in previous analyses by Popov et al. (2015) within the Schizopholis-Botsfordia Fauna, which is confined to tropical Gondwana, plus a single Laurentian faunal assemblage from Greenland (Fauna A4; Fig. 12.1). The latter abnormality probably was caused by the fact that Laurentian faunas from the lower to middle part of Cambrian Stage 4 are poorly known and strongly underrepresented, both in terms of geographical distribution and generic richness.

Cluster B includes faunal assemblages of the so-called Eothele Fauna (Fig. 12.1; Popov et al., 2015) as a separate Subcluster B₂. Cluster B is composed of the individual faunas from the Australasian sector of Gondwana and Laurentian faunas. The latter form a distinct third-order subcluster, which also includes the faunal assemblage from Sonora (B8), clearly showing the Laurentian biogeographical signature. Subcluster B₁ includes two individual faunas (A3 and A10; Fig. 12.1) from the Australian Sector of Gondwana, which in the previous analysis by Popov et al. (2015) was included within the Schizopholis-Botsfordia Fauna. It is possible that these Australian faunas are among the precursors of the Eothele Fauna. The new biogeographic analysis confirms the earlier suggestion by Popov et al. (2015) that the Eothele Fauna evolved late in Cambrian Stage 4 due to increased faunal migration within the southern tropical latitudes directed from Australasian Gondwana to Laurentia. Remarkably, the contemporaneous faunas of Siberia (A6), which formed an isolated continent mainly in the northern tropical latitudes, and North African sector of Gondwana (Fig. 12.1; B1, located in temperate southern latitudes) retained their individuality. Yet both contain Eothele, which was a short-lived taxon that proliferated close to the end of Cambrian Stage 4.

The second cluster analysis, which only comprises the Wuliuan Stage (Fig. 12.2), includes 41 genera representing 16 individual faunas mainly based on data published by Popov et al. (2015), with addition of new data on the linguliform faunas from Sonora and Siberia (Appendix). The results of the analysis also mainly preserved the general pattern that was found in the analysis published earlier by Popov et al. (2015); however, there are notable differences addressed below.

As in the earlier biogeographical analysis by Popov et al. (2015), two major clusters (A and B) are recognized (Fig. 12.2). Cluster A includes diverse and highly endemic faunas of Kazakh terranes (C1 and C2), which are characterized by diverse micromorphic brachiopod taxa, predominantly acrotretides. However, the Baltic fauna (C8; Fig. 12.2) surprisingly appears within this cluster—a possible explanation could be that, unlike the Kazakh faunas, which represent genuine faunal associations, the Baltic fauna is based on a composite list, while characters of individual Baltoscandian faunal associations and their biofacies differentiation remain inadequately known. Thus, the observed grouping reflects heterogeneity of the original data set used in the analysis. Nevertheless, this clustering also suggests a relative isolation of linguliform brachiopod faunal assemblages that inhabited the Kazakh terranes and Baltic continent in relation to other contemporaneous faunas.

Other Wuliuan faunas form Cluster B, which is subdivided into two second-order subclusters (Fig. 12.2; B1 and B2). The Wuliuan linguliform faunal assemblage from Sonora, Caborca Terrain (C16) appears within the large third-order Subcluster B1a, together with numerous faunas from tropical Gondwana and associated volcanic arcs and microcontinents; this subcluster also includes the Siberian fauna (C10; Fig. 12.2). The faunal assemblage from Sonora shows closest similarity to the fauna of the Alai peri-Gondwana terrane.

The only other Wuliuan fauna from Laurentia, from the Pioche Shale of Utah (C5), occurs within the third-order Subcluster B1b of the peri-Iapetus faunas together with faunal assemblages Novaya Zemlya (C4; considered as part of Baltica continent) and New Brunswick (Avalonia), which was attached to the North African sector of Gondwana through the Cambrian (Cocks and Torsvik, 2002; Fig. 12.3). A cluster of the Wuliuan peri-Iapetus faunas also was recognized earlier by Popov et al. (2015), but that also included the Wuliuan fauna of Spain (C7), which was attached to the North African sector of Gondwana. In the new analysis, the latter fauna appears at the base of second-order Subcluster B (Fig. 12.3).

The biogeographical affinities of Laurentian faunas of Wuliuan age remain poorly resolved because of our insufficient knowledge. Further progress in biogeographical studies of the Cambrian faunas is not possible without proper documentation of individual faunas from Laurentia and Siberia, two major Cambrian continents that were distinctly isolated from the Gondwana supercontinent. A better understanding of biofacies differentiation of the Cambrian linguliform brachiopod faunas also should be taken in consideration in future analyses of their biogeographical affinities.