Comment on: Fürsich et al., 2023, Miocene instead of Jurassic: the importance of sound fieldwork for paleontological data analysis

Abstract

We published a series of papers regarding the oldest turritellids, naticids, their paleoecological interaction, and gastropod biozonation, which are of Oxfordian in age, from the Jhura pond section, Kutch, western India. Recently, an Oxfordian age was challenged by Fürsich et al. (2023) and they argued for a Cenozoic age. The authors reproduced a local geological map based on regional data where the Jhura pond section sediments were overlying the Bhuj Formation. In the original regional data, there was no Bhuj Formation and the introduction of the Bhuj Formation served to show that Jhura pond section sediments were “allochthonous”. Other lines of argument against our conclusions (e.g., identification of associated bivalve fauna, foraminiferal assemblage, and geological context) were brought forward. There were additional inconsistencies, such as the reworking of Oxfordian fossils, in their comment/opinion pieces. The only hard evidence was the report of a microfaunal assemblage, but the taxa were identified at the generic level and most of the genera appear in the Jurassic or even earlier.

Here we provide detailed and concrete evidence explaining features at the Jhura pond section, such as the subvertical nature of the beds, the ooid-bearing lithologies, the presence of various Oxfordian fossils, the difference in turritellids, naticid assemblages, and differences in the diversity curves between the present beds and the lower Miocene Chhasra Formation of Kutch. Detailed paleoecological analyses (both gastropods and bivalves) speak for two paleocommunities. We, therefore, reiterate that the present Jhura pond section sediments are Oxfordian in age and validate all the interpretations and conclusions that we have made in our previous papers.

Non-technical Summary

Turritellid gastropods are prey and naticid gastropods are predators and their oldest interaction was reported by us from the Oxfordian (157 million years ago). Recently, the Oxfordian age was disputed on the basis of mostly comments/opinion pieces. A map was reproduced based on data that are not correct. The only hard evidence provided, was the microfaunal assemblage. Against these claims, we hereby furnish several concrete examples, such as locality information, mode of occurrence, lithology (Fe-ooid-bearing beds), indigenous Oxfordian fossils, and the distinct assemblage of turritellid and naticid gastropods between the present and the Miocene beds of the Chhasra Formation, Kutch. Furthermore, the diversity curves and paleoecological analyses (both gastropods and bivalves) speak for two distinct and separate assemblages. We, therefore, reiterate our early claims about the age and evolution of prey–predator interaction and the development of the conchiolin layer in corbulid bivalves and other inferences that we have described in our previous publications.

Introduction

“One gets very skeptical about other people's records in palaeontology.”

— Ager, 1973, p. 15.

Recently, we published a series of papers (Das et al., 2018, 2019; Bardhan et al., 2021; Saha et al., 2021) on the Oxfordian turritellid gastropod-dominated marine invertebrate assemblages from Jhura, Kutch, western India. Fürsich et al. (2023) challenged an Oxfordian age of our fossil materials and advocated for a Miocene age. They tried to establish that our locality information and mode of occurrence of the beds are incorrect. Their conclusion, however, is flawed, as we demonstrate in this comment.

Locality information

Fürsich et al. (2023, p. 342, fig. 1) provided a geological map of the Jhura Dome and mentioned that this map had been “modified after Biswas and Deshpande, 1970.” The map of Biswas and Deshpande (1970, p. 117) is a regional geological map (see also Fig. 1) on which the different domes are drawn in hand sketches and shown in very spotty occurrences. We are not sure how a geological map with all of the local details (Fürsich et al., 2023, fig. 1) could be reproduced from such a simplified regional map. Careful observation of the regional map of Biswas and Deshpande (1970) and field study of the area reveal the presence of only three formations: Jhurio (Bathonian), Jumara (Callovian–Oxfordian), and Jhuran (Kimmeridgian); the Bhuj Formation is not present there. The introduction of the Bhuj Formation in the newly traced map is therefore wrong, and erroneously led Fürsich et al. (2023) to argue that the Jhura pond section is above the Bhuj Formation and therefore, “allochthonous” and not connected to the surrounding Oxfordian strata. Fürsich et al. (2023) further claimed that Biswas (1993, p. 225) found Miocene strata nearby the pond area, which is incorrect. Rather, Biswas (1993, p. 225) mentioned the presence of gently dipping Miocene strata to the south of the Jhura Dome. Another inconsistency in Fürsich et al.'s (2023) map is that the Jhura camp area is shown within the Bhuj Formation, yet they previously reported finding the ammonite Peltoceratoides Spath, 1924, and some gastropod species there, which are Oxfordian in age (Alberti et al., 2011, 2013a). This was done perhaps to avoid an isolated Bhuj Formation near the Jhura pond section. We here reproduce the map of the Jhura Dome, which Mitra and Ghosh (1964) and Mitra (1978) provided (Fig. 2). Clearly the area of the Jhura pond section (1 km SE of Jhura village) falls on the Dhosa Oolite Member, supporting our assignment of an Oxfordian age. However, we regret that the GPS data provided in our original text was incorrect (it should have been 23°25′35.5″N, 69°36′58.2″E), although the actual locality information provided (i.e., 1 km SE of Jhura village) was correct as described.

Figure 1. (1) Map of India showing the position of Kutch area marked by red rectangle. (2) Reproduction of original regional geological map of Kutch mainland of Biswas and Deshpande (1970). Position of Jhura Dome is indicated by the red square. (3) Enlargement of the Jhura Dome area in (2), with Jhura Dome marked by yellow outline. Note the sketchy nature of the Jurassic beds of the Jhura Dome. The only three formations are discernible: Jhurio, Jumara, and Jhuran.

Figure 2. Geological map of Jhura dome. Mitra and Ghosh (1964) first provided the geological map of Jhura dome along with the scale. The enlarged version of that geological map of the Jhura dome was provided by Mitra (1978) but without scale. We have redrawn the geological map of the Jhura dome from Mitra (1978) and added the map scale from Mitra and Ghosh (1964, fig. 1). The red star marks the Jhura pond section collection locality of Mitra and Ghosh (1964). Bed 1: Green-brown, oolitic limestone with sandstone bands. Bed 2: Alternating layers of gypseous shale/sandstone. Bed 3: Calcareous sandstone. Bed 4: Shale. Bed 5: Sandy limestone. Bed 6: Brown shale. Bed 7: Sandy limestone. Bed 8: Hard, compact sandstone. Bed 9: Shale. Bed 10: Red, massive sandstone. Bed 11: Green shale. Bed 12: Ferruginous sandy limestone. Bed 13: Shale. Bed 14: Ferruginous limestone. Bed 15: Brown shale. Bed 16: Brown shale with sandstone. Bed 17: Golden oolite. Bed 18: Clayey, calcareous sandstone. Bed 19: Sandy limestone. Katrol = Katrol Formation.

Fürsich et al. (2023, p. 341) alleged that Mitra and Ghosh (1979) “failed to provide precise locality information” from where they collected turritellid specimens along with the Oxfordian ammonites. In fact, Mitra and Ghosh (1964, 1979) did provide the precise locality information of their collecting site within the Dhosa Oolite Member: they reported Turritella jadavpuriensis Mitra and Ghosh, 1979, precisely from the uppermost part of the Dhosa Oolite Member. Mitra and Ghosh (1964, p. 193) reported the species as “Turritella jadavpuria n. sp.”, which was formally described in 1979. Moreover, Mitra and Ghosh (1979, p. 120) also provided the locality information of the fossil site, as mentioned in their systematics of T. jadavpuriensis under the heading ‘Locality’ — “About one kilometre, south-east of village Jhura, Kutch.” Despite the wrong co-ordinate, we wonder why one of the authors (SB) of Fürsich et al. (2023) failed to locate our pond section.

Recently, Prof. Kalyan Halder of Presidency University, Kolkata, India, a well-known worker of the Cenozoic fossils of Kutch, was conducting fieldwork there and, upon our request, he visited the Jhura pond section and located it without any difficulty. One of us, Subhendu Bardhan (SB) has been visiting Kutch since 1975 and first published on Jhura materials in 1979 (Bardhan et al., 1979). Dr. Bardhan, along with his Ph.D. students during the early 1990s (one of them is Dr. D. Mukherjee, who was a co-author of Fürsich et al., 2020), used to take long traverses several times up to the Jhura core. Dr. Bardhan encountered turritellid-bearing rocks in the adjacent Kaila River beds (he did not pursue this assemblage at that time since the main focus was ammonites and brachiopods [Dr. Subhendu Bardhan, personal communication, 2023]).

Fürsich et al. (2023) also mentioned the recent construction of many small ponds from where the typical Oxfordian turritellid species, such as T. jadavpuriensis (Fürsich et al., 2023, figs. 3.1, 3.2), were collected. It is true that the strike extension of the Jhura pond section towards the north was leveled for expansion of agricultural land during 2016. Consequently, it now appears that the turritellid-bearing beds do not have a patchy outcrop but have a larger areal extent.

Mode of occurrence

Our studied section shows steeply dipping beds (Fig. 3), as also mentioned by Fürsich et al. (2023). This may be due to local as well as regional factors. There is a reverse fault through which the Kaila River is flowing. Chowksey et al. (2011) and Kothyari et al. (2021) also recorded high-angle dipping Mesozoic rock underlying Quaternary sediments near the Kaila River (see Kothyari et al., 2021, figs. 6–8). The classical Jurassic rocks in the mainland of Kutch are found in a series of E–W trending anticlinal domes, including Jara, Jumara, Keera, Jhura, Habo, etc. (see Biswas and Deshpande, 1970, fig. 1; also Fig. 1 herein). Northern parts of these domes have been affected by the Kutch Mainland fault (KMF) and igneous activity (Biswas, 2005). In many cases, the northern parts of those domes are missing (e.g., Jara dome; Subhendu Bardhan, personal observation, 2023). In Jumara and Keera, the northern parts of the Jurassic strata (partly preserved), including the Dhosa Oolite Member (e.g., in Keera), dip at a high angle (Mitra et al., 1979; Datta, 1992). The Jhura pond section is also situated at the northern limit of the Jhura Dome, north of which there are no outcrops (only the cultivatable land within the Banni Plain [Grassland]). So, it is not surprising that the beds are subvertical.

Figure 3. Field photographs of the Jhura pond section, 1 km SE of Jhura village (23°25′35.5″N, 69°36′58.2″E). (1) Subvertical alternation of shale-sandstone with abundant turritellid specimens; photograph taken 3 January 2014; (2) subvertical fine-grained sandstone bed yielding sparse turritellids (see 3); photographs taken 24 January 2015. Prof. K. Halder confirmed on 6 February 2023 that the both beds are still present at the Jhura pond section.

Lithology of the Jhura pond section

We mentioned in our paper (Das et al., 2018) that detailed stratigraphic information is available for the Jhura pond section. It includes several shale–sandstone beds with occasional limestone bands that resemble the upper part of the lower Oxfordian Dhosa Oolite Member (see Das et al., 2018). Fe-ooids are found in varying amounts in these beds (Fig. 3.1). These Fe-ooids are also found among Turritella jadavpuriensis shell fragments (Fig. 4.4, 4.5). Similarly, many Fe-ooids have adhered to a naticid species, Euspira jhuraensis Das et al., 2019 (Das et al., 2019, fig. 4; also see below).

Figure 4. Petrographic characters of Fe-ooid-bearing rocks from the Jhura pond section, strongly resemble those of the Oxfordian Dhosa Oolite from the Bhakri area, 7 km SE of the Jhura pond section. (1, 2) Thin sections from the Jhura pond area. (3) Thin section from the Bhakri area. (4, 5) Presence of ooids on turritellid fossils (specimen nos. ISI/g/Jur/T 11003 and ISI/g/Jur/T 11004) from the Jhura pond section. Scale bars = (1–3) 25 μm; (4, 5) 1 cm.

While describing the details of the characteristic texture of the Dhosa Oolite, Datta (1992, p. 47, fig. 21) mentioned that “the textural composition suggests intermixing of materials from contrasting depositional environments. The ooids are likely to be derived from high energy parts while the lime mud is a typical product of quiet environment.” Later, Fürsich et al. (1992), Alberti et al. (2013b), and Ramkumar et al. (2013) shared the same view of the external source of ooids and quartz grains within the micritic sediment. The Fe-ooids are mostly elliptical, have numerous concentric rings, and their nuclei are occupied by bioclasts and quartz grains. We here present microphotographs of several Fe-ooid-bearing units in the Jhura pond section (Fig. 4.1, 4.2). Clearly, these resemble the Dhosa Oolite texture noted by the above-mentioned workers (see Fürsich et al., 2020, fig. 61) as well as the Dhosa Oolite in the Bhakri area (Fig. 4.3) (S. Sarkar, personal communication, 2023). It is pertinent to mention that the entire Lower Miocene succession (Khari Nadi and Chhasra formations) is completely devoid of oolitic sediment (Biswas, 1992; Kumar et al., 2009; Catuneanu and Dave, 2017). This fact strongly corroborates our correlation of the studied stratigraphic segment with the Dhosa Oolite Member.

Question of reworking of the Oxfordian fossils

Fürsich et al.'s (2023) other assumption, on the basis of which they criticized our age determination, is that the presence of many Jurassic fossils in the Jhura pond section is due to reworking. While the potential for time averaging is always to be taken seriously in any fossil assemblage (Fürsich and Aberhan, 1990), claims of reworking at a biostratigraphic scale (sensu Kidwell and Bosence, 1991) must be evaluated with particular care. In this case, the presumed mechanism of reworking (Miocene transgression) would have been expected to rework fossils from rocks of all ages over which it passed. Why then are only Oxfordian fossils found? Mitra and Ghosh (1964) reported two Oxfordian ammonites, Peltoceras kumagunense Spath, 1931, and Paryphoceras rugosum Spath, 1928, from the Dhosa Oolite bed from which they collected two turritellid species (Turritella jadavpuriensis and Turritella jhuraensis Mitra and Ghosh, 1979). Mitra and Ghosh (1979, p. 119) also mentioned that “the horizon bearing T. jadavpuriensis and T. jhuraensis is directly overlain by the Kimmeridgian horizon.” During our careful search, we also discovered one broken ammonite specimen belonging to Collotia sp., aff. C. fraasi (Oppel, 1865) (Fig. 5.1–5.3). We included this occurrence in our initial submission of our paper (Das et al., 2018), but because we were unable at the time to provide full reference of the describing author due to lack of access to literature, we deleted it at the request of the editor. Additionally, we found one belemnite specimen morphologically close to Belemnopsis tanganensis (Futterer, 1894) (see Spath, 1927, p. 9, pl. 1, figs. 3a, b, 4) (Fig. 6).

Figure 5. Additional Jurassic fossils from the Jhura pond section. (1–3) Collotia sp., aff. C. fraasi (Oppel, 1865) (specimen no. ISI/Amm/Jur/Col 1), (1) lateral, (2) ventral, and (3) apertural views. (4–6) Kutchithyris aff. K. euryptycha Kitchin, 1900 (specimen no. ISI/Brac/Jur/Kut 1), (4) dorsal, (5) ventral, and (6) anterior views. Scale bars = 1 cm.

Figure 6. (1) Field photograph of ooid- and turritellid-bearing sandstone showing belemnite specimens (2, 3, see arrows). (2) Belemnite specimen resembling Belemnopsis tanganensis (Futterer, 1894) (specimen no. ISI/Bel/Jur/K 1); (3) longitudinal section of another belemnite specimen (see arrow).

During our recent revisiting of the photographs and outcrops, we found one unidentifiable but unmistakable longitudinal cross-section of a belemnite (see Fig. 6.3) and several broken belemnite fragments. Besides cephalopods, we also found a single specimen of the brachiopod Kutchithyris aff. K. euryptycha Kitchin, 1900, in our old collections (Fig. 5.4–5.6). This species is most abundant in the Oxfordian Dhosa Oolite Member (Mukherjee et al., 2017). These occurrences clearly argue for an Oxfordian age determination.

Identification of turritellid species

The family Turritellidae occasionally has been reported from pre-Cretaceous strata, but all of these reports (including one by Fürsich, 1984) have been discounted on further investigation (see Allmon, 2011, p. 163). Claims of Jurassic turritellids should, therefore, be treated with caution, as we acknowledged in our original paper (Das et al., 2018). Fürsich et al. (2023) mentioned that the Lower Miocene sediments of Kutch contain abundant turritellids, which of course does not imply that any horizon in this area bearing abundant turritellids is of Miocene age. Recent compilations of turritelline gastropods record approximately 1,700 fossil and living species and subspecies names, which may translate into perhaps 800 valid fossil species (see Allmon, 2011). We listed both the turritellid-dominated assemblages (TDA of Allmon, 2007) and turritellid-rich assemblages (TRA) from the fossil record since the Early Cretaceous (Bardhan et al., 2021, supplementary table 3). Turritellids are excellent biostratigraphic markers within basins (e.g., Woodring, 1930, 1931; Gardner, 1935; Stenzel, 1940; Wheeler, 1958; Kauffman, 1977; Sohl, 1977; Saul, 1983; Squires, 1988), but also show notorious homoplasy across time and geography (Merriam, 1941; Marwick, 1957; Kotaka, 1978; MacNeil and Dockery, 1984; Allmon, 1994, 1996).

In our original paper (Das et al., 2018), we were careful to document all diagnostic shell characters, including whorl profile, growth-line shape, protoconch form, and early apical ontogeny. Figure 7.1, 7.2 shows the original syntype of Turritella jadavpuriensis proposed by Mitra and Ghosh (1979), which was reposited in the Central Palaeontological Laboratory, Geological Survey of India (GSI), and had been declared lost. We designated a replacement neotype (Das et al., 2018, fig. 7). Turritella jadavpuriensis is the most dominant species within the Jhura turritellid community (75% of the total turritellid shells). It has both inflated and slender variants, of which the inflated variant, shown in Figure 7.3 and 7.4, is the most common. Figure 7.5 and 7.6 shows for comparison the most abundant turritellid species in the Miocene of western India, Zaria angulata (Sowerby, 1840). This comparison shows distinct differences in terms of whorl angulation, number of spirals, the relative strength of primary spirals, and depth of suture. Turritella jadavpuriensis ranges up to 6.5 cm in height, almost double the size of Z. angulata (detail of the apical sculpture formula is available for T. jadavpuriensis, see Das et al., 2018, but such information is not available for Z. angulata). Another important difference is that T. jadavpuriensis has prosocline growth lines while Z. angulata has opisthocline growth lines.

Figure 7. (1, 2) The original syntype specimens of Turritella jadavpuriensis Mitra and Ghosh, 1979 (G.S.I. type nos. 19629 and 19630), now lost; images reproduced from Mitra and Ghosh (1979). (3, 4) The replacement paraneotype specimen of T. jadavpuriensis (specimen no. ISI/g/Jur/T 5) of Das et al. (2018, figs. 7.1, 7.2). (5, 6) Most dominant Miocene species, Zaria angulata (Sowerby, 1840) (specimen nos. ISI/G/T/MIO/K/U 192 and ISI/G/T/MIO/K/U 200). Scale bars = (1, 2) 2 cm; (3–6) 1 cm.

Figure 8 shows other turritellid species of the Oxfordian and the Miocene, again clearly showing differences. Turritella amitava Das et al., 2018 (see Fig. 8.2) has diagnostic characters (e.g., small-sized shell with subquadrate whorls, equidistant primary spirals, spiral B weak to obsolete, faint and discontinuous secondary spirals, and the growth line formula 4-4-S-P) not found in any species of Miocene age (see Fig. 8.4–8.6). It is true that the Oxfordian Turritella jhuraensis (Fig. 8.1) resembles the Miocene Turritella assimilis Sowerby, 1840 (Fig. 8.4) in having similar shell size, shell outline, whorl profile (flat to convex), and suture, but in ornamentation, they differ: T. assimilis has a higher number of secondary spirals compared to T. jhuraensis (for detail, see Das et al., 2018).

Figure 8. Comparison of three species from the Jhura pond section (1–3) and the Lower Miocene of Kutch (4–6) (see text for detailed comparisons) (1) Turritella jhuraensis Mitra and Ghosh, 1979 (no. ISI/g/Jur/T 32); (2) Turritella amitava Das et al., 2018 (no. ISI/g/Jur/T 45); (3) Turritella dhosaensis Das et al., 2018 (no. ISI/g/Jur/T 52). See also Das et al. (2018, figs. 10.1, 10.4, 14.5); (4) Turritella assimilis Sowerby, 1840 (no. ISI/G/T/MIO/C/U 12635); (5) Turritella kachchensis Vredenburg, 1928 (no. ISI/G/T/MIO/K/U 7555); (6) Haustator tauroperturritus de Montfort, 1810 (no. ISI/G/T/MIO/K/U 65). Scale bars = 1 cm.

Repository and institutional abbreviation

All specimens (both from the Oxfordian and the Miocene of Kutch) are stored in the Museum of Geological Studies Unit, Indian Statistical Institute, Kolkata, India, and are numbered following the institutional abbreviation: ISI/g/Jur/ for Jurassic gastropods; ISI/B/Jur/ for Jurassic bivalves; ISI/Amm/Jur/ for Jurassic ammonites; ISI/Bel/Jur/ for Jurassic belemnites; ISI/Brac/Jur/ for Jurassic brachiopods; ISI/G/T/MIO/ for Miocene turritellids; and ISI/G/N/MIO/ for Miocene naticids.

Contrasts in naticid assemblages between the Jurassic and Miocene of Kutch

The naticid baüplan is very simple and morphologically naticids are generalists. The problem of naticid classification is further complicated by the presence of widespread homoplasy and convergence (Kowalke and Bandel, 1996; Bandel, 1999). While identifying naticids from Kutch, we made detailed character analyses of 63 species within 17 genera belonging to the four subfamilies (i.e., Naticinae Forbes, 1838; Polinicinae Finlay and Marwick, 1937; Sininae Woodring, 1928; and Gyrodinae Wenz, 1941) and we identified several diagnostic characters of each subfamily (see Das et al., 2019, table 2). We identified one species (Gyrodes mahalanobisi Das et al., 2019) belonging to the subfamily Gyrodinae (Fig. 9.1, 9.2). Gyrodes mahalanobisi has distinct subsutural wrinkles, which are absent in other genera (see Das et al., 2019, figs. 1.3, 1.7, 3). We also described two species of Euspira Agassiz in Sowerby, 1837, that belong to the subfamily Polinicinae — one (Euspira jhuraensis Das et al., 2019; Fig. 9.3) from the Jhura pond section and another (Euspira lakhaparensis Das et al., 2019; Fig. 9.4) from the oolitic bed of the uppermost Tithonian, ~2.5 km northeast of Lakhapar (see Das et al., 2019). Species in the subfamily Polinicinae are large with at least three nucleus whorls and have a parietal callus that covers the umbilicus (Fig. 9.3). In contrast, the Lower Miocene beds of Kutch (both Khari Nadi and Chhasra formations) have a monospecific assemblage, containing only Natica obscura Sowerby, 1840, belonging to the subfamily Naticinae (Fig. 9.5, 9.6). Natica obscura has a tightly coiled shell and partially open umbilicus (see Harzhauser et al., 2009; Goswami et al., 2020). Clearly, Oxfordian and Miocene assemblages differ at both subfamily and genus levels.

Figure 9. Naticid species showing the subfamily- and the genus-level differences between the Oxfordian (1–4) and the Miocene (5, 6) assemblages of Kutch. (1, 2) Gyrodes mahalanobisi Das et al., 2019 (nos. ISI/g/Jur/N 8 and ISI/g/Jur/N 1); (3) Euspira jhuraensis Das et al., 2019 (no. ISI/g/Jur/N 77); note numerous ooid grains inside the aperture of the shell; (4) Euspira lakhaparensis Das et al., 2019 (no. ISI/g/Jur/N 99). From Das et al., 2019, figs. 1.5, 1.2, 4.1, and 6.1. (5, 6) Natica obscura Sowerby, 1840 (no. ISI/G/N/MIO/K/U 20). Scale bars = 1 cm.

Ancillary taxa

We identified a number of other gastropods and bivalves that Fürsich et al. (2023) also alleged to be of Cenozoic age, but their assessment is considered to be incorrect. Among the Jhura pond section gastropod genera, many of them have been reported previously from the Cretaceous (Stoliczka, 1868; Taylor et al., 1983). It is pertinent to mention that turritellids and naticids also were reported previously from the Cretaceous and we extended the range down into the Upper Jurassic (Das et al., 2018, 2019). We found a solitary specimen of Murex Linnaeus, 1758, and we had hoped to collect more specimens in subsequent fieldwork, but much of the outcrop was destroyed by the aforementioned expansion of agricultural land. Muricids are clearly rare in the fauna as evinced by the total absence of muricid-like drill holes within the turritellid assemblages, whereas in the Miocene, muricid-like holes are abundant (see below). Among the ancillary bivalves, Tellina Linnaeus, 1758, previously had been reported from the Permian and Jurassic (Howse, 1848; Blanford, 1870) and Anadara Gray, 1847, from the Cretaceous (Jaccard, 1869).

Indocorbula was introduced by Fürsich et al. (2000, p. 138), based on material from the Jurassic of Kutch. Species are highly variable both in shape (rostrate to nearly circular) and nature of the ornamentation (both number and strength). The escutcheon is bordered by a ridge, the lunule is large and less conspicuously bordered, and the umbones show a slight offset. From the nature of shape and variation in ornamentation as well as bordered escutcheon and offset beaks (Fig. 10), we placed our specimens certainly in Indocorbula, but our disarticulated specimens were either broken or filled in by matrix, so we provisionally assigned them to Indocorbula sp. In the case of Palaeonucula sp., most of the specimens are articulated; disarticulated shells were either broken or covered by matrix (except one). Some of the specimens are shown in Figure 10.8–10.11.

Figure 10. (1–7) Indocorbula sp. showing different diagnostic morphological characters (shape and ornamental variations, keeled escutcheon [5], offset beaks [2], and bipartite chondrophore [7] with large cardinal posterior socket) of the genus. Specimens numbers: (1, 2) ISI/B/Jur/R13/C 60; (3) ISI/B/Jur/C 10; (4–6) ISI/B/Jur/R13/C 2; (7) ISI/B/Jur/C 19. (8–11) Palaeonucula sp. (8) right valve (no. ISI/B/Jur/P 65); (9) left valve (no. ISI/B/Jur/P 68); (10) dorsal view (no. ISI/B/Jur/P 90); (11) internal view (right valve; no. ISI/B/Jur/P 75). Scale bars = (1–7) 1 cm; (8–11) 500 μm.

Bivalve predation

In the present Oxfordian bivalve assemblage, the infaunal bivalves (e.g., Indocorbula sp. and Palaeonucula sp.) were drilled (Bardhan et al., 2021). Drilling intensity (DI) was 6.20 and 4.72, respectively. Drill holes were naticid-like. There was no preference for prey selection; bivalves were equally targeted like turritellid gastropods (4.64). But in the Lower Miocene, predator–prey dynamics were completely different. In the Khari Nadi Formation, turritellid gastropods were drilled by naticid predators while epifaunal Ostrea sp. and Chlamys sp. had muricid-like drill holes. Drilling Intensities were 5.77 for turritellids, 5.95 for Ostrea sp., and 7.48 for Chlamys sp. (Bardhan et al., 2014). During deposition of the Burdigalian Chhasra Formation, there was a clear shift of prey selection towards bivalves. Chattopadhyay and Dutta (2013) documented bivalve assemblage level DI, which was 20 at the species level. The highest DI was recorded in Ostrea angulata (Lamarck, 1819) (35.4); Chlamys sp. had a DI of 13. Turritellids had an assemblage-level DI of 5.77. Bivalves were preferred prey, whereas only two specimens of Ostrea were drilled and no Chlamys specimen was found to be drilled in the Jhura pond section assemblage. Although they show a low DI (9.49), turritellids were the main target for the predation.

Diversity

Individual-based rarefaction is widely used to compare biological diversity between localities or temporal bins in ecology and paleobiology (Gotelli and Colwell, 2001; Alroy et al., 2008). Figure 11 shows the rarefied diversity curves for the Lower Miocene Chhasra Formation and the specimens from the Jhura pond section (data on the Chhasra Formation are from Goswami et al., 2020; data from the Oxfordian Jhura pond section are from Bardhan et al., 2021). Rarefaction was performed with the iNext package (Chao et al., 2014; Hsieh et al., 2022) in the R-platform (R Core Team, 2021).

Figure 11. Rarefied (solid line) and extrapolated (broken line) diversity curves for the Lower Miocene Chhasra Formation (in red) and the Oxfordian Dhosa Oolite Member (in blue), with their estimated variance. The extrapolation was performed at N = 20000.

As per Fürsich et al. (2023), we can hypothesize that the gastropod community diversity of the Jhura pond section should be similar to that of the Chhasra Formation. Our results contradict this expectation and suggest that the gastropod community of the Lower Miocene Chhasra Formation is nearly twice as diverse as the Jhura pond section, further supporting the conclusion that the Jhura pond section specimens do not represent the Lower Miocene Chhasra Formation.

Microfaunal assemblage data

Fürsich et al. (2023) mentioned that the analysis of two samples from their new locality yielded a microfaunal assemblage of Miocene age. No photographs of the specimens from these samples were provided, and identification was done only at the genus level. In our opinion, the limited information provided do not support Fürsich et al.'s (2023) conclusion (e.g., Astacolus de Montfort, 1808, is also found in the Jurassic; Biswas, 1993; Gaur and Talib, 2009). Biswas et al. (2022, p. 72) clearly mentioned that the base of the Lower Miocene Chhasra Formation is characterized by “marker Turritella–Lepidocyclina band.” Lepidocyclina Gümbel, 1870, is conspicuously absent in the Fürsich et al. (2023) microfaunal assemblage. Catuneanu and Dave (2017) described biozonation within the Chhasra Formation based on the presence of different species of Miogypsina Sacco, 1893. Again, Miogypsina is not recorded in the microfaunal assemblage of Fürsich et al. (2023).

Regarding the “Cenozoic age” of Fürsich et al.'s (2023) microfaunal assemblage, all of the microfossils (except one, Ammonia Brünnich, 1772) have a pre-Cenozoic origin (e.g., Quinqueloculina d'Orbigny, 1826, ranges from the Triassic to the Quaternary; Paleobiology Database, PBDB: https://paleobiodb.org/navigator/; March, 2023). Quinqueloculina, Astacolus, and Brizalina Costa, 1856, have been reported from the Jurassic of Kutch (Bhalla and Abbas, 1978; Biswas, 1993; Talib et al., 2007; Gaur and Talib, 2009) and the nearby Jaisalmer Basin (Jain and Garg, 2014). Fürsich et al. (2023) challenged our age determination on the basis of some microfossil genera, such as Astacolus, in favor of a Miocene age, but they themselves reported Astacolus from the Jurassic of Jhura and other localities (Fürsich et al., 2013, 2020). The presence of Ammonia may be explained as being a genus that may have had a longer range. It is worth mentioning that Bhalla and Abbas (1975) reported Cenozoic foraminiferas from Jurassic rocks of the adjacent Habo dome.

Paleoecology of two assemblages

Fürsich et al. (2023) appear particularly troubled by the implications of a Jurassic age for the Kutch fauna for our understanding of patterns of molluscan evolutionary radiation during the Mesozoic Marine Revolution (MMR). Vermeij (1977, 1987) envisaged the MMR as the diversification of many mainly durophagous predatory groups (fish, crab, etc.), which exerted intense predation pressure, one of the fallouts of which was the infaunalization of benthic prey. Vermeij (1987) mainly worked on gastropod adaptation against durophagous predation pressure. Besides these durophagous predators, drill holes appeared on prey during the Mesozoic (Koken, 1892; Fürsich and Jablonski, 1984; Zardini, 1985), including in the Jurassic fauna (Klompmaker et al., 2017). Yet in the absence of Jurassic body fossils of the predators, these holes remained difficult to interpret. Harper et al. (1998) provided a convincing record of naticid-like drill holes on the bivalve Neocrassina Fischer, 1887, from the Upper Jurassic. Similarly, some of us also reported naticid drill holes from the same Dhosa Oolite Member of Bhakri, which is 7 km southeast of the Jhura pond section (Bardhan et al., 2012). The current discovery of the naticid body fossil from the Jurassic was therefore not improbable. We are happy to note, but not surprised, that recently Prof. Fürsich along with others reported predatory drill holes from a Lower Jurassic bivalve of southern Germany (Karapunar et al., 2021).

Drilling predation exerted tremendous pressure on the molluscan benthic community especially gastropods and bivalves. The predatory gastropods are mainly muricids and naticids, both of which, until recently, were thought to have evolved during the Cretaceous. These predators evolved uncommon predatory skills over evolutionary time scales and became highly escalated (sensu Vermeij, 1987) in selecting prey size and sites for drilling on the molluscan prey. Predation intensity reached a peak during the Miocene when the DI became higher, especially in the western Tethys (Kelley and Hansen, 2006). Although DI varies even within the same time span, a broad generalization can be made of higher and higher DI through the ages.

Now in Kutch, turritellids (different species) are found in the Miocene and Jurassic. Naticids also likewise are separated at the subfamily and genus levels (in the Oxfordian, we found two genera, Gyrodes Conrad, 1860, and Euspira, whereas in the Miocene turritellids had a monospecific naticid predator, Natica obscura). Different predators show different kinds of behavioral stereotypy, and thus differences are apparent in the Miocene and the Jhura pond assemblages. Figure 12.1 shows the most frequently found Turritella jadavpuriensis (75% of total turritellids) from the Oxfordian Jhura pond section, which bears characteristic naticid drill holes that are distributed throughout the whorls indicating that the predators were not very efficient. On the other hand, Figure 12.2 shows that the prey–predator size relationship was already well established. Larger predators consumed large prey and smaller predators consumed smaller prey (Fig. 12.2), supporting Kitchell et al.'s (1981) view of energy maximization. Figure 12.3 and 12.4 shows the site and size stereotypy of naticid-like drill holes in the most dominant Miocene species. Figure 12.3 shows that most drill holes are restricted to the middle and the upper parts of the whorls unlike the distribution of drill holes all through in T. jadavpuriensis (Fig. 12.1). Figure 12.4 also shows prey and predator size stereotypy, similar to T. jadavpuriensis (R ² = 0.627). For Zaria angulata, however, both prey and predator sizes were small and predation intensity was significantly lower (DI = 4.71) than that of T. jadavpuriensis (DI = 8.93). Furthermore, some of us (Goswami et al., 2020) documented numerous muricid-like drill holes in Z. angulata. There are three muricid species found in the Chhasra Formation (Goswami et al., 2020). In general, muricid drill holes show poor site stereotypy like the present one (Fig. 12.5), but size stereotypy was significantly well established (R ² = 0.488; Fig. 12.6). Not a single muricid-like drill hole is found in our Oxfordian turritellid specimens. We recorded only one specimen of muricid as ancillary taxa. Clearly, these paleoecological interactions of prey and predator of the Miocene and Jurassic demonstrate two separate paleocommunities.

Figure 12. (1, 3) Vertical distribution (not to scale) of naticid-like drill holes on turritellid species. (1) Oxfordian Turritella jadavpuriensis and (3) Miocene Zaria angulata. (5) Vertical distribution (not to scale) of muricid-like drill holes on Z. angulata. (1, 3, 5) Solid circles indicate drill holes at the apertural side; hollow circles indicate abapertural drill holes. (2, 4, 6) Bivariate plots of Outer Drillhole Diameter (ODD), as a proxy of predator size, versus turritellid prey size (height). Relationship between predator size (ODD) and prey size (height) in (2) Oxfordian T. jadavpuriensis and (4) Miocene Z. angulata (for naticid-like drill holes); relationship between predator size (ODD) and prey size (height) in (6) Miocene Z. angulata (for muricid-like drill holes).

Ethics

In a scientific debate, authors should show restraint in making personal comments directed at others. We are flabbergasted by reading some comments in Fürsich et al. (2023), which we think were unwarranted. Examples include:

1. (1) “a group of paleontologists from Kolkata” Fürsich et al. (2023, p. 341)—How does the biogeography of workers matter here?

2. (2) The use of “allegedly” comes from upper Tithonian rocks by Fürsich et al. (2023, p. 342)—Why allegedly? We mentioned that it comes from the uppermost oolitic band from the Lakhapar section (Das et al., 2019, p. 675).

3. (3) Use of “importance of sound fieldwork” in the title of Fürsich et al. (2023)—30 years of fieldwork by Fürsich et al. (2023) escaped the notice of abundant turritellid-bearing beds! Any worker who has studied the classical Jurassic domes in the Kutch mainland knows that there are no Bhuj and Lower Cretaceous rocks near Jumara, Jhura, etc. These younger units are only exposed on the western part (e.g., towards Lakhapar).

Conclusions

Based on the evidence presented here, we reconfirm that our fossil materials from the Jhura pond section of the Jhura Dome area are Oxfordian in age and are distinctly different—faunistically and paleoecologically—from the Miocene of Kutch. The turritellid assemblage at the Jhura pond section differs from those of the Miocene of Kutch in many respects, including diversity. No Miocene beds in Kutch have ooids. On the contrary, most of the turritellid-bearing beds have varying amounts of Fe-ooids in the Jhura pond section, and ooids are present in the matrix of our reported turritellid and naticid fossils from this section. The only hard evidence for a Miocene age presented by Fürsich et al. (2023) was the presence of microfaunas. But these microfaunas are not documented by images and nearly all of the reported genera are long ranging (e.g., Astacolus ranging from the Permian to Miocene). Prey–predator interaction, and especially stereotypy, provides many deep insights into the behavior of fossil communities. Our paleoecological analyses show significant differences between the size and site stereotypy of the Oxfordian and the Miocene naticid predators. Muricid-like drill holes are abundant in Miocene turritellid faunas whereas not a single muricid-like drill hole is present in the Oxfordian assemblage. Moreover, the targets for prey selection also were different.

We strongly reaffirm our claims such as autochthony, stratigraphic position, diagnostic fossils, and data analyses about the Oxfordian age of the Jhura pond section strata. We therefore uphold all the interpretations and conclusions we have drawn in our previous research and publications (Das et al., 2018, 2019; Bardhan et al., 2021; Saha et al., 2021).