The Kutch basin developed due to the fragmentation of Gondwana during the Middle Jurassic and hosted diverse endemic fauna, of which brachiopods are one of the chief constituents. The dominant brachiopod faunal element is the terebratulid genus Kutchithyris Buckman. The genus is represented throughout the exposed Middle Bathonian to Oxfordian sequence in Kutch and is also reported sporadically from outside Kutch. The systematics of this small but distinct clade is in a state of flux. The present paper focuses on revising the systematics of the genus and its three dominant species, namely, K. acutiplicata, K. propinqua and K. euryptycha, based on numerous specimens collected from the field with precise stratigraphical and sedimentological background and the type materials. They constitute an evolving lineage, and have been known from the Upper Bathonian rocks of the Pamirs, where they are cited as one of the celebrated examples of rapid speciation. A detailed comparison of the specimens from these two areas reveals that the speciation took place in Kutch and involved cladogenesis. Thus, it provides a good example of the punctuational model of evolution.
Introduction
The Mesozoic sequence of Kutch in western India, known for its fossil treasures, has often been a favourite hunting ground for palaeontologists in the past and present. The Kutch basin emerged as a result of intense tectonic activities associated with the fragmentation of Gondwana during the Bajocian to Bathonian (Biswas, 1991; Singh et al., 1982). This newly opened-up basin provided a virgin area for faunal invasion, thereby prompting speciation events in diverse groups. Kutchithyris Buckman, 1918, a terebratulid brachiopod genus represented by K. acutiplicata as the type species, made a sudden appearance in the Middle Bathonian of Kutch with no obvious ancestor and is in fact a fleeting fossil (cf. Ager, 1984; Mukherjee et al., 2000). The genus Kutchithyris is represented by 12 species in Kutch and spans a time interval of about 13 million years from the Middle Bathonian to Oxfordian (unpublished).
Taxonomic study of Kutchithyris of the Kutch sequence has a long history starting with Kitchin (1900) and Buckman (1918). The genus constitutes an important faunal element of the Indo-Madagascan brachiopod assemblage from evolutionary as well as biogeographic stand-points. However, its taxonomy, though having had a tortuous history (see Mitra, 1974, 1978; Mitra and Ghosh, 1973), still remains in a state of flux and needs major revision. In the present study we try to rationalize the systematic position of different species described under Kutchithyris. It now becomes clear that the inadequate sample size used in the previous studies precluded a proper under-standing of intraspecific variability; further, imprecise stratigraphical and sedimentological data lay behind their classification, involving much subjective splitting. Besides, internal morphological study, a powerful tool for generic discrimination, was not adequately applied (see also Cooper, 1983). We here redefine the genus Kutchithyris and provide an emended diagnosis. Three species, viz., K. acutiplicata, K. propinqua, and K. euryptycha are redescribed. They are not only dominant elements in the Kutchithyris lineage of the study areas and stratigraphically important, but they have also wider biogeographical distribution across different faunal provinces. In the Pamirs, these three species constituting an important evolutionary lineage have been cited as one of the classical examples of rapid evolution (Raup and Stanley, 1985). Naturally, they deserve special attention and are discussed here in detail.
The Kutchithyris species have not previously been regarded in the context of facies distribution in Kutch. A detailed facies analysis reveals varying lithological associations of the different species. Their temporal distribution patterns indicate an environmental gradient from carbonate ramp to argillaceous mid-shelf. While the two older species, K. acutiplicata and K. propinqua, had a strong calcareous facies association, K. euryptycha had a heterolithic association.
Geological setting
The Mesozoic sediments of Kutch were deposited during repeated marine transgressions in a pericratonic rift basin developed by the fragmentation of Gondwana (Biswas, 1977, 1991). The rocks, ranging in age from the Bathonian to Aptian are subdivided into four major divisions, namely, the Patcham, Chari, Katrol and Bhuj formations in ascending order (Mitra et al., 1979; Krishna, 1984). The Patcham and Chari formations are the principal brachiopod fossil-bearing units and are of interest in the present study. In the mainland, the Patcham Formation is exposed at Jumara, Keera and Jhura, while the Chari Formation, besides being exposed at these sections, also crops out at the other remaining domes (Figure 1).
Figure 1
Studied area, geographical localities and distribution of Jurassic rocks (stippled) in Kutch, India.
A detailed lithological succession of the Patcham and Chari formations found in the four major sections is shown in Figure 2 (modified after Bardhan et al., 1994), in which the distribution of the species studied is indicated.
Figure 2
Stratigraphical sections at Jhura, Jumara, Keera and Jara (modified after Bardhan et al., 1994). Numbers inside sections indicate bed numbers. Dotted lines indicate age boundaries. Arrowheads indicate horizons of Kutchithyris acutiplicata (Kitchin), solid arrows indicate horizons of Kutchithyris propinqua (Kitchin) and broken arrows indicate horizons of Kutchithyris euryptycha (Kitchin). Keys: 1, limestone; 2, shale; 3, Golden Oolitic limestone; 4, coquina bed; 5, ironstone; 6, sandstone; 7, calcareous sand-stone; 8, oolitic limestone; 9, coral biostrome; 10, siltstone/mudstone.
Facies and fauna
A high resolution facies analysis and systematic recording of vertical facies transitions have resulted in building up a comprehensive stratigraphical framework (Figure 3) with requisite details in standard sedimentological terms, which helps in delineating the important events of palaeogeographical shifts in the course of sequence building. It reveals that fluctuating sea levels not only brought about changes in the lithofacies associations but also influenced temporal changes in the faunal compositions, especially in the case of the species of Kutchithyris.
Figure 3
Characteristic facies assemblage within beds comprising different members of Patcham and Chari formations. Numbers indicate members.
Revisionary work at Jhura is still going on and preliminary investigation reveals that all major facies associations are traceable and the major part of the Patcham Formation is exposed here. A brief description of the facies associations and faunal distribution is given below.
Patcham Formation
This formation exposed in Jumara is considered as a single unit (informal Upper Member) in view of its general monotony. However, detailed facies analysis reveals two distinct facies.
Facies PA.
This facies consists of limestone-marl alternation. The limestone is wackestone/floatstone. It is massive and has tabular geometry, and the average thickness is about 9 cm. A parallel-sided bioclastic packstone (1–3 cm thick) is locally intercalated (also see Fürsich and Oschmann, 1993). Their basal contact is sharp, whereas the upper contact is slightly diffused. Bivalve shells within them show a dominantly concave-up attitude. The marl is 15 cm in average thickness, darker in colour than the limestone and is laterally persistent, but with broadly undulated boundaries. This facies is dominated by diverse genera of corals and sponges and also by brachiopods including four species of Kutchithyris, viz., K. acutiplicata, K. propinqua, K. katametopa and K. planiconvexa.
Facies PB.
This facies (Figure 3) is made up of rudstone layers (average thickness is 6 cm) which are parallel-sided and contain fossils largely preserved intact. Some beds are dominated by corals, others by sponges and brachiopods. Ripped up mud clasts are common. This facies is mainly present in Bed 1 of the lower part of the Patcham Formation at Jumara, alternated with facies PA and is a coralline limestone (Figure 2). It hosts different species of brachiopods, i.e., Kutchithyris acutiplicata, K. propinqua, K. katametopa, K. planiconvexa. The next unit, Bed 2 at Jumara, represented solely by facies PA, is marked by the disappearance of K. acutiplicata and the continuation of K. propinqua. The depositional environment of these two facies has been interpreted as warm, agitated and fully marine and the fauna grew near the fair-weather wave base (Datta, 1992; Fürsich et al., 1994, Mukherjee et al., 2002).
Our preliminary investigation has found K. acutiplicata right from Bed 1, the oldest exposed carbonate facies in Jhura, which is a white limestone with shale and grayish-yellow slabby limestone alternations (Figure 2). The next unit, Bed 2, in the lower part of the Patcham Formation at Jhura is massive Golden Oolite and shale alternation (Figure 2). The Golden Oolite is thick and characterized by symmetrical wave-ripples of large wave-length. It sparsely yields specimens of K. acutiplicata. K. propinqua suddenly appears in Bed 2 and is represented by a few specimens quite distinct from K. acutiplicata.
Chari Formation
The Chari Formation can be divided into five informal members (Figure 3) which are broadly correlatable not only across the mainland; some of them can even be traced up to Patcham island. However, some degree of lateral variation within individual members exists. Detailed description of the members in each section and their formalization will be dealt in a later communication. The lowest member (Member 1) has the interformational demarcation plane at its base. It can be traced in all the four domes. The lower beds consist of oolitic limestone, sometimes overlain by slabby limestone and shale, limestone, ironstone alternation horizon. Its upper boundary lies at the base of a sandstone layer forming the next member. Member 2 consists of a massive ridge forming sandstone layers that can be traced easily in Jumara, Jara, Keera and Jhura. The next member, Member 3, has a shale, limestone, ironstone alternation horizon in its lower part overlain by a sandstone bed which is a marker horizon in all the four domes. The upper boundary is defined by the top of the sandstone bed. Member 4 consists of shale-wackestone alternation sometimes interrupted by ironstone beds. The upper boundary lies at the base of an oolitic limestone unit (Member 5) which has bored limemudstone pebbles at its lower part, indicating an omission surface (Datta, 1992; Fürsich et al., 1992). The oolitic limestone is highly fossiliferous and a marker horizon in all the four domes. The upper boundary of Member 5 is defined by the Katrol Formation.
The facies changes are very rapid vertically as well as laterally. The faunal characters, especially the brachiopod associations, vary markedly with the facies changes. Brachiopods, particularly terebratulids including Kutchi-thyris, are generally rare in argillaceous facies, but they are ubiquitous in calcareous and arenaceous sandstone and more so in fine-grained varieties where clusters of K. breviplicata have been found. The lithocharacters of the six facies (CA to CF) are summarized below (modified after Datta, 1992).
Facies CA.
This facies is multistoreyed cross-stratified (both chevron and hummocky) bioclastic limestone (packstone/grainstone) with local presence of ooids. There are at least three levels where bored pebbles (wackestone) are sporadically strewn. At Jumara and Keera the facies has an erosive contact with the underlying marly limestone of the Patcham Formation. K. propinqua is found in this facies and appears abundantly in Bed 2 of the lower levels of the formation at Keera (Figures 2, 3). The facies can be divided into two distinct subfacies on the basis of abundance of the ooids. Brachiopods are prolific in the ooid-poor part and considerably decrease in number in the ooid-rich part. This facies is also a product of a high-energy milieu, the deposition taking place within the wave zone and it possibly acted as a barrier bar (Biswas, 1981; Datta, 1992).
Facies CB.
This facies is a white limestone (wackestone) with a few corals and diverse sponges preserved in live attitudes (Figure 3). The environment has been interpreted as a sponge meadow in shallow water, but below the wave base. The facies is found in Bed 4 exposed in Jumara and in Bed 4 at Jhura (Figure 2). In Jumara the first appearance of CB is also marked by the first appearance of K. euryptycha. It also hosts other brachiopod species, especially K. propinqua.
Facies CC.
This facies is marked by repeated alternation of shale, white limestone (wackestone) and reddish or brownish limestone (packstone/grainstone) and also occurs more than once in the stratigraphical sequence (Figure 3). Chevron cross-stratification is occasionally present in the grainstone but the packstone and wackestone are relatively massive. The facies, representing a shoaling up parasequence, was possibly deposited in a restricted lagoon near the wave base and is well developed in all the four sections (e.g., Bed 5 at Jumara, Bed 5 at Jhura, Bed 8 at Keera, Bed 1 at Jara; Figure 2). Brachiopods, especially terebratulids, are rare and mostly broken.
Facies CD.
This facies is represented by lenticular beds of thick shale interbedded with thin, parallel-sided, dark brown, ferruginous limestone. This facies is present only in Bed 6 at Jumara (Figure 2) amidst the shale-limestone alternation background. It was formed possibly due to ponding within a swale. Terebratulids are almost absent.
Facies CE.
This facies is yellowish-grey sandstone present in all the four sections (Figure 3), and occurs more than once in the stratigraphical sequence and increases in thickness towards the landward side, i.e., towards Jhura (e.g., Beds 7, 9 at Jhura, Beds 8, 10 at Jumara, Beds 5, 7 at Keera, Beds 2, 4 at Jara; Figure 2). It is commonly multistoreyed, massive with an upward-coarsening sequence and devoid of bioturbation, and it also contains large-scale cross-beds. The sandstone represents a shoaling-upward phase, followed by renewed deposition of fine-grained siliciclastics (Fürsich and Oschmann, 1993). Terebratulids present include K. euryptycha and are very common. The sandstone facies is repeated as reworked concretions and bored pebbles (protected shelf of Fürsich and Oschmann, 1993) in the late Callovian (Bed 12 at Jumara, Bed 12 at Keera, Bed 9 at Keera; Figure 2) and thus points to fluctuating sea-level and nonsequence with erosive phases (Fürsich et al., 1992).
Facies CF.
This facies is present throughout the mainland of Kutch with its characteristic lithological and faunal associations and found in Bed 15 at Jhura, Bed 15 at Jumara, Bed 11 at Keera and Bed 9 at Jara. It is a heterolithic facies represented by repeated alternation of oolitic limestone and gray shale. The top is characterised by a distinct conglomeratic sub-facies. At its basal part, bored limestone pebbles are found in various concentrations. Small-scale synsedimentary deformation structures like fault and slump folds are fairly common within the conglomeratic sub-facies (Figure 3). Deposition of this facies took place in a deeper offshore setting but during high energy episodes such as storms, induced by a major transgressive event (Singh, 1989; Fürsich et al., 1992). Numerous species of terebratulids including K. euryptycha are found in it.
Stratigraphical distribution of the three Kutchithyris species is shown on the standard chronostratigraphical zonation based on endemic ammonite species (Figure 4).
Figure 4
Range chart of Kutchithyris acutiplicata (Kitchin), Kutchithyris propinqua (Kitchin) and Kutchithyris euryptycha (Kitchin) as documented in Western Kutch. Zones and subzones are modified from Datta (1992) and Bardhan et al. (2001).
From the above discussion, it is revealed that the depositional milieus of the Patcham and Chari formations are different although both are the products of initial carbonate platform deposition. The transition from the storm-dominated shallow environment of the Patcham Formation to the overlying low-energy mid-shelf environment of the Chari Formation also reflects the compositional change of the Kutchithyris community. The two older species K. acutiplicata and K. propinqua became extinct stepwise in “palaeontological relay fashion” as the basin became progressively shallower during the Patcham time. K. propinqua appeared immediately before the Bathonian-Callovian transition which marks a global sea-transgression (Haq et al., 1987) and this is also evident in the regional scenario. Fürsich and Oschmann (1993) showed that three orders of sea-level changes indicate gradual deepening, punctuated occasionally by shallowing trends within the Patcham-Chari Formations. K. euryptycha occurs only in the Chari Formation and is poorly represented or totally absent in the facies, indicating a deeper bathymetry (Facies CC), while it is abundant in Facies CE and CF, which are the products of a shallower environment. Kutchithyris finally disappears from Kutch during the Oxfordian-Kimmeridgian transition that heralded a sharp rise in global sea level (Datta, 1992; Haq et al., 1987; Fürsich and Oschmann, 1993).
Systematic palaeontology
The major morphological characters considered here for bivariate analyses (Table 1) are shown in Figure 5. The dimensions however are not provided since specimens are plentiful. They will be available upon request.
Table 1
Ontogenetical growth patterns of three Kutchithyris species.
Figure 5
Major morphological characters for measurements used in statistical analyses. A. Dorsal view. B. Side view. C. Commissural view. Keys: L, distance from anterior to posterior; W, total width of shell; PMW, position of maximum width from posterior margin; F, foramen diameter; PMT, position of maximum thickness; PLD, distance of origination of plica from posterior margin; T, thickness of valves; CL, curved length along pedicle valve; PLW, width of plica.
Superfamily Terebratuloidea Gray, 1840
Family Terebratulidae Gray, 1840
Genus Kutchithyris Buckman, 1918
Generic diagnosis.
Small to large, unequally biconvex valves. Anterior commissure highly variable both onto-genetically and interspecifically, rectimarginate, uniplicate to sulciplicate. Adult lateral commissure simple, straight, with a ventral convexity anteriorly. Umbo short, incurved, permesothyrid. Foramen small to large, oval to circular. Radial capillation present on shell surface. Loop long, almost half of dorsal valve length. Crural process posterior to midloop and long terminal points. Transverse band narrow.
Discussion.
Kitchin (1900) described altogether 20 species of terebratulids from Kutch and placed all under the comprehensive genus Terebratula. Later, Buckman (1918) assigned Kitchin's species to Kutchithyris, which comprised 10 species. Of these, six were reported from Kutch and the remaining four from England (see Buckman, 1918). Buckman's description of Kutchithyris was based mainly on the external shell characters. Mitra and Ghosh (1973) emended the generic diagnosis of Kutchithyris by including the internal characters of the shell. Ovcharenko (1969) reported K. acutiplicata and K. euryptycha from the Upper Bathonian deposits of the southeast Pamirs. He also showed the internal structure of the two species.
Middlemiss (1980) placed some species from the Lower Cretaceous of Morocco in Kutchithyris, but Cooper (1983) correctly showed that the external morphology of the Moroccan species differs considerably from that of Kutchithyris, the Moroccan species being spherical.
Cooper (1989) described two doubtful species of Kutchithyris ranging from the Upper Bathonian (Middle Dhruma Formation) to Middle Callovian (Tuwaiq Mountain Formation) of Saudi Arabia. The measurements of shell parameters show that they have much larger thicknesses compared to the Kutch species and hence do not belong to Kutchithyris (unpublished data).
Kutchithyris acutiplicata (Kitchin, 1900)
Terebratula acutiplicata Kitchin,1900, p. 6–9, pl. 1, fig. 1–7.
Kutchithyris acutiplicata (Kitchin). Buckman, 1918, p. 113, pl. 20, fig. 17; Ovcharenko, 1969, figs. 1–3; Mitra and Ghosh, 1973, p. 185, fig. 3, 4K-R; Cooper, 1983, p. 96, pl. 63, fig. 3.
Terebratula (Kutchithyris) cf. acutiplicata (Kitchin). Trechman, 1923, p. 284, pl. 16, figs. 5–7.
Kutchithyris cf. acutiplicata (Kitchin). Allan, 1945, p. 1–22.
Kutchithyris hendersoni Marwick, 1953, p. 85, pl. 15, figs. 13, 14.
Types.
Kitchin (1900) selected a series of specimens to describe Terebratula acutiplicata. We here designate G. S. I. Type No. 6601 (Kitchin, 1900, fig. 1) as the lectotype (Figure 6.7a–d) and Nos. 6596–6600, 6602–6603 and 15588 as paralectotypes (Figure 6.8–6.10). The types are kept at the Repository of the Geological Survey of India, Kolkata.
Figure 6
Kutchithyris acutiplicata (Kitchin) from Patcham Formation of Jumara. All ×1. 1a–c. B/Jul/7 dorsal, side and anterior view. 2a–c. B/Jul/3, dorsal, side and anterior views. Note radial stripes in anterior portion (a). 3a–c. B/Jul/114, dorsal, side and anterior views. 4a–c. B/Jul/170, dorsal, side and anterior views. 5a–c. B/Jul/172, dorsal, side and anterior views of a young individual. Note gentle curve of lateral commissure and rounded plica. 6a–c. B/Jul/73 dorsal, side and anterior views. Magnified view of left dorsal valve showing radial striations (d). 7a–d. Lectotype, G.S.I. Type No. 6601, from “Upper Patcham Beds” (Bed 1, Jumara, Figure 2), dorsal, side, anterior and ventral views. 8a–c. G.S.I. Type No. 6600, dorsal, side and anterior views. 9a–c. G.S.I. Type No. 6599, dorsal, side and anterior views. 10a–d. G.S.I. Type No. 6603, dorsal, side, anterior and ventral views.
Material.
223 specimens including the types. Ju 1a/1–208 from Bed 1, Jumara, Jh a/1–2 from Bed 1 and Jh 2 a/1–4 from Bed 2, Jhura. They are kept at the Jadavpur University Museum. G. S. I. Type nos. 6596–6603, 15588 from Upper Patcham Beds (Bed 1 of Jumara in this paper).
Description.
Shell outline changes through growth, oval in juvenile stage (L > 15 mm), subpentagonal in middle stage (L = ca. 20 mm), pentagonal in adult stage (L = ca. 30 mm). Shell shape as seen in lateral profile also changes from lenticular to subglobose through growth and becomes more convex during shell growth. Maximum width lies at about 1/3 length from anterior margin and greatest thickness at about mid-length. Generally, convexity of brachial valve lies posteriorly (Figure 6.1b, 6.3b), but broader and flattened variants are less convex and narrower and globose forms strongly inflated (Figure 6.10b). In extreme variants, pedicle valve relatively more convex (Figure 6.9b). Anterior commissure strongly biplicate with two sharp and strong plicae at about mid-length. Distance of plica from posterior margin (PLDI) shifts allometrically (Table 1). Crest of plica rounded in younger stage but becomes angular in adults. In adult shell, ventral valve develops a sharply raised median ridge that starts about 2/3 length from posterior end, which is also end of early oval stage, flanked on both sides by two lateral elevations, separated from central ridge by a lateral depression. Central ridge with a flattened crest, gradually widens anteriorly; lateral ridges less elevated than central one.
Hinge line gently to strongly curved. Commissure simple until it reaches a point about 2/3 length from posterior end where it sharply turns to ventral side almost at a right angle. It forms two well-marked plicae with characteristic M-shape.
Beak with a broad base, incurved; lateral ridge absent. Foramen large, permesothyrid, circular to oval in outline and inclined. Foramen often bordered by a rim (Figure 6.1a, 6.4a). Cardinal margin terebratulid, but some specimens show submegathyrid cardinal margin in younger stage.
Shell occasionally shows distinct traces of weak and fine radial 'striping' usually in anterior part. Growth lines strongly approximated near anterior commissure.
Pedicle collar present; cardinal process simple, teeth mallet-shaped, inserted in socket, denticula very weakly developed. Hinge plates concave, crural bases recurved from hinge plates, loop 14.1 mm long and almost half length of dorsal valve, crural process posterior to mid loop, transverse band narrow arch, terminal points moderately long.
Discussion.
Upper Bathonian deposits of the southeastern Pamirs contain K. acutiplicata (Ovcharenko, 1969). The specimens from the Pamirs have a broad, biplicate shell and a well-developed furrow on the brachial valve. We concur with Ovcharenko's identification because the Pamir specimens are difficult to distinguish from those of K. acutiplicata from Bed 1 in Jumara.
Trechman (1923) described a few specimens from the Upper Bathonian of Totara, Kawhia, New Zealand and identified them as Tereburatula (Kutchithyris) cf. acutiplicata. The shell dimensions and other valve characters of the New Zealand specimens are similar to those of K. acutiplicata from Kutch. Trechman (1923) also mentioned that most of his specimens bears resemblance to those figured by Kitchin (1900). In our view the New Zealand specimens are to be K. acutiplicata.
Marwick (1953) described Kutchithyris hendersoni from the Upper Temaikan Stage of the Kawahia Series in Totara Peninsula, New Zealand. He distinguished his species from K. acutiplicata by the uniformly less inflated shells and with strong anterior folding. In spite of the distortion, the shell shape and the characteristics of beak and plication are similar to those of K. acutiplicata. (Marwick, 1953, pl. 15, figs. 13, 14). We consider that K. hendersoni also to be identical to K. acutiplicata.
Sahni and Bhatnagar (1958) described Kutchithyris jaisalmerensis from an unspecified horizon in the Callovian sequence of Jaisalmir, western India. It resembles K. acutiplicata in the characters of beak and anterior plication. According to Sahni and Bhatnagar (1958), K. jaisalmerensis has dorsal muscle impressions similar to those of the genus Selerithyris and the anterior plication ranges from rectimarginate, uniplicate to strongly biplicate during ontogeny. However, our inspection of the type specimens revealed that K. jaisalmerensis has neither the rectimarginate nor uniplicate stage in the anterior commissure, and the uniplicate stage also does not appear in the specimens of K. acutiplicata from Kutch. The most remarkable difference between the two species, however, lies in the development of multiple narrow folds that produce fimbriation. This feature in K. jaisalmerensis is reminiscent of another coeval Kutch species, K. katametopa (Kitchin 1900).
Occurrence.
Kitchin (1900) worked on the material collected by others in the Geological Survey of India, and did not specify the horizons from where the fossils were collected. He merely described the fossils as coming from “Upper Putchum Beds” of his “Putchum Group”. However, the state of preservation and the nature of the enclosing matrix definitely indicate that they came from Bed 1 of the Patcham Formation exposed in Jumara. Beside Bed 1 in Jumara this species occurs in Bed 1 and Bed 2 in Jhura.
Kutchithyris propinqua (Kitchin, 1900)
Terebratula propinqua Kitchin, 1900, p. 10, pl. 2, figs. 2–4
Terebratula aurata Kitchin, 1900, p. 18, pl. 14, figs. 1–4.
Terebratula cf. acutiplicata Kitchin, 1900, p. 9, pl. 2, fig. 1.
Terebratula sp. indet. Kitchin, 1900, p. 16, pl. 3, figs. 1, 2.
Terebratula cf. aurata Kitchin, 1900, p. 21, pl. 4, figs. 5, 6.
Kutchithyris propinqua (Kitchin). Buckman, 1918, p. 113, pl. 19, fig. 18.
Kutchithyris aurata (Kitchin). Buckman, 1918, p. 113, pl. 19, fig. 19.
Types.
Kitchin (1900) used a series of specimens to describe Terebratula propinqua and Terebratula aurata. We here designate G. S. I. Type No. 6620 (Figure 8.8a–d) as the lectotype of K. propinqua and Nos. 6605–6606, 6619, 6621–6624 as paralectotypes (Figure 8.7, 8.9).
Figure 7
Serial transverse sections of Kutchithyris acutiplicata (Kitchin). Numbers indicate grinding increments in mm.
Figure 8
Kutchithyris propinqua (Kitchin) from Patcham and Chari formations of Kutch. All ×1. 1a–c. B/Ju4/34, from Bed 4, Jumara, dorsal side and anterior views, respectively. 2a–c. B/Ju2/29, from Bed 2 Jumara, dorsal side and anterior views. 3a–c. B/Ju4/8, from Bed 4 Jumara, dorsal, side and anterior views. 4a–c. B/Ju4/2, from Bed 4 Jumara, dorsal, side and anterior views. 5a–c. B/Ju4/49, from Bed 4, dorsal, side and anterior views of a large individual. 6a–c. B/Ju4/37, from Bed 4 Jumara, dorsal side and anterior views of a young individual. Note rounded anterior commissure and absence of ridge at 1.68 cm of length. 7a–d. G.S.I. Type No. 6606 from “lower beds of Chari Group”, Keera (Bed 2, Keera, Figure 3) dorsal, side anterior and ventral views, respectively. 8a–d. G.S.I. Type No. 6620, lectotype, from “Upper Putchum Beds” of Jumara (Bed 1), dorsal, side, anterior and ventral views. 9a–c. G.S.I. Type No. 6619, from “lower beds of Charee Group”, Keera, (Bed 2, Keera, Figure 3), dorsal, side and anterior views.
Material.
118 specimens. Jul p/1–9, Ju2 p/1–23, Ju4 p/1–54 were collected from Beds 1, 2 and 4 in Jumara; K2p/1–19 from the uppermost Golden Oolite (Bed 2 in Figure 3a), Keera; Jh 2 p/1–3, Jh 3 p/1 from Beds 2 and 3 at Jhura. G. S. I. Type numbers 6605–6606 from “Upper Putchum beds” in Jumara, (= Beds 1–4 in Figure 2b), 6619–6624, 15587 from Golden Oolite limestone, “lower beds of Charee Group” (= Bed 2 in Figure 3a), Keera and are kept at the Repository of the Geological Survey of India, Kolkata.
Description.
Shell large (maximum length 51 mm), highly variable in form from rounded pentagonal to subpentagonal or subcircular both ontogenetically and individually. Length always greater than width and thickness, maximum width lies at middle, position of maximum thickness lies at one-third length from posterior end. Shell biconvex, pedicle valve more convex, curvature (CL) increases allometrically with growth (log CL = 1.59 log L + 0.6728). Dorsal valve with two weak, blunt, broadly rounded plicae, starting at about one-third length from anterior margin, with plicae highly variable in width and height. Plicae develop later in ontogeny after shell attains ca. 23 mm in length and widens with growth (Table 1). A short and shallow median sulcus present between two plicae. Ventral valve more convex than dorsal and in some specimens (Figure 8.7d) tends to form a posterior median carination, which becomes flatter and narrower anteriorly forming a median ridge; median ridge inconspicuous or scarce in some variants.
Lateral commissure forms a wide curve towards ventral valve, and anterior commissure biplicate with low and rounded plicae. Beak weakly developed, slightly incurved. Foramen circular, medium in size, inclined. Lateral ridges present in some variants (Figure 8.9a).
Surface devoid of any ornamentation except growth striae, represented by series of constrictions in anterior end of adult specimens; in some specimens radial striping similar to K. acutiplicata present near anterior end (Figure 8.3a).
Young specimens, according to Kitchin (1900), almost circular in outline with equal length and width. Maximum width lies at middle, which shifts towards anterior margin during later ontogeny. Shell margins very sharp with a narrow beak. Ventral valve with a low posterior carina in some variants, which dies out anteriorly. Biplication of anterior commissure appears between 18 and 23 mm length from posterior end.
Pedicle collar present, and deltidial plates broad and shallow. Cardinal process low, and hinge plates slender or bladelike in shape and dorsally inclined. Loop 14.55 mm long, about half of brachial valve length. Crural process posterior of mid-loop. Transverse band slightly broader than that in K. acutiplicata.
Discussion.
The present species shows strong allometric growth in many respects during ontogeny. The shell outline varies from circular to subpentagonal, the beak is narrower in the early growth stage, and the position of maximum thickness migrates from the middle to the anterior. The anterior commissure is rectimarginate in the juvenile while weaker and shorter plications are present in the adult.
The adult specimens of K. propinqua are similar to those of K. acutiplicata at the intermediate stage (see Figures 6.5a–c, 8.1a–c). The plica widens in a positively allometric manner both in K. propinqua and K. acutiplicata (Table 1). The central ridge on the pedicle valve is less strongly developed in K. propinqua than in K. acutiplicata and appears only in the later growth stage of the former species (see Figures 6.2c, 6.7d and 8.5c). Also, the shell outline is subpentagonal and the shell curvature is relatively low in K. propinqua, therefore approximating those features of the immature specimens of K. acutiplicata (see Figures 6.5a, 8.5a). Additionally, the beak rises from the broad base, is less elevated and has a distinctively smaller foramen in K. propinqua, recalling the early ontogeny of K. acutiplicata (see Figures 6.5a–b, 8.3a–b). Finally, the characteristic crowding of growth striae starts in the early growth stage in K. acutiplicata, whereas it is narrowly restricted only to the last third of the length from the anterior end in K. propinqua (see Figures 6.2, 6.5, 8.2 and 8.5). These imply that K. propinqua is a paedomorphic offshoot of K. acutiplicata, possibly through neoteny. However, the adult specimens of K. propinqua differ from K. acutiplicata in their larger size, weaker and shorter plication, lateral commissure with a broad curvature, and less shell curvature.
Terebratula aurata Kitchin (1900) from the Golden Oolite of Keera exhibits ontogenetical shell transformation and has shell features which are the same as K. propinqua. Kitchin (1900) mentioned that these two forms were closely related. Detailed morphometrical analyses based on a large number of specimens as well as the type specimens reveal that these two species are difficult to distinguish. Moreover, our systematic sampling in Kutch also shows that the two species occur in the same stratigraphical interval and T. aurata does not occur in beds younger than those bearing K. propinqua, as stated by Kitchin. Many specimens from the Golden Oolite of Keera, the type strata of T. aurata, are identical to those found in Bed 4 of Jumara. Thus we conclude that T. aurata is a junior synonym of K. propinqua.
Kitchin (1900) found several specimens different from T. acutiplicata and tentatively identified them as Terebratula cf. acutiplicata. They are characterized by a more rounded and less angular fold and broad and shallow dorsal lateral depression with rounded troughs. Thus, these shell characters evidently indicate that T. cf. acutiplicata should be referred to the coeval species K. propinqua.
Occurrence.
Kitchin (1900) reported T. propinqua from the “Upper Putchum Beds” and "T". aurata from the Golden Oolite in Keera. Our specimens are from Beds 1–5 in Jumara, Bed 2 of Jhura and Beds 1–2 in Keera.
Kutchithyris euryptycha (Kitchin, 1900)
Figure 10
Kutchithyris euryptycha (Kitchin) from Chari Formation of Kutch. All ×1. 1a–c. B/Ju4/25, from Bed 4 Jumara, dorsal, side and anterior views, mark radial stripes (b). 2a–c. B/Ja6/18, from Bed 6, Jara, dorsal, side and anterior views. 3a–c. B/Ja6/6, from Bed 6 Jara, dorsal, side and anterior views. 4a–c. B/Ja4/37, from Bed 4 Jara, dorsal, side and anterior views. 5a–c. B/Jh9/4,from Bed 4 Jhura, dorsal, side and anterior views, note development of uniplicate commissure at L = 1.6 cm. 6a–c. B/Jh15/20, from Bed 15, Jhura, dorsal side and anterior views. 7a–d. G.S.I. Type No. 6617, dorsal, side, anterior and ventral views. 8a–c. G.S.I. Type No. 6635, dorsal side and anterior views. 9a–c. G.S.I. Type No. 6634, dorsal, side and anterior views. 10a–d. G.S.I. Type No. 6658, dorsal, side, anterior and ventral views.
Figure 11
Serial transverse sections of Kutchithyris euryptycha (Kitchin). Numbers indicate grinding increments in mm.
Terebratula sella var. Sowerby, 1840, p. 328, pl. 22, fig. 12.
Terebratula sella Sowerby. Wynne, 1880, p. 88.
Terebratula jumarensis Kitchin, 1900, p.13, pl. 3. figs. 3, 6.
Terebratula euryptycha Kitchin, 1900, p. 25, pl. 5, figs. 3–11;
Barrabé, 1929, p. 25, pl. 21, fig 16.
Lophrothyris euryptycha (Kitchin). Buckman, 1918, p. 114, pl. 21, fig. 13a; Mitra, 1960, p. 89, pls. 15–20.
Kutchithyris euryptycha (Kitchin). Ovcharenko, 1969, figs. 4–6.
Types.
Kitchin (1900) did not select the holotype of T. euryptycha. We here designate one of his specimens, G. S. I. Type No. 6638, as the lectotype.
Material.
290 specimens including the types. Ju4 e/1–55 from Bed 4, Ju 10 e/1–46 from Bed 10, Ju 12 e/1–11 from Bed 12, Ju15 e/59–72 from Bed 15 in Jumara; Jh 9 e/1–62 from Bed 9, Jh 15 e/1–50 from Bed 15 in Jhura; K7 e/1–2, 40–57 from Bed 7 in Keera; Ja1 e/1–13 from Bed 1, Ja9 e/51–58 from Bed 9 Jara; G.S.I Type no, 6614, 6617 from Upper Patcham Beds of northwest Jumara, 6630–6638 from above the Macrocephalus beds to the top of the Charee Group in Jhura.
Description.
Shell small, 31.2 mm in maximum shell length, consistently longer than wide. Shell highly variable, circular in young, oval to subpentagonal in middle and elliptical in adult. Shell convexity variable, increasing allometrically during ontogeny (log CL = 1.209 log L + 1.2684; Table 1). Maximum width and thickness of shell at middle. Dorsal valve with a more or less marked longitudinal dorsal plica or fold. Some variants with a plica very low in height, giving a flattened appearance, with an anterior commissure almost rectimarginate. Plica variable in width, narrow to broad. Ventral valve more convex than brachial, with a sulcus variable in strength, corresponding to dorsal plica; sulcus broader and shallower in relatively wider and thinner shells than in narrower and thicker shells. In some cases ventral valve forms an anterior linguiform projection (Figure 10.2, 10.4, 10.9 and 10.10). Shell surface bears fine radial striae as in K. acutiplicata and K. propinqua, and growth lines become crowded near anterior margin in adult.
Lateral commissure near posterior end curves towards ventral side, then proceeding anteriorly with a wide curve, giving rise to form anteriorly a single central arch, broadly or narrowly rounded and even straight in some variants (Figure 10.3c). Anterior commissure rectimarginate to uniplicate.
Beak broad to narrow, suberect to slightly incurved. Lateral ridges absent. Foramen circular, moderate in size, truncating beak in vertical plane. Deltidial margin some-times slightly thickened. Pedicle collar present and cardinal process without lobe. Teeth linguiform in shape and denticula weakly developed. Hinge plates thin, highly concave; crural bases sharply recurved from hinge plates, crural plates distinctly converging. Loop 9.3 mm long, about half of brachial valve, and transverse band short and narrow.
Discussion.
Kitchin (1900) described Terebratula euryptycha based on a specimen which Grant (1840) previously identified as Terebratula sella var. from the beds immediately overlying the “Macrocephalus beds” of Waagen (1875). Kitchin (1900) also mentioned that it ranges right up to the top of his Charee Group. Buckman (1918) placed the species in his new genus Lophrothyris based on the external shell characters such as strong development of uniplica, short and obliquely truncated beak close to the umbo and slightly divergent muscle tracks. He selected Lophrothyris lophus Buckman as the type species of the genus but gave no description either of the muscle scars or of the internal structure for this species. Later, Mitra (1960, 1978) also allocated T. euryptycha to Lophrothyris following Buckman (1918), but did not compare the internal structure between the two species. When Muir-Wood (1937) reported two specimens of L. euryptycha from Jhaler in Attock district, Pakistan, she was sceptical about the inclusion of T. euryptycha within Lophrothyris, as no detailed investigation had been made of the internal characters of these two species. Almeras and Moulan (1988) were also uncertain of the inclusion of T. euryptycha within Lophrothyris.
We examined shell character and internal structure of K. euryptycha in detail based on 290 specimens from Kutch. K. euryptycha has an internal structure which is the same as that of the type species of the genus, K. acutiplicata (Figures 7, 11). Moreover, K. euryptycha has a shell outline very similar to that of the smaller specimens of K. acutiplicata less than 15 mm in length. To sum up these similarities, K. euryptycha can be safely assigned to Kutchithyris.
Terebratula jumarensis Kitchin (1900) from Bed 4 in Jumara is similar to K. euryptycha in overall shell morphology. Our detailed morphometrical analysis shows that the two species are conspecific. Kitchin (1900) found K. euryptycha from horizons (Beds 10–15) much higher than that of T. jumarensis and this stratigraphical hiatus may have prompted him to assign it to a different species. Our systematic collecting reveals that this species sustained stasis for about 11 million years. Both T. jumarensis and T. euryptycha appeared in the same volume of Kitchin (1900) and T. jumarensis has page priority. However, we prefer to select K. euryptycha as the valid name since this name is more familiar and deeply entrenched in the literature.
The adult specimens of K. euryptycha in turn are closely comparable with the medium-sized specimens of K. propinqua as they have very incipient (Figure 10.1a–c) or no plications (Figure 10.3, 10.7) in the anterior commissure. The young individuals of K. propinqua (Figure 8.6a–c) and the adult individuals of K. euryptycha are hardly distinguishable since they have a nearly elliptical shell outline, broad and rather flattened dorsal valve, and narrow and weakly incurved to erected beak. However, the adult individuals of K. euryptycha have a well-marked longitudinal dorsal arch and a gently rounded curve in the lateral commissure at the middle. Furthermore, the sharpness of the angular bend decreases markedly in K. propinqua while the position of the angular bend shifts posteriorly until it becomes completely rounded at the mid-point of the shell length in K. euryptycha. Since K. euryptycha is stratigraphically younger, with a slight over-lapping with K. propinqua in Bed 4, and it resembles the younger specimens of K. propinqua, we suggest that K. euryptycha is a paedomorphic derivative of K. propinqua, as K. propinqua was paedomorphically derived from K. acutiplicata. Consequently, they constitute an evolutionary lineage.
Ovcharenko (1969) recorded specimens of K. euryptycha from the Upper Bathonian deposits of the southeastern Pamirs. His specimens are represented by distended uniplicate shells without the median furrow. He observed that K. euryptycha has a brachial loop shorter than that of K. acutiplicata. They resemble the coeval specimens of K. euryptycha from Bed 4 of Jumara.
Barrabé (1929) reported a specimen of Terebratula euryptycha from the Callovian in the west of Ankidave, Madagascar, which Muir-Wood (1937) regarded as a broader form of T. euryptycha with a less incurved umbo. Such a variant is also seen in the specimens from Kutch and is particularly similar to those from Bed 7 (Jara) and Bed 9 (Jumara).
Wynne (1880) identified a specimen from the Jurassic of Shekh Budin Hills, Trans-Indus extension of the Salt Range as Terebratula sella. It is most probably referable to Kutchithyris euryptycha.
Occurrence.
Beds 4–5 in Jumara, Beds 9–15 in Jhura, Beds 1–9 in Jara and Beds 7–11 in Keera. Kitchin (1900) did not mention the specific horizon from which ‚T’. jumarensis was collected but merely mentioned “Upper Putchum Beds of north west Jumara” as the locality. He found T. euryptycha from “above the macrocephalus beds to top of Chari Group”. From his description, it is inferred that his specimens come from Bed 10 to 15 of Jumara (Figure 2).
Discussion
Interestingly, the present three species form an evolutionary lineage with a long geological history from the Middle Bathonian to Oxfordian and have also been recorded from contemporaneous sequences in the Pamirs, where the lineage is cited as a classical example of the punctuational model of evolution (Ovcharenko, 1969; Raup and Stanley, 1985). Ovcharenko (1969) described two species of Kutchithyris, namely, K. acutiplicata and K. euryptycha from the Upper Bathonian of the southeastern Pamirs, where K. acutiplicata and K. eurytpycha were reported to occur in the lower and the upper parts of a 1–1.5 m thick marl bed, respectively. Between these two parts of the layer, there is an only 10 cm-thick interval that contains both species along with some transitional forms. According to Ovcharenko (1969), the transitional forms appeared in extremely small numbers (hybridization of two species, sensu Geary, 1992) and were not found elsewhere— a rare insight at that time, when the punctuational model of evolution (Eldredge and Gould, 1972) was not in vogue. Ovcharenko claimed to document a rapid mode of speciation in a small, isolated population. Generally, the forms depicting such a speciation event elude preservation due to the brief time intervals and small areas involved and also to the smallness of the sample size. Consequently, the apparent discontinuity in the fossil record of a lineage is taken as the absence of the intermediate forms. Thus, the Pamir appears to be a rare site preserving the record of an allopatric speciation event with the transitional forms having survived the vagaries of fossilization. In Kutch, the same ancestor-descendant lineage K. acutiplicata-K. euryptycha has been found along with the transitional forms and all of them are represented by plenty of individuals, enabling us to comment on their population structures. Detailed taxonomical treatment in the present study reveals that the so-called transitional forms are not only present within the Kutch brachiopods, but also that they appeared much earlier (Middle Bathonian) and occupy an intermediate strati-graphical interval. They were recognised much earlier by Kitchin (1900) and Buckman (1918) as a distinct species and named Kutchithyris propinqua (Kitchin). It evolved rapidly from K. acutiplicata. The species occurs in profusion in Kutch and spans a considerable geological range. In the Pamirs, the specimens are fewer in number and the lineage occupies a much lesser thickness, about 10 cm, thus perhaps indicating a stratigraphical condensation. In fact, it is also observed that K. propinqua has stratigraphical distributions overlapping with both the ancestral species K. acutiplicata and the descendant species K. euryptycha and the evolving lineage follows cladogenesis (sensu Gould and Eldredge, 1993), involving branching speciation rather than phyletic transformation as suggested by Ovcharenko (1969).
Acknowledgments
One of the authors (D.M.) is grateful to the Director General, Geological Survey of India, for granting permission to publish the paper. T. C. Lahiri, Geological Survey of India, critically read the manuscript and provided helpful suggestions. We are grateful to Tomoki Kase for his suggestions and for help in preparation of the Table. M. K. Mukherjee, Indian Statistical Institute, helped in preparation of figures and K. Halder, Burdwan University, helped in photography. We are grateful to the two reviewers for their critical comments. S. B. received financial aid from the Department of Science and Technology, India (ESS/23/VES/022/98).
