BENTHIC FORAMINIFERAL SYSTEMATICS AND MORPHOGROUPS IN THE MIDDLE TO LATE JURASSIC ROCKS OF KACHCHH BASIN, INDIA: IMPLICATIONS FOR PALAEOECOLOGICAL CHANGES Open Access

The Kachchh Basin was formed by the breakup of Peninsular India from Gondwanaland (Biswas, 2016; Saraswati et al., 2018; Kothyari et al., 2021; Quasim et al., 2021). In the south, the Kachchh Mainland trends east to west, and parallel to it in the north, a sequence of high ridges is aligned, whereas the Wagad highland is to the east, occupying the region between the Kachchh Mainland and the Island Belt (Rai & Jain, 2013; Wasim et al., 2017; Kumar et al., 2022; Fig. 1).

Kachchh is a significant landmark for studying the Jurassic rocks at the southern tip of the Neo-Tethys (Alberti et al., 2019). The Jurassic rocks of Kachchh are known for their better preservation of mega- and microfossils, attracting many palaeontologists towards these strata (Rahiminejad & Hassani, 2016; Talib et al., 2017; Khan et al., 2020). Foraminifera are some of the most important microfossils used for reconstructing the palaeoenvironmental conditions of the area (Suokhrie et al., 2021; Ansari et al., 2022; Narayan et al., 2022). The Jurassic foraminifera are commonly considered provincial instead of global (Fürsich et al., 2016; El-Sabbagh et al., 2017), and their distribution and abundance are controlled by many physical and chemical factors in an environment (El-Sabbagh et al., 2017). Their diversity and distribution in the geological column and sensitivity to various environments, such as the coastal and marginal marine to deeper zones of the ocean and tropical to polar settings, make recent and ancient environmental studies possible (Rahiminejad & Hassani, 2016; Ansari et al., 2021a; Caratelli & Archuby, 2023). Benthic foraminifera have reliably represented past bathymetry, feeding strategies, oxygen availability, salinity, transgression-regression phases, and sea-level changes throughout time (Leckie & Olson, 2003; Jain & Farouk, 2017; Farouk et al., 2020; Minhat et al., 2020; Ansari et al., 2022). According to El-Menhawey et al. (2020) and Walker et al. (2023), sea-level fluctuations can be reconstructed using the link between benthic foraminifera and environmental variables.

Well-preserved formations containing micro- and megafossils are exposed in the Kachchh region, and comprehensive studies are done on biostratigraphy and palaeobiogeography using Jurassic foraminifera (Talib et al., 2012; Kumar et al., 2021). These works indicate that little emphasis has been placed on the palaeoenvironmental conditions of the area. Here we provide a detailed analysis of Jurassic foraminifera of Habo Dome, Kachchh, Gujarat (Fig. 2), by combining the systematics and the biotic and statistical proxies to interpret the changing depositional environment from the upper Bathonian to lower Oxfordian ages.

The geology of Kachchh is depicted in Figure 1, showing the Jurassic sequence of Kachchh, India. These rocks unconformably overlie the Precambrian basement, which envelops approximately fifty percent of the Kachchh region (Biswas, 1993; Ansari et al., 2021a). Stolickzka (in Waagen, 1871, 1873–75) categorised the succession of the Jurassic of Kachchh as ‘Pachcham’, ‘Chari’, ‘Katrol’, and ‘Umia’ groups in stratigraphic order based on the order of superposition and mineralogical and palaeontological characteristics. However, Waagen (1873–75) precisely divided the boundaries of the subdivision of the Kachchh Jurassic using the collection of Stoliczka (1867) based on ammonites (Talib et al., 2017; McLachlan & Haggart, 2018). Here, the Pachcham, Chari, and Katrol formations are easily discernible, but the Umia Formation is not visible (Fig. 1).

The Habo Dome, named after the Habai Mata Temple situated at the foot of the hills near Habai village (Fig. 2), is one of the prominent domes at the eastern part of the central ridge, located between 23°22′7″N and 23°2′40″N and 69°50′59″E and 69°53′20″E, about 40 km northeast of Bhuj (Talib et al., 2016; Kumar et al., 2021). This dome, which covers an area of roughly 75 square kilometres within the Jurassic rocks of Kachchh, is approximately 15 km long and 5 km wide. Habo Dome includes Dharang, Jhikadi, Rudramata, and Lodai areas (Talib et al., 2017; Irshad et al., 2019; Fig. 2B). The Habo region is known for its excellent exposure and abundance of mega- and microfossils (Ahmad et al., 2015). The total thickness of this dome is ∼366 m, and the Chari Formation is found to be extensively fossiliferous. The exposed layers of Jurassic sedimentary rocks in the study region include sandstones, limestones, and shales.

The clastic sedimentation of Habo Dome shows the three environments: tidal flat lagoonal, tidal fluvial, and foreshore-offshore facies (Irshad et al., 2019). Sediments were transported through bulk flows, channelised-unchanneled flows, and suspended settling to form the three facies’ combinations. Matrix-supported conglomerates, including large-size clasts and fining upward sequences, are characteristics of bulk flow (Selley, 2000). Alternating high- and low-energy facies, such as thick- or thin-bedded sandstone (coarse-, medium-, and fine-grained) intercalated with shale, are recognised by unchanneled movements (Boggs, 2009). Channelised and suspended settling are characterised by ripple marks and shale deposition, respectively (Allen & Chambers, 1998; Nicholas, 2009; see Table 2 of Irshad et al., 2019).

The Bathonian Age Pachcham Formation comprises black micritic limestone facies exposed in the Dharang region (Fig. 3). This facies is noticeably laminated and strongly bioturbated, with foraminifera (microfossils), Modiolus, Bositra, and Lopha (megafossils; Bhalla & Abbas, 1978), as well as Lockeia, Sabularia, Thalassinoides, and Chondrites (trace fossils; Patel et al., 2008). The matrix-supported conglomerate with trace fossils suggests low-energy marine deposition throughout a stagnant phase with minimal sediment supply during the transgression (Aurell et al., 2003). The ammonoids within fine-grained oolitic limestone beds and bioturbated deposits support the transgressive phase (Ahmad et al., 2015). According to Mishra (2024), the Pachcham Formation is marked by a basal sequence of coarse-grained limestone followed by fine-grained carbonate mudstone with microfossils, indicating an early rise in sea level below storm wave base to open shelf environment (Fürsich et al., 2005). It is also explained by the gradual slope of the carbonate shelf due to the transgressive deposit of carbonate mudstone, containing organic-rich anoxic facies (Mishra & Tiwari, 2006). The higher proportion of smectite further supports coral lithology in deeper conditions (Jain et al., 2019). However, limestone has high total organic carbon (TOC) values (∼4 wt%) and low organic carbon values as compared to shale, suggesting a high-producing fauna in a reducing environment (Srivastava & Bhaumik, 2021).

The Early Callovian section of the Chari Formation combines a trivial argillaceous sequence with sandstones introduced as Dharang Shales and Jhikadi Shales (Irshad et al., 2019; Fig. 3). This stratum shows the microfacies change from mudrock microfacies (fine-grained clastic) to sandstone microfacies (coarse-grained clastic). In this timespan, sandstone bodies suggest terrigenous accumulation and non-marine sediments caused by increased rainfall and freshwater runoff (Fürsich et al., 1994). The regressive phase is indicated by the coarsening-upward sequence of sandstone, while the coral boulder beds suggest a typical marine environmental condition under the transgressive phase (Mishra et al., 2024). A noticeable move from Bathonian to Callovian is characterised by a shift in deposition around the mid-shelf zone from carbonate to terrigenous sediments (Fürsich et al., 2001) and a decline in faunal diversity in the shallowing interval (Fürsich et al., 2005). The TOC content of the intercalated shales ranges between 1.35% and 7.9%, with an average of 1.75%, indicating a shallow marine environment under anoxic conditions (Arora et al., 2015).

The Middle Callovian section of the Chari Formation comprises limestone with intercalated beds of shales and increasing kaolinite (Fig. 3). The presence of kaolinite indicates the sea-level rise with abundant sediments (Fürsich et al., 2005). Fine-grained siliciclastic rocks that dominate the Callovian record a deepening of the basin with transgression (Singh, 1989; Fürsich et al., 1991; Dromart et al., 2003). The abundant megafauna like Praesaccella, Plaleonucula, Mesosaccella, and Nuculoma suggest a high food influx within the sediments, but less diversity of benthic species was seen in Callovian than in Bathonian (Fürsich et al., 2004).

In the Late Callovian to Oxfordian Age Chari to Katrol formations, carbonates were in little supply (Fig. 3). Low-relief areas became more vulnerable to erosion due to decreasing sea levels (Alberti et al., 2013), resulting in more smectite to the basin due to transgressive-regressive cycles during the deposition of the Chari Formation (Fürsich & Oschmann, 1993). Zavar (2006) suggested that ooid deposits developed during the Oxfordian Age in England, France, and Poland due to rising sea levels. However, the accretion of ooid and Fe-oolite is characterised by regression and agitated water conditions along the edge of the Paris Basin (Curial & Dromart, 1998; Geiger & Schweigert, 2006). The sediments of the Oxfordian Age represent the Dhosa Oolite Member, which emerged through the transgression phase, but their concluding burial turned up during the regression due to shallow brackish or freshwater influence (Garg & Jain, 2012; Ramkumar et al., 2013), resulting in a basin filled with marginal marine to non-marine deposits by the Early Cretaceous (Fürsich et al., 2004).

For the present study, a thick and well-exposed succession of Jurassic rocks on the northern side of Habo Dome was sampled (Fig. 2B). This area was mainly chosen because the dome has steep slopes on its northern side and is near Bhuj, the headquarters of Kachchh district. Due to reasonably deep cutting by Nala, more than half of the sequences of Jurassic rocks of Habo Dome are very well exposed in a comparatively small circumscribed area. Based on the lithological variation, the samples were collected bed-by-bed for the foraminiferal study. Through GPS, the accurate and exact locations were recorded ( Appendix 1). From bottom to top, the Pachcham to Katrol formations of Habo Dome are divided into fourteen litho-units, H–0 to H–13 (a total of 64 samples; Fig. 2A). Out of these, only 23 samples (see Fig. 4) yielded foraminifera, whereas the remaining 39 samples were barren. A composite lithology (Fig. 2A) was prepared by integrating all the sections mentioned above. In order to prevent mixing, all 64 samples were carefully packed in thick covers.

Conventional methods were used to prepare samples: 300 g of crushed samples were immersed in water with sodium carbonate and boiled continuously for 4–6 hours. After that, careful washing with a water shower was carried out over a set of sieves with 35-, 63-, and 125-micron mesh (Ansari et al., 2022). Then, the sieve samples were put in an electric oven for drying. The dried samples were divided into two fractions: <63 and 63–125 microns, then transferred to plastic vials and labelled. A stereo-zoom binocular microscope was used to examine dried samples. Henceforth, 300 foraminiferal tests were chosen using a Random Picking chart with a fine-pointed sable hairbrush and poking needle. For the examination, foraminiferal tests were sorted and arranged on microfaunal slides. Some specimens were cleaned further with an ultrasonic cleaner.

Species identifications were made by the use of the literature (literature cited in the Systematics section) and the Ellis and Messina Catalogue of Foraminifera (online). Scanning Electron Photomicrography was done for one representative specimen of all the described species by SEM (model JEOL–JSM 6510 LV) at the University Sophisticated Instruments Facility (USIF), Aligarh Muslim University, Aligarh, India.

Multivariate techniques offer greater sensitivity and power than individual analyses (Gauch, 1982). Clustering and ordination describe biological patterns independently of environmental correlations. They give a clear picture of the community pattern and rate the strength of biological-environmental links (Jennings et al., 2004). Multivariate analysis plays a crucial role in palaeoecological reconstructions and palaeoclimatological dynamics (Gupta, 1997). Techniques such as principal component analysis (PCA), principal coordinates analysis (PCoA), and correspondence analysis (CA) help reconstruct palaeoenvironments by interpreting microfossil data. They can also infer sea-level changes by examining changes in marine proxies (Horton et al., 1999).

In micropalaeontological research, cluster analysis is one of the most frequently used methods to evaluate how organisms are related to communities (Schmiedl et al., 1997; Mendes et al., 2004; Kuhnt et al., 2007; El-Menhawey et al., 2020). The PAST software (palaeontological statistics) of Hammer et al. (2001) was used in this technique to determine the relationship between fauna and environmental parameters (Abdelhady & Fürsich, 2015; Fajemila et al., 2020; Buragohain & Ghosh, 2021; Mazumder & Nigam, 2021).

Non-metric multidimensional scaling (NMDS) is used to illustrate the data similarities or dissimilarities. In a low-imensional area, NMDS begins with an orientation matrix for each data set. Depending on each method, we examined the same data twice to determine which was best for the observed data, as the two procedures differ genetically. The NMDS findings are illustrated in Figures 5 and 6 with medium stress values (0.15 to 0.20), looking for a better result even though it makes fewer speculations regarding the type of data (Kreft & Jetz, 2010). The poor configuration shows a high-stress value, whereas a low-stress value represents the ‘goodness of fit’ between the components and positions (Kruskal, 1964a).

Based on the foraminiferal assemblages, the palaeoecological conditions may be interpreted using the diversity index and morphogroup concepts (Reolid et al., 2008a; Talib et al., 2016, 2017; Ansari et al., 2021b, 2022). The test features, including chamber arrangement (e.g., trochospiral, planispiral), aperture (rounded, radiate, slit-shaped, lipped), and coiling (sinistral/dextral), are used to interpret the palaeoenvironmental conditions with the feeding strategy and lifestyle of the taxa (Tyszka, 1994; Murray et al., 2011). Estimating a species’ relevance within the community is necessary for diversity measures in which productivity, numbers, and biomass play an essential role (Canales & Henriques, 2008). Of these, the Fisher α-diversity (Fisher et al., 1943) of foraminiferal assemblages is incorporated in the present study (Wasim et al., 2020, 2021). We further integrate abiotic factors—the terrigenous materials, stratigraphy, sedimentological characteristics, clay minerals, sedimentary structures, and microfacies analysis—to interpret the changing oxygen levels and fluctuating sea levels.

The studied section of Habo Dome, Kachchh, based on 64 samples, contains foraminiferal assemblages represented by 86 species ( Appendix 1). Six suborders comprise the assemblages of foraminifera. Out of these, Lagenina comprises 16 genera and 61 species, or 70.93% of the total species. Textulariina with three genera and 16 species, form 18.60%. Robertinina with one genus and five species, represents 5.81%. Spirillinina with one genus and two species constitutes 2.32%. Involutina and Rotaliina both have one genus and one species and comprise 1.16% of the total species represented.

Out of 12 families, Vaginulinidae is the dominant family among the studied foraminiferal assemblages, having 52 species belonging to nine genera (60.46%). This is followed by the family Lituolidae, which has 12 species belonging to one genus (13.95%). Nodosariidae has five species of four genera (5.81%), and Epistominidae has five species belonging to one genus (5.81%). Spirillinidae, Robuloididae, Haplophragmoidadae, and Haplophragmidae all have two species of one genus (2.32% each). Ichthyolariidae, Lagenidae, Planulinidae, and Involutinidae have one genus and one species (1.16% each). Calcareous species are dominant, with a calcareous vs. agglutinated ratio of 4.37:1.

Here we document new reports of 27 species from the Indian subcontinent, namely: Ammobaculites fragmentaria, A. rhaeticus, Prodentalina vetustissima, Pseudonodosaria mutabilis, Pyramidulina obscura, Lingulina lordosa, Frondicularia inversa, Lenticulina crepidula, L. dunkeri, L. exgaleata, L. inermis, L. major, L. tumida, Marginulinopsis matutina, M. pauliniae, Saracenaria saxonica, Saracenaria aff. S. cretacea, Astacolus deformis, A. scalpatus, A. schloenbachi, Marginulina garretti, M. scapha, M. sporta, Citharina raricosta, Vaginulina excentrica, Epistomina anterior, and Planulina spissicostata, out of which only one genus, Ammobaculites is agglutinated.

The 27 species reported in India for the first time are described here and illustrated in Figure 7. The remaining 59 species are illustrated in Figures 8 and 9. In the present study, genus classification is based on Loeblich & Tappan (1987), whereas species within each genus are arranged alphabetically (Poole & Wade, 2019). All the discussed specimens have been deposited in the micropalaeontology collection of the Department of Geology, Aligarh Muslim University, Aligarh, India.

Order FORAMINIFERIDA Eichwald, 1830
Suborder TEXTULARIINA Delage & Hérouard, 1896
Superfamily LITUOLACEA de Blainville, 1827
Family LITUOLIDAE de Blainville, 1827
Genus Ammobaculites Cushman, 1910
Type species: Spirolina agglutinans d'Orbigny, 1846

Ammobaculites fragmentaria Cushman, 1927 
Fig. 7.1 

• Ammobaculites fragmentaria Cushman, 1927, p. 130, pl. 1, fig. 8.

• Ammobaculites fragmentarius Cushman, 1927. McNeil & Caldwell, 1981, p. 158–159, pl. 12, figs. 6, 7. Koke & Stelck, 1984, p. 276–277, pl. 1, fig. 42.

268 specimens.

Length = 0.99–0.78 mm, width = 0.14–0.08 mm, thickness = 0.13–0.09 mm.

Ammobaculites fragmentaria was first reported by Cushman in 1927 from the Cretaceous of Canada. The present specimens closely resemble the original ones. This species differs from A. coprolithiformis in its more significant proportion of uncoiled chambers, tapering test, and peculiar flaky wall structure. The specimen by Shahin & El-Baz (2021) from Egypt has a broad coiling with five chambers and a short-height uncoiled part.

Ammobaculites rhaeticus Kristan-Tollmann, 1964 
Fig. 7.2 

• Ammobaculites rhaeticus Kristan-Tollmann, 1964, p. 37, pl. 4, figs. 8–13.

27 specimens.

Length = 0.92–0.56 mm, width = 0.19–0.11 mm, thickness = 0.13–0.11 mm.

Ammobaculites rhaeticus was initially described by Kristan-Tollmann (1964) from the Upper Triassic of Austria (the Rhaetian strata). The present specimens resemble the original ones in size and coiled part, whereas the uncoiled part has a broader top and no brick-like chambers. This species differs from Ammobaculites cobbani Loeblich & Tappan 1950 in having considerably more chambers in the uncoiled part, through the lower end chamber, the round, not the elongated aperture, and the different nature of the shell. Compared to Ammobaculites parallelus Ireland 1956, the present species has much broader and lower chambers, gradually becoming more prominent towards the top.

Suborder LAGENINA Delage & Herouard, 1896
Superfamily ROBULOIDACEA Reiss, 1963
Family ICHTHYOLARIIDAE Loeblich & Tappan, 1986
Genus Prodentalina Norling, 1968
Type species: Dentalina terquemi d'Orbigny, 1850

Prodentalina vetustissima (d'Orbigny, 1850)
Fig. 7.3 

• Dentalina vetustissima d'Orbigny, 1850, p. 1–427.

• Dentalina exilis (d'Orbigny), 1850. Franke, 1936, p. 31, pl. 2, fig. 25. Riegraf, Luterbacher & Leckie, 1984, p. 697, pl. 5, figs. 144–146.

59 specimens.

Length = 1.16–0.60 mm, width = 0.11–0.07 mm, thickness = 0.06–0.03 mm.

Prodentalina vetustissima was originally described by d’Orbigny (1850) from Paris. Our specimens somewhat resemble those described by Riegraf et al. (1984) from Morocco but differ in that the first chamber is more inflated with oblique sutures and weaker chambers. Prodentalina vetustissima originally has a more pronounced circular section, but the specimen under study is smaller and has an ovate transverse section.

Superfamily NODOSARIACEA Ehrenberg, 1838
Family NODOSARIIDAE Ehrenberg, 1838
Genus Pseudonodosaria Boomgaart, 1949
Type species: Glandulina discreta Reuss, 1850

Pseudonodosaria mutabilis Reuss, 1863 
Fig. 7.4 

• Pseudonodosaria mutabilis Reuss, 1863, p. 58, pl. 5, fig. 7–11.

• Pseudonodosaria mutabilis Reuss, 1863. Bolli, Beckmann & Saunders, 1994, p. 23, figs. 8.8, 10.1–6. Abu-Zied, 2007, p. 775, pl. 9, figs. D, E.

Three specimens.

Length = 1.10–0.63 mm, width = 0.54–0.32 mm.

Reuss (1863) reported it from the Upper Hills Gault in the Lower Cretaceous of Germany. It has also been reported from the Santonian of California. Studied specimens are similar to those described by Abu-Zied (2007) from Egypt. Pseudonodosaria mutabilis is close to P. humilis in chamber arrangement and growth but differs in the number and shape of chambers.

Superfamily NODOSARIACEA Ehrenberg, 1838
Family NODOSARIIDAE Ehrenberg, 1838
Genus Pyramidulina Fornasini, 1894
Type species: Pyramidulina eptagona Fornasini, 1894

Pyramidulina obscura (Reuss, 1845)
Fig. 7.5 

• Nodosaria obscura Reuss, 1845, p. 26, pl. 13, fig. 7–9.

• Nodosaria obscura (Reuss), 1845. Dieni & Massari, 1966, p. 110, pl. 3, fig. 3. Shahin, 2000, p. 533, fig. 11.37. Abu-Zied, 2007, p. 775, pl. 9, figs. F, G.

Six specimens.

Length = 0.71–0.55 mm, width = 0.19–0.10 mm.

Pyramidulina obscura was first described by Reuss (1845) from the Lower Cretaceous of Germany. Our specimens resemble those described by Abu-Zied (2007) from Egypt. This species is less abundant in the Jurassic of Kachchh and differs from Abu-Zeid’s specimen in having a slender test, deflated chambers towards the aperture, and obscure sutures.

Genus Lingulina d'Orbigny, 1826
Type species: Lingulina carinata d'Orbigny, 1826

Lingulina lordosa Loeblich & Tappan, 1950 
Fig. 7.6 

• Lingulina lordosa Loeblich & Tappan, 1950, p. 51, pl. 13, fig. 24a, b.

• Lingulina lordosa Loeblich & Tappan, 1950. Tingley & Sawyer, 2015, p. 2, pl. 1, fig. 30.

Six specimens.

Length = 0.78–0.47 mm, width = 0.34–0.22 mm, thickness = 0.23–0.11 mm.

Lingulina lordosa was first described by Loeblich & Tappan (1950) from the Upper Jurassic of North America. Present specimens appear similar to those represented by Tingley & Sawyer (2015) from Canada. This species may be distinguished from L. cordiformis Terquem by being approximately one-third smaller and lacking the huge terminal chamber, which, in the type of L. cordiformis, has a height greater than the preceding three chambers combined. Lingulina lordosa differs from L. cordiformis in possessing a radial aperture on a short neck rather than an elongated aperture. The present species may be separated from L. gottingensis Franke (1936) by the compressed rather than inflated test and by having arched instead of straight sutures. It is separated from Frondicularia excavata Terquem (1866) by its much larger size, radiate aperture, and less strongly arched sutures.

Genus Frondicularia Defrance, 1826
Type species: Renulina complanata Defrance, in de Blainville, 1824

Frondicularia inversa Reuss, 1844 
Fig. 7.7 

• Frondicularia inversa Reuss, 1844, p. 211.

• Frondicularia inversa Reuss, 1844. Cushman, 1946, p. 86, pl. 34, figs. 11, 12. Frizzell, 1954, p. 98, pl. 12, fig. 3. Sliter, 1980, p. 384, pl. 7, fig. 17, 18.

Two specimens.

Length = 1.86–1.10 mm, width = 0.95–0.56 mm.

Reuss (1844) first described Frondicularia inversa from the Cretaceous sediments of Germany. Our specimens are similar to those described by Sliter (1980) from the North Atlantic. Frondicularia inversa resembles Frondicularia angustata, but the test is narrow, the chambers are thicker in the middle, and there are broader ridges. This specimen also differs from the sample of Bagg (1897), which has very few chambers.

Family VAGINULINIDAE Reuss, 1860
Genus Lenticulina Lamarck, 1804
Type species: Lenticulites rotulatus Lamarck, 1804

Lenticulina crepidula (Fichtel & Moll, 1803)
Fig. 7.8 

• Nautilus crepidula Fichtel & Moll, 1803, p. 107, pl. 19, figs. g–i.

• Cristellaria (Planularia) crepidula (Fichtel & Moll), 1803. Frentzen, 1941, p. 345, pl. 5, figs. 6, 7. Kristan-Tollmann et al., 1980, p. 130, pl. 26, fig. 8.

50 specimens.

Length = 0.56–0.32 mm, width = 0.40–0.28 mm, thickness = 0.09–0.05 mm.

Lenticulina crepidula was originally explained by Fichtel & Moll in 1803 from Germany. The present specimens are similar to those reported by Kristan-Tollmann et al. (1980) from Germany. This form is somewhat close to Lenticulina stilla (Terquem), but the latter is thrice as long and differs in having prominently raised sutures.

Lenticulina dunkeri (Reuss, 1863)
Fig. 7.9 

• Cristellaria dunkeri Reuss, 1863, p. 73, pl. 8, fig. 6.

• Lenticulina dunkeri (Reuss), 1863. Meyn & Vespermann, 1994, p. 137, pl. 25, fig. 11, 12. pl. 26, fig. 1, 6. Szinger, 2008, p. 136, pl. 2, fig. 14.

Three specimens.

Length = 0.85–0.56 mm, width = 0.73–0.39 mm, thickness = 0.69–0.49 mm.

Lenticulina dunkeri was initially described by Reuss in 1863 from the Early Cretaceous of Germany. Only three specimens are found in the present material of Kachchh. Habo Dome specimens resemble those described by Szinger (2008) from Hungary, which has a prominent umbo.

Lenticulina exgaleata Dieni, 1985 
Fig. 7.10 

• Lenticulina exgaleata Dieni, 1985, p. 343.

• Lenticulina exgaleata Dieni, 1985. Canales & Henriques, 2013, p. 191, fig. 4.2. Silva, Canales, Henriques & Ureta, 2020, p. 55, fig. 2k.

92 specimens.

Length = 0.85–0.64 mm, width = 0.70–0.39 mm, thickness = 0.65–0.41 mm.

Present specimens are similar to those reported by Canales & Henriques (2013) from Portugal, and the original was described by Dieni (1985) from Sardinia, Italy, in the Middle Jurassic. This species resembles Lenticulina helios, but L. helios has an umbonal boss.

Lenticulina inermis (Terquem, 1862)
Fig. 7.11 

• Cristellaria inermis Terquem, 1862, p. 447, pl. 6, fig. 5.

• Lenticulina inermis (Terquem), 1862. Riegraf, Luterbacher & Leckie, 1984, p. 698, pl. 6, fig. 158.

14 specimens.

Length = 0.59–0.28 mm, width = 0.51–0.25 mm, thickness = 0.13–0.08 mm.

A few specimens are reported from the present material, which is very close to those described by Riegraf et al. (1984) from the Lias of Morocco. Initially, it was described by Terquem (1862) from the Lower Jurassic of France.

Lenticulina major (Bornemann, 1854)
Fig. 7.12 

• Cristellaria major Bornemann, 1854, p. 40, pl. 4, fig. 13.

• Lenticulina major (Bornemann), 1854. Cordey, 1963, p. 46, fig. 3, text-figs. 4–9.

One specimen.

Length = 0.70–0.28 mm, width = 0.45–0.25 mm, thickness = 0.11–0.06 mm.

A single specimen of Lenticulina major was reported from Habo Dome of Kachchh, India. The present specimen is similar to the original one, as described by Bornemann (1854) from Germany. This is a highly variable species in its external morphology, with an initial coil that can be prominent or obscured. The periphery is rounded and entire to lobate. There is a variation in the degree of inflation of the final chamber.

Lenticulina tumida Myatlyuk, 1949 
Fig. 7.13 

• Lenticulina tumida Myatlyuk, 1949, p. 114, pl. 3, fig. 9.

• Lenticulina tumida Myatlyuk, 1949. Bielecka & Styk, 1981, p. 30, pl.3, fig. 4. Görög, Toth & Wernli, 2012, p. 107, pl. 3, fig. 2. Smoleń & Lwanczuk, 2018, p. 267, figs. 8. B, C.

22 specimens.

Length = 0.45–0.29 mm, width = 0.34–0.25 mm, thickness = 0.13–0.09 mm.

Twenty–two Lenticulina tumida were recorded from Kachchh material and initially expressed by Myatlyuk in 1949 in the upper Jurassic to lower Cretaceous from Russia. The present specimen is similar to those reported by Bielecka & Styk (1981) from the Polish basin, Görög et al. (2012) from the Callovian of Hungary, and Smoleń & Lwanczuk (2018) from the Callovian of the Polish lowland. The specimens from the Villainy Mountains of Hungary described by Görög et al. (2012) are only about half the size of those described by Bielecka & Styk (1981). Our specimen differs from the holotype in having flush sutures, a prominent umbonal boss, and a thick keel.

Genus Marginulinopsis Silvestri, 1904
Type species: Cristellaria bradyi Goes, 1894

Marginulinopsis matutina (d'Orbigny, 1850)
Fig. 7.14 

• Cristellaria matutina d'Orbigny, 1850, p. 242.

• Cristellaria (Astacolus) antiquata (d'Orbigny), 1850. Franke, 1936, p. 105, pl. 10, fig. 14. Bartenstein & Brand, 1937, p. 172, pl. 6, fig. 31. Riegraf, Luterbacher & Leckie, 1984, p. 685, pl. 7, figs. 167, 168.

One specimen.

Length = 0.74–0.52 mm, width = 0.48–0.26 mm.

Only one specimen of Marginulinopsis matutina was reported from Kachchh material and was initially reported by d’Orbigny in 1850. The present specimen is similar to those described by MacFadyen (1936) from the Argovian of British Somaliland, Franke (1936) from Lias of France, Bartenstein & Brand (1937) from the Upper Bajocian of northwest Germany, and Riegraf et al. (1984) from the Jurassic of Morocco.

Marginulinopsis pauliniae Terquem, 1866 
Fig. 7.15 

• Marginulina pauliniae Terquem, 1866, p. 427, pl. 17, fig. 5.

• Marginulina oolithica (Terquem), 1866. Bartenstein & Brand, 1937, p. 160, pl. 2A, fig. 11, p. 161, pl.  2B, fig. 37, p. 162, pl. 3, fig. 38. Riegraf, Luterbacher & Leckie, 1984, p. 686, pl. 7, fig. 166.

Seven specimens.

Length = 0.78–0.52 mm, width = 0.52–0.26 mm.

Very few specimens of Marginulinopsis pauliniae were reported from the present material, initially described by Terquem in 1866 from the Lower Jurassic of France. The present specimen is identical to those reported by Terquem & Berthelin in 1875 from Lias (Lower Jurassic) of France, Franke (1936) from Lias of France, Bartenstein & Brand (1937) from the Upper Bajocian of northwest Germany, and Riegraf et al. in 1984 from Jurassic of Morocco.

Genus Saracenaria Defrance, 1824
Type species: Saracenaria italica Defrance, 1824

Saracenaria saxonica (Bartenstein & Brand, 1951)
Fig. 7.16 

• Lenticulina saxonica Bartenstein & Brand, 1951, p. 284.

• Saracenaria saxonica (Bartenstein & Brand), 1951. Sliter, 1980, p. 394, pl. 12, figs. 10–14.

11 specimens.

Length = 0.74–0.52, width = 0.42–0.25 mm, thickness = 0.32–0.17 mm.

This species was first reported by Bartenstein & Brand in 1951 from the Upper Jurassic to Lower Cretaceous of north-western Madagascar. The present specimens differ slightly from the originals because the sutures do not extend from the centre to the periphery.

Saracenaria aff. S. cretacea Dailey, 1970 
Fig. 7.17 

• Saracenaria cretacea Dailey, 1970, p. 100–111.

• Saracenaria cretacea Dailey, 1970. Sliter, 1980, p. 105, pl. 12, figs. 1, 2.

24 specimens.

Length = 0.78–0.48 mm, width = 0.42–0.26 mm, thickness = 0.37–0.12 mm.

Saracenaria cretacea was first reported by Dailey (1970) from the Cretaceous of California. Habo specimens closely resemble those described by Sliter (1980) from the Mesozoic rocks of Morocco. Distinguishing characteristics of this species are a large initial planispirally coiled portion, followed by uniserial chambers characterised by little increase in height and a slight narrowing when viewed from the side, and a variably costate ventral margin. The species closest to the present one is Saracellaria reesidei Fox, from the Lower Coloradoan of Wyoming, whereas it lacks the large initial planispiral coil and gently curved sutures of S. cretacea. Saracellaria cretacea resembles S. aculeata Espitalie and Sigal most in outline. Still, it is distinct from that species in most other aspects, such as the perfect triangle shape in the transverse section.

Genus Astacolus de Montfort, 1808
Type species: Astacolus crepidulatus de Montfort, 1808

Astacolus deformis (Bornemann, 1854)
Fig. 7.18 

• Cristellaria deformis Bornemann, 1854, p. 41, pl. 4, fig. 35.

• Astacolus deformis (Bornemann), 1854. Riegraf, Luterbacher & Leckie, 1984, p. 698, pl. 6, fig. 155.

Six specimens.

Length = 0.68–0.39 mm, width = 0.25–0.16 mm, thickness = 0.19–0.13 mm.

A few Astacolus deformis are documented from Kachchh material, first reported by Bornemann in 1854 from the Lower Jurassic of Germany. The present specimens are similar to those reported by Riegraf et al. (1984) from Morocco. This species resembles Cristellaria centralis Terquem from the Upper Bajocian of France in size and general appearance but is less compressed and has more horizontal and less depressed sutures.

Astacolus scalptus (Franke, 1936)
Fig. 7.19 

• Cristellaria (Astacolus) scalpta Franke, 1936, p. 105–106, pl. 10, figs. 19, 20a,b.

• Astacolus scalptus (Franke), 1936. Canales & Henriques, 2008, p. 165, pl. 3, fig. 4. Canales & Henriques, 2013, p. 192, fig. 4.10.

26 specimens.

Length = 0.66–0.28 mm, width = 0.41–0.17 mm, thickness = 0.16–0.09 mm.

This species was initially described by Franke (1936) from the Lower Jurassic of France. Present specimens of Astacolus scalptus are somewhat identical to those reported by Canales & Henriques (2008, 2013) from the Jurassic of Portugal. The Portugal specimens are more inflated, have depressed sutures, and have more width than the Habo Dome specimens.

Astacolus schloenbachi (Reuss, 1863)
Fig. 7.20 

• Cristellaria schloenbachi Reuss, 1863, p. 65, pl. 6, fig. 14, 15.

• Astacolus schloenbachi (Reuss), 1863. Kovatcheva, 1968, p. 16, pl. 3, fig. 9. Meyn & Vespermaann, 1994, p. 186, pl. 41, figs. 16, 17. Szinger, 2008, p. 136, pl. 2, fig. 15.

17 specimens.

Length = 0.72–0.30 mm, width = 0.28–0.16 mm, thickness = 0.15–0.08 mm.

A few specimens of Astacolus schloenbachi are reported from Habo Dome, Kachchh. Our specimens resemble those that Szinger (2008) described from the Early Cretaceous of Hungary, but they were initially described by Reuss (1863) from Germany. However, compared to Hungary, the specimen’s test from Kachchh is more inflated. It was also reported by Kovatcheva (1968) and Meyn & Vespermann (1994).

Genus Marginulina d’Orbigny, 1826
Type species: Marginulina raphanus d'Orbigny, 1826

Marginulina garretti Cushman & Ellisor, 1945 
Fig. 7.21 

• Marginulina garretti Cushman & Ellisor, 1945, p. 555, pl. 73, figs. 21–24.

One specimen.

Length = 0.51–0.27 mm, width = 0.23–0.11 mm, thickness = 0.10–0.06 mm.

One specimen of Marginulina garretti is described from the Habo material of Kachchh. It was initially described by Cushman & Ellisor (1945) from the Anahuac Formation, Texas. This species differs from Marginulina vaginata Garrett and Ellis in the more elongated test, the more significant number of adult globular chambers, and the much more numerous costae. It is named in honour of Mr. J. B. Garrett.

Marginulina scapha Lalicker, 1950 
Fig. 7.22 

• Marginulina scapha Lalicker, 1950, p. 12, pl. 1, figs. 7a, b.

• Marginulina scapha Lalicker, 1950. Gordon, 1967, p. 452, pl. 2, figs. 1–5. Canales & Henriques, 2013, p. 192, fig. 4.13. Silva, Canales, Sandoval & Henriques, 2017, p. 8, fig. 3w.

One specimen.

Length = 0.58–0.36 mm, width = 0.20–0.16 mm, thickness = 0.16–0.13 mm.

A lone specimen collected from the present material is similar to the holotype. The differences may be observed in inflated initial parts and flush sutures. Marginulina scapha was initially explained by Lalicker in 1950 from the Middle Jurassic (Bathonian) in southwestern Montana. Lalicker defined his species from that of Schwager based on the extensive uniserial part, and the curving (other than straight) sutures in M. scapha were remarked by Gordon (1967). Canales & Henriques (2013) also documented M. scapha from the Middle Jurassic (Bajocian) of Cape Mondego (Portugal). These specimens also have flush sutures.

Marginulina sporta Lalicker, 1950 
Fig. 7.23 

• Marginulina sporta Lalicker, 1950, p. 14, pl. 2, figs. 3a,b.

11 specimens.

Length = 0.86–0.56 mm, width = 0.50–0.32 mm, thickness = 0.20–0.15 mm.

Eleven specimens of Marginulina sporta are described from Habo Dome Kachchh. Our illustrated specimen is similar to the original one from Montana described by Lalicker (1950) but differs in the arrangement of coiled chambers. The test length of Lalicker’s sample is comparatively less than that of Kachchh’s specimen. This species is somewhat similar in outline to Cristellaria candonensis d’Orbigny, which differs in having more chambers in the uniserial portion and in being more slender.

Genus Citharina d’Orbigny, 1839
Type species: Vaginulina (Citharina) strigillata Reuss, 1846

Citharina raricosta (Fursenko & Polenova, 1950)
Fig. 7.24 

• Vaginulina raricosta Fursenko & Polenova, 1950, p. 56, pl. 5, figs. 5–8.

Three specimens.

Length = 0.89–0.61 mm, width = 0.46–0.31 mm, thickness = 0.21–0.15 mm.

This species belongs to the large Vaginulina harpa Roemer group. Vaginulina harpa was described by Roemer from the Neocomian Hills Clay. Comparing its extremely short description with the features of V. raricosta, it is observed that the latter differs by its much more elongated shape. In this species, the number of chambers is usually eight to nine, and only in rare cases does it reach fourteen, whereas in typical V. harpait is around sixteen. Furthermore, in the latter species, judging from Roemer’s figure, the septa are more slanted, and there is no apertural neck. The number of costae is a very significant difference between the two species.

Genus Vaginulina d'Orbigny, 1826
Type species: Nautilus Iegumen Linne, 1758

Vaginulina excentrica (Cornue, 1848)
Fig. 7.25 

• Cristellaria excentrica Cornue, 1848, p. 14, pl. 2, figs. 11–13.

26 specimens.

Length = 0.53–0.25 mm, width = 0. 18–0.11 mm, thickness = 0.12–0.05 mm.

This species is similar to Vaginulinopsis excentrica but different in test coiling and distinct sutures. Our specimens are identical to the original one described by Cornue (1848).

Suborder ROBERTININA Loeblich & Tappan, 1984
Superfamily CERATOBULIMINACEA Cushman, 1927 
Family EPISTOMINIDAE Wedekind, 1937
Genus Epistomina Terquem, 1883
Type species: Epistomina regularis Terquem, 1883

Epistomina anterior (Bartenstein & Brand, 1951)
Figs. 7.26a–b

• Epistomina caracolla anterior Bartenstein & Brand, 1951, p. 239–339, pl. 12A, fig. 341.

• Epistomina anterior (Bartenstein & Brand), 1951. Sliter, 1980, p. 369, pl. 22, figs. 1–6.

71 specimens.

Length = 0.70–0.45 mm, width = 0.55–0.28 mm, thickness = 0.32–0.14 mm.

The present specimens are similar to those of Sliter (1980) from the eastern North Atlantic. This species was first reported by Bartenstein & Brand (1951) from the Late Jurassic to Early Cretaceous of northwestern Madagascar. This species differs from Jurassic Epistomina uhligi in having a more angled periphery, a more evenly biconvex test, and a more prominent umbo on the umbilical side.

Superfamily PLANORBULINACEA Schwager, 1877
Family PLANULINIDAE Bermudez, 1952
Genus Planulina d'Orbigny, 1826
Type species: Planulina ariminensis d'Orbigny, 1826

Planulina spissicostata Cushman, 1938 
Fig. 7.27 

• Planulina spissicostata Cushman, 1938, p. 69, pl. 3.

• Planulina spissicostata Cushman, 1938. Ghoorchaei, Vahidinia & Ashoori, 2012, p. 42, pl. 3, figs. 16, 17.

Three specimens.

Length = 0.46–0.42 mm, width = 0.40–0.30 mm, thickness = 0.20–0.17 mm.

Very few specimens of Planulina spissicostata were reported from Kachchh, India. Initially described by Cushman (1936), the Habo Dome specimens are similar to those described by Ghoorchaei et al. (2012) from Iran. This species is also described by Hedberg & Pyre (1944) and Neagu (1972) from the Early Cretaceous of northeastern Hedberg Anzoátegui, Venezuela, and Eastern Carpathians, Romania, respectively.

Several researchers (Nagy, 1992; Tyszka, 1994; Nagy et al., 1995; Reolid et al., 2008a,b, 2014, 2019a,b; Ansari et al., 2021b; Wasim et al., 2021) have proposed Jurassic foraminiferal morphogroup schemes based on test morphology (test outline, chamber arrangement, and mode of coiling), microhabitat (epifaunal, shallow infaunal, and deep infaunal), and feeding strategies (suspension-feeder, herbivore, bacterivore, omnivore, etc.; Fig. 10). In particular, calcareous foraminifera reveal an unmatched degree of complexity in the tests like structure, morphology, and chemistry (Audet et al., 2023). Many researchers have utilised various symbols to classify morphogroups based on various test attributes (Ansari et al., 2021b). The morphogroups are mainly separated into two groups: agglutinated and calcareous. The agglutinated foraminifera in this study fall into morphogroups C2 and D. The calcareous foraminifera in this study fall into morphogroups G, H, J1, J2, and K in the morphogroup scheme from Reolid et al. (2008a; Fig. 10).

Morphogroup C consists of a multilocular, elongated, subcylindrical test form with an infaunal living habitat and detritivore-bacterial scavengers (Fig. 10). Within this morphogroup, only the C2 subgroup is found in the present assemblages. Elongated uniserial forms with a planispirally or streptospirally coiled early stage represent subgroup C2. Ammobaculites and Haplophragmium are listed in Subgroup C2, which reflects a shallow infaunal lifestyle and bacterial scavengers (detritivores; Fig. 10). With a tiny coil and a flaring test, agglutinated Ammobaculites with coarse to medium silt demonstrate accessibility to the shelf environment (Gaillard, 1983). Small Ammobaculites from the Jurassic Period (Barnard et al., 1981; Reolid et al., 2008a) are representative of a shallow water habitat (Fig. 10) and a bacterial feeding strategy (Nagy, 1992; Tyszka, 1994), whereas large forms depict a comparatively deeper shelf zone (Reolid et al., 2008a). According to Jain et al. (2019), Ammobaculites are a sign of low-level oxygen conditions and a shallow infaunal habitat (Fig. 10). Ammobaculites from the Lower Kharga Shale Member suggest a marshy habitat (Gerlach et al., 2017; Orabi, 2020).

Morphogroup D comprises multilocular tests that are rounded to planoconvex and have a spiral chamber arrangement. Genera of this morphogroup live in an epifaunal environment and are herbivorous, detritivores and bacterivorous. Haplophragmoides represent this group in the present assemblages (Fig. 10). This genus is interpreted as having epifaunal active herbivore, detritivore, and bacterivore habits.

The calcareous morphogroup G consists of only Epistomina, which has a plano-convex trochospiral test (Fig. 10). They are grazing herbivores that feed on seaweed epifauna. Most Epistomina species are considered opportunistic taxa that live in oxygen-depleted (dysoxic) outer shelf habitats (Bernhard, 1986; Koutsoukos et al., 1990; Sagasti & Ballent, 2002; Ballent et al., 2006). The outer shelf environment in the lower subtidal zone is thought to be represented by Epistomina (Reolid et al., 2008a; Talib et al., 2017). Epistomina, according to Le Galvez (1958) and Colpaert et al. (2017), is sensitive to the substrate, preferring fine and muddy sea bottoms.

Morphogroup H contains calcareous taxa with planispiral discoidal flattened forms consisting of two chambers, represented here by Spirillina and Trocholina, with Trocholina (Fig. 10) dominating at the specimen level (proloculus and a secondary long undivided tubular chamber). These taxa are considered epifaunal to grazing herbivores and phytodetritivores that feed on primary weed fauna.

Lagoonal algal vegetation contains modern Spirillina (Davies, 1970; Brasier, 1975). According to several authors, Spirillina’s depth habitat changed from a shelf to a marginal marine environment during the Jurassic period (Reolid et al., 2008a). As a vagile epifaunal habitat, Bajocian Spirillina prefers the shallow photic setting found in North Sea deltas (Nagy, 1992). Upper Jurassic Spirillina indicates that shelf and deep bathyal environments are well represented by low sedimentation rates (Reolid et al., 2008a; Farahani et al., 2018).

Morphogroup J comprises calcareous foraminifera with extended multilocular tests suited to a shallow infaunal environment. Subgroup J1 comprises elongated, subcylindrical, and uniserial tests suited to shallow infaunal lifestyles, such as bacterial scavengers, herbivores, and active deposit-feeders. This subgroup is represented in the existing assemblages by Prodentalina, Falsopalmula, Pseudonodosaria, Pyramidulina, Lingulina, Marginulinopsis, Marginulina, and Lagena. Shallow forms of infauna are regarded as Prodentalina and Pseudonodosaria (Reolid et al., 2019a). Genus Pseudonodosaria was assumed to be a shallow-to-deep infaunal form with nourishing strategies ranging from deposit-feeder to grazing omnivore and/or bacterial swamper (Koutsoukos et al., 1990; Tyszka, 1994; Reolid et al., 2012b; Farahani et al., 2017). Infaunal taxa Prodentalina (Józsa et al., 2018) can be found on the open sea outer shelf region (Olóriz et al., 2002, 2006; Reolid et al., 2008a).

Subgroup J2 is denoted here by Frondicularia, Saracenaria, Astacolus, Citharina, Vaginulinopsis, Planularia, and Vaginulina (Fig. 10). It comprises flattened forms with an active deposit-feeding and grazing omnivore trophic behaviour, as well as a shallow infaunal microhabitat. Saracenaria, Astacolus, Citharina, Planularia, and Vaginulina represent the shallow infaunal ecosystem (Talib et al., 2017; Reolid et al., 2019a). Gebhardt (2020) suggests that Vaginulinopsis is a shallow infaunal species that feeds on bacteria and debris present in pelitic sediments. Citharina, for example, is an ornamented genus that indicates low oxygen levels, while Astacolus and Vaginulina have smooth tests that show normal oxygen conditions (Bernhard, 1986). The bulk of the genera in this group, including Frondicularia, Saracenaria, Astacolus, Vaginulinopsis, Planularia, and Vaginulina, are unornamented in the present assemblages.

Morphogroup K is characterised here by Lenticulina and Planulina, active deposit feeders or grazing omnivores with multilocular, biconvex, and planispiral forms with an epifaunal to deep infaunal microhabitat (Fig. 10). A variety of Jurassic microhabitats were favourable to the worldwide genus Lenticulina.

In the Jurassic period, Lenticulina was commonly documented. The high diversity of Lenticulina during the Mesozoic suggests greater ecological tolerance, with the possibility of a rise upward in the sediments to enhance oxygen intake in dysoxic situations (Bhalla & Abbas, 1984; Talib et al., 2017; Wasim et al., 2017; McNeil & MacEachern, 2023). Compared to modern Lenticulina, which prefers epifaunal microhabitats (Murray, 1991; Farahani et al., 2017), Jurassic Lenticulina favoured infaunal life habitats (Tyszka, 1994; Reolid et al., 2008a; Jain et al., 2019). The abundance of Lenticulina, Astacolus, and Nodosaria suggests an open marine environment along the continental shelf (Hughes, 2004). During the Callovian, Lenticulina adapted to high food influx and shallower depths, favouring mesotrophic environments and muddy substrates with stronger wave action, rather than the quieter, clearer, tropical, and oligotrophic conditions of the Bathonian (Jain et al., 2019).

Ammobaculites hagni (average of 31.37%; Fig. 4) dominates this assemblage, which occurs in rocks distinguished by a propensity for intercalations of limestone and shale in the Jhikadi Member of the Chari Formation (Fig. 2). The assemblage has a moderate BFOI value of 42.85%, dominating infaunal species that account for an average of 85.71% of the population. Lenticulina nodosa (29.27%), Ammobaculites variabilis (28.64%), Ammobaculites fragmentaria (26.06%), Astacolus anceps (20.13%), Ammobaculites raphiformis (12.39%), and Prodentalina vetustissima (10.53%) are other dominant species.

This study found a comparatively high abundance of the genus Ammobaculites, which comprises the family Lituolidae. According to Murray (1991), high proportions of this genus indicate brackish water. Jurassic and Cretaceous sediments containing Ammobaculites show they can withstand low oxygen levels (Koutsoukos et al., 1990). Ammobaculites, infaunal deposit feeders, are limited to enclosed brackish shelf seas, brackish lagoons and estuaries, and shallow brackish waters of tidal swamps (Murray, 1991).

Epistomina ghoshi (average of 49.65%; Fig. 4) dominates the assemblage, which occurs in the marl-clay strata from the Lower Dharang and Lower Jhikadi members. The assemblage has a high average BFOI value (66.66%) and is comprised of only epifaunal species. Additionally, Lenticulina subalata (28.15%), Epistomina erecta (14.92%), Trocholina conica (9.97%), Lenticulina helios (6.82%), and Lenticulina quenstedti (6.10%) dominate the assemblage.

It has been observed that the genus Epistomina is more prevalent in moderately deep waters with muddy sea bottoms (Bernier, 1984; Stam, 1986; Samson, 2001; Olóriz et al., 2003) and fine-grained sediments in shallow waters with mean oxygen levels ranging from low to high (Bartenstein & Brand, 1937; Riegraf et al., 1984; Tyszka, 1994; Sagasti & Ballent, 2002).

Lenticulina subalata (average of 73.25%; Fig. 4), which has a propensity for a silty substrate, dominates assemblage 1, and only epifaunal species characterized this assemblage. The other dominant taxa are Epistomina ghoshi (10.63%), Lenticulina quenstedti (10.49%), Lenticulina exgaleata (8.75%), and Lenticulina helios (5.80%) arranged in decreasing order of average relative abundance. Lenticulina subalata is dominant in assemblage 2 with an average of 48.24%, followed by Epistomina ghoshi (11.46%), Astacolus anceps (7.79%), Lenticulina muensteri (4.06%), and Lenticulina helios (2.38%). Both assemblages show the 80% BFOI value due to being dominated by epifaunal taxa.

Lenticulina can adapt to environmental change, which explains why it is found in virtually all the investigated assemblages. Furthermore, the great diversity of the genus and relative abundance suggest that it thrived in an oxic environment (Canales & Henriques, 2008), as varied Lenticulina dominant assemblages imply high amounts of dissolved oxygen (Jones & Charnock, 1985; Nagy et al., 2009; Smoleń, 2012; Talib et al., 2016, 2017; Wasim et al., 2017, 2021). Hughes (2004) suggested that assemblages dominated by Lenticulina and Astacolus develop in open sea settings below the wave base in Middle and Late Jurassic sediments of Saudi Arabia, demonstrating higher percentages of these taxa connected with maximum flooding surfaces from southern Spain (Reolid et al., 2008a).

PCA, which simplifies large data matrices with multiple variables into smaller components, was applied to determine the relationship between species and the sampling station of the community (Capotondi, et al., 2015). Figure 5 shows the concordance map of the four assemblages reflected by the cluster analysis of the benthic foraminiferal taxa. In the PCA plot, the first principal component (PC1) represents the direction of maximum variance in the dataset, while the second principal component (PC2) captures the second-largest variance (Ferraro et al., 2009). These components are orthogonal and serve to highlight the most significant patterns in the data. The graph shows a relatively high-stress value (0.20), indicating there is a strong distinction between the four observed connections, with the PCA axis showing the minimum overlap among them (Kruskal, 1964b). Variance explained refers to the proportion of the total variance in the data that is captured by each principal component (PC). The percentage of variance explained by each principal component indicates how much of the overall variability in the dataset is represented by that component. The PC1 explains 51.82% of the variance, and this means that 51.82% of the total variation in the data can be attributed to the direction defined by PC1. Similarly, PC2 explains 21.91% of the variance, and 21.91% of the variability in the data is captured by PC2. These percentages help to understand the relative importance of each principal component in representing the structure of the data. Cluster I (Ammobaculites hagni assemblage) represents a transgressive phase, indicating deeper water depths. This assemblage reflects conditions where food influx was relatively low to medium while oxygen levels remained high, creating favourable conditions for the species present in this cluster. The position of this cluster on the PCA plot suggests a significant environmental shift towards deeper, more oxygenated waters during the transgression. Cluster II (Epistomina ghoshi assemblage) also corresponds to a transgressive episode but in the deeper part of the middle shelf. The lower to medium food availability in these oxic conditions is indicated by the clustering pattern on the PCA plot, where this group of species thrives in stable but less nutrient-rich conditions. This environmental phase aligns with the transition of the water column during the transgression, particularly in deeper shelf areas. Cluster III (Lenticulina subalata assemblage 1) reveals a regression phase marked by higher food influx and declining oxygen levels. The species within this cluster are indicative of a shift towards shallower, more eutrophic conditions, where food was abundant but oxygen was limited, as seen from its position on the PCA plot. The environmental implications of this cluster suggest a response to shallower waters and nutrient-rich, low-oxygen settings typical of a regressive phase. Cluster IV (Lenticulina subalata assemblage 2) shows another transgressive phase, representing the outer neritic depth zone. This assemblage is associated with higher oxygen levels compared to Cluster III, as seen in its position on the PCA plot. The species within this cluster are adapted to more oxygenated conditions typical of deeper waters. This transgressive phase is characterised by a shift towards better-oxygenated environments, indicating a return to conditions similar to those observed in the deeper part of the middle shelf but with a distinct depth-related environmental difference (Jain et al., 2019; Audet et al., 2023; Fig. 5).

These environmental shifts in the PCA plot clearly correspond to the observed assemblages, with each cluster representing specific ecological phases. The differences in food influx and oxygen availability across these clusters reflect the complex interplay of environmental conditions during transgressive and regressive phases. The analysis of these patterns helps us understand how these assemblages responded to changing environmental conditions during the study period, providing deeper insights into the past ecological dynamics.

Cluster analysis of the presence-absence data was conducted on Bathonian to Oxfordian foraminifera assemblages (see Kumar et al., 2021) in order to examine palaeoecological patterns. The dissimilarity between cluster pairs was measured using the Forbes coefficient, which assesses faunal resemblance efficiently, even in cases where sample sizes are not uniform (Alroy, 2015). Using the average linkage method, samples were categorised into clusters. The Forbes coefficient is used to quantify dissimilarity in PCoA (Fig. 6). However, the stress value is relatively low (0.15), and the axis variance is 51.82% (for the X-axis, PCo1) and 21.91% (for the Y-axis, PCo2), representing the high variability within the Kachchh region (Abdelhady & Fürsich, 2015). In addition to the discontinuous groupings produced by clustering, PCoA ordination offers a way for visualising multivariate similarity that displays gradients in faunal composition (Arefifard & Clapham, 2021; Greco et al., 2021; Fig. 6). At the genus level, the abundance of foraminifera were binned and translated into relative proportions. The Bray Curtis dissimilarity between the various samples was estimated from the generated dataset. It used two-dimensional principal coordinates analysis to visualise the similarity pattern, indicating systematic differences in composition by site and depth (Greco et al., 2021).

In Figure 6A, Cluster I (Ammobaculites hagni assemblage) is positioned in the PCoA plot where the X-axis explains 49.70% of the variance, and the Y-axis accounts for 16.10% of the variance. The distribution of samples in this cluster indicates the dominance of a single taxon in environments with moderate depths. The position of the cluster suggests that as depth increases (Milker et al., 2009), there is a shift in the composition of the assemblage. This implies that Ammobaculites hagni is most abundant at moderate depths, but deeper settings show more variation in assemblage composition. The clustering pattern supports the idea that environmental conditions, particularly water depth, significantly influence the taxon distribution.

In Figure 6B, Cluster II (Epistomina ghoshi assemblage) exhibits greater variability, as reflected by its positioning in the PCoA plot, where the X-axis represents 46.80% of the variance, and the Y-axis explains 18.30%. This high variability suggests that the assemblage is sensitive to fluctuations in the environmental conditions within the middle shelf region. The spread of this cluster in the PCoA space may be indicative of changing conditions, such as variations in food availability and oxygen levels, leading to a broader distribution of samples in this cluster. This greater spread reflects the environmental instability that Epistomina ghoshi assemblage might experience in such areas (Salonen et al., 2019).

In Figure 6C, Cluster III (Lenticulina subalata assemblage 1) is shown in the PCoA plot, with the X-axis explaining 52.20% of the variance and the Y-axis indicating 20.20% of the variance. The distribution of this cluster indicates a shift towards shallower water depth with higher food influx and decreasing oxygen levels, typical of a regressive phase (Tyler & Kowalewski, 2014). The clustering pattern reveals that this assemblage is adapted to eutrophic conditions, where nutrient availability is high but oxygen levels are low. The samples in this cluster are closely grouped, indicating a more uniform environmental response across the sample in shallower regions, as seen in the PCoA plot.

In Figure 6D, Cluster IV (Lenticulina subalata assemblage 2) is positioned in the PCoA plot where the X-axis explains 51.10% of the variance, and the Y-axis accounts for 19.60% of the variance. This cluster represents another transgressive phase, marked by relatively outer neritic waters. The cluster shows a tighter grouping compared to Cluster III, indicating more stable environmental conditions with higher oxygen availability compared to the shallower regressive conditions seen in Cluster III. The PCoA plot suggests that the Lenticulina subalata assemblage 2 is associated with more favourable conditions for the taxa, likely linked to deeper, more oxygenated regions (Kruskal, 1964b).

Correspondence analysis (CA) is used to determine the palaeobathymetry of the examined section and to identify the environmental patterns in species abundances (Petró et al., 2018). This method enables bathymetric interpretation of strata and system tracts, explaining water level alterations by converting the obtained data scores into curves (Herkat, 2007). CA is a useful tool for detecting faunal variations in data collection where a species is confined to an ecological habitat and its overlapping areas form a cumulative sequence (Bonham-Carter et al., 1986). The relationships between assemblages and environmental characteristics were assessed (TerBraak & Verdonschot, 1995; Singh et al., 2021; Fig. 6). Salinity, depth, the organic content of the sediment, and the distribution of the sediment grain size (clay, silt, and sand) were among the environmental elements taken into consideration. In this study, the Ammobaculites hagni assemblage (Cluster I) is linked to moderate depth, while Epistomina ghoshi (Cluster II) shows higher variability in deeper shelf regions (fluctuating conditions). The Lenticulina subalata assemblages (Clusters III and IV) reflect a shift towards shallower and outer neritic depths, respectively.

In Figure 6, PCoA and CA are correlated. Figure 6A represents the agglutinated taxa with moderate depth, suggesting normal salinity (Nagy et al., 2010; Wu et al., 2015; Haller et al., 2019). Cluster I is characterised by burrowing microfauna like Ammobaculites (Fig. 4), which live in coarse-grained, bioturbated sediments, indicating deeper depth and a high oxygen level (Barnard et al., 1981), and the BFOI has decreased values, which points to a much more substantial climate change inside the basin (Fürsich et al., 2005). The PCoA and CA ordinations both show high oxygen availability and deeper depth. Figure 6B arises from Cluster II (Epistomina ghoshi assemblage), which reflects normal marine conditions in deeper depths (Shipp & Murray, 1981). Several researchers believe Epistomina was typical of habitats on outer shelves in the lower subtidal zone (Bernier, 1984; Meyer, 2000; Samson, 2001; Fig. 6). Epistomina is also supposed to be linked to the highest flooding surfaces (Henderson & Hart, 2000; Oxford et al., 2000; Olóriz et al., 2003) and prefers fine and muddy substrates (Le Galvez, 1958). However, Gordon (1970) noted that the Epistomina water depth estimates can be highly varied when this genus becomes prominent, and various studies have interpreted both shallow and deeper water settings. Barnard et al. (1981) and Barnard & Shipp (1981) proposed a shallow-water habitat with standard marine conditions for E. ghoshi. Epistomina is considered a representative of a moderately deep open marine environment in the present assemblages, occupying both deeper parts of the middle shelf and shallower parts of the outer shelf, and it is impossible to distinguish between the middle and outer shelves solely based on epistominid dominance.

In Figures 6C and 6D, Lenticulina subalata 1 and 2 are the dominant assemblages, respectively. PCoA ordination shows the normal oxygen value (Fig. 6C), whereas the CA plot indicates the medium oxygen supply and shallower depth. Similarly, PCoA analysis shows the medium food influx as well as the medium oxic condition, and the CA also follows the same pattern as PCoA (Fig. 6D).

Foraminiferal assemblages with a predominance of calcareous species, the family Vaginulinidae, and the suborder Lagenina indicate shallow water open marine environmental conditions above the CCD (Canales & Henriques, 2008; Gaur & Talib, 2009; Bhat et al., 2016; Talib et al., 2016, 2017; Wasim et al., 2017; Reolid et al., 2019a, 2019b; Ansari et al., 2021b). Therefore, the foraminiferal assemblages in this study are interpreted to reflect the mid-to-outer shelf in the Habo region of Kachchh. The study sequence was divided into five palaeoecological units (Fig. 3) representing various bathymetry and environmental circumstances according to morphogroup study of the foraminiferal assemblages.

This oldest unit comprises 31 species in ten genera, of which nine are calcareous groups and only one genus is agglutinated with a calcareous/agglutinated ratio of 9:1. Genus Lenticulina has comparatively high diversity, indicating a high level of dissolved oxygen (Jones & Charnock, 1985; Nagy, 1992; Tyszka, 1994; Reolid et al., 2008a,b, 2010, 2012a,b; Smoleń, 2012; Talib et al., 2016; Ansari et al., 2021b).

The Fisher Index in this unit ranges from 1.896 (sample H–0.1) to 3.806 (sample H–2.8) while reaching a maximum of 5.62 in H–2.2, with an average of 4.13 for palaeoecological unit 1, indicating a mid-shelf setting for this unit (Canales & Henriques, 2008; Jain et al., 2023; Fig. 3).

This unit is dominated by calcareous morphogroup J2 (average of 55.80%) with no ornamentation, suggesting normal oxygen values (Bernhard, 1986). At the generic level, the shallow infaunal group (55.85%) is dominant, which shows low food availability in the infaunal microhabitat of the outer shelf (Peng & Li, 2023). Most of the evidence, as mentioned above, points towards deposition in the mid-shelf region with normal salinity and well-oxygenated waters of a shallow basin in a transgressive phase (Fig. 3).

These litho-units (H–3, H–4, and H–5) are devoid of foraminifera as well as of megafossils. The lack of fauna may be linked to palaeoenvironmental changes that caused a biotic crisis, which resulted in dysoxic or possibly anoxic conditions unsuited for life and deposition in a shallow regressive environment (Fig. 3, also see the ‘Facies Analysis’ section).

This unit contains 32 species organised into 14 calcareous genera. The dominance of calcareous forms suggests normal salinity (Valchev, 2003). Genus Lenticulina is dominant, with an average relative abundance of 65.6%. The dominance of Lenticulina with low species diversity implies shelf conditions with normal oxygen levels (Wasim et al., 2021).

The Fisher Index values vary from 3.04 (H–6.2) to 7.14 (H–6.10), with an average of 4.38 over this unit, indicating a mid-shelf setting somewhat deeper than the Palaeoecological unit 1 (Fig. 3).

The dominance of calcareous morphogroups J2 (32.57%) with no ornamentation indicates normal oxygen values, having stable environmental conditions at sea bottom, stabilisation, and maturity of the benthic foraminiferal habitat (Reolid et al., 2008a). At the generic level, the shallow infaunal group is dominant (59.09%), which implies better oxygen and high food flux in the epifaunal microhabitat (Reolid et al., 2008a). Based on the data presented above, we conclude that this unit was deposited in the mid-shelf area in the normal salinity and oxygenation waters of a gradually deepening basin during a transgression phase (Fig. 3).

Of the 43 species belonging to 22 genera in Palaeoecological Unit 4, three genera belong to agglutinated groups, whereas 19 belong to the calcareous group, with an agglutinated/calcareous ratio of 1:6.33, suggesting normal salinity (Wasim et al., 2021). The majority of Ammobaculites species have coarse grains, suggesting deeper shelf settings (Barnard et al., 1981; Ansari et al., 2021b).

The Fisher Index varies from 9.85 (H–7.4) to 1.99 (H–8.9) and is >5 in the majority of the samples with an average of 5.77, showing a well-oxygenated environment, the culmination of a transgressive phase, and suggesting deposition in the deeper environment of the outer shelf (Canales, 2001). The dominance of subgroup C2 indicates a deeper water shelf, and the shallow infaunal group (69.08%) is dominant at the generic level, which suggests stable environmental conditions at the sea bottom with a high degree of oxygen and high food availability in the epifaunal microhabitat of the outer shelf (Barnard et al., 1981; Reolid et al., 2008a). In view of this, the deposition of palaeoecologic unit 4 took place in the outer shelf region with normal salinity, a stable environment with low energy, and high oxygenated waters in the deepest part of a transgressive phase (Fig. 3).

Megafossils and foraminifera are absent from this unit. The absence of fauna may be related to palaeoenvironmental changes, triggering a biotic crisis and probably creating dysoxic or even anoxic conditions unsuitable for life. Deposition took place in a shallow, regressive environment (Fig. 3; also see the ‘Facies Analysis’ section).

The Pachcham and Chari formations of Habo Dome yielded 86 foraminiferal species from 64 samples, of which 27 species were reported for the first time from the Indian subcontinent. The family Vaginulinidae is dominant, comprising 52 species, followed by nine genera with 60.46% of total assemblages. The calcareous species are dominant, having hyaline forms with a calcareous to agglutinated ratio of 4:1.

A variety of palaeoecological techniques, including clustering (PCA, PCoA, and CA), biodiversity index, and morphogroup analysis show the fluctuating environmental conditions of the studied sequence. Based on these techniques, the selected section of Middle to Late Jurassic rocks of Habo Dome, Kachchh, may be divided into five palaeoecological units.

Palaeoecological unit 1 indicates that the deposition of the Pachcham Formation and the earliest part of the Chari Formation uncovered in the Habo Dome was deposited in the mid-shelf region with normal salinity and highly oxygen-rich waters in a progressively deepening basin during a transgressive phase. Palaeoecological unit 2 records environmental changes towards dysoxic and even anoxic water conditions, which suggests deposition in a shallow regressive environment causing the disappearance of fauna, including foraminifera, which resulted in a barren interval. Palaeoecological unit 3 was deposited in the middle shelf region with normal salinity and oxygenated waters in a progressively deepening basin during a transgressive phase. The basin attained maximum depth and stable environmental conditions during this time. Palaeoecological unit 4 suggests deposition in an outer shelf region with normal salinity, a stable environment with low energy, and high oxygenated waters in the deepest section of a transgressive period. Palaeoecological unit 5 indicates environmental changes with a lack of oxygen and food availability, and the deposition of sediments that took place under shallow regressive environments and resulted in barren intervals.

Overall, the Pachcham and Chari formations visible at Habo Dome, Kachchh, were deposited in an open marine shelf environment ranging from the middle to outer shelf conditions, with normal to low salinity and normal to high oxygenated waters. However, in a disturbed marine shelf, the location of deposition changed between the middle and outer shelf zones, as shown by fluctuating shorelines.

The authors are grateful to the Chairperson, Department of Geology, Aligarh Muslim University, Aligarh, India, for encouragement and for providing research facilities in the Micropalaeontology Lab of the Department. Thanks are also due to the Coordinator, University Sophisticated Instruments Facility (USIF), Aligarh Muslim University, Aligarh, India, for SEM photography and to the district authorities of Kachchh for logistic support during the fieldwork. The authors express their sincere gratitude to Prof. R. Mark Leckie, another anonymous reviewer, and the editor for their constructive comments and suggestions, which greatly improved the manuscript. The authors would also like to acknowledge Prof. Raj K. Singh, School of Earth, Ocean and Climate Sciences, IIT Bhubaneswar, India, for his valuable assistance in generating the multivariate plots. In addition, the authors are thankful to the University Grant Commission (UGC), New Delhi, India, for providing the first author with a fellowship (Ref. No. 201610160297/UP02600095/ECN-JUN20C01887).

 Appendix 1. Benthic foraminiferal species assemblage data for the samples collected in the Kachchh Basin, India, GPS locations of samples, and sample statistics. The appendix can be found linked to the online version of this article.