Harris et al., eds., 2006, The Triassic-Jurassic Terrestrial Transition. New Mexico Museum of Natural History and Science Bulletin 37. 77 VERTEBRATE FAUNAL TURNOVER DURING THE TRIASSIC-JURASSIC TRANSITION: AN INDIAN SCENARIO
SASWATI BANDYOPADHYAY AND DHURJATI PRASAD SENGUPTA
Geological Studies Unit, Indian Statistical Institute, 203 Barrackpore Trunk Road, Kolkata 700108, India, E-mail: [email protected] and [email protected]
Abstract—The Gondwana basins of India yield unique Permo-Mesozoic vertebrates. Among these, the Pranhita- Godavari (P-G) basin has an almost continuous faunal succession spanning the Late Permian to Early Jurassic, and possibly into the Middle Jurassic. In the P-G basin, nine successive vertebrate-bearing horizons have been identified – these include five Triassic and three Jurassic biotic zones. The faunal assemblages of the Late Triassic (Early to Late Norian) and Early Jurassic (Hettangian to Sinemurian) zones of the P-G basin clearly exhibit evidence of faunal turnover from the Late Triassic to Early Jurassic. The elements of the Late Norian Lower Dharmaram fauna, which includes aetosaurs, phytosaurs, and small prosauropods, is replaced by large prosauropods and a sphenosuchian in the Hettangian Upper Dharmaram Formation, which is, in turn, succeeded by the overlying Sinemurian to Pliensbachian Lower Kota Formation that includes sauropods, triconodonts, and “symmetrodonts.” The Upper Dharmaram fauna does not contain aetosaurs or phytosaurs, and the Triassic-Jurassic boundary lies at the base of the upper part of the Dharmaram.
INTRODUCTION Triassic and Jurassic continental deposits of India are found mainly in the Gondwana successions that are preserved in a number of discreet basins of peninsular India. These intracratonic basins are nucleated along preexisting zones of weakness in the Precambrian basement (Chakraborty et al., 2003). Until recently, these were well known for their rich coal reserves and plant fossils rather than their faunal remains. Recovery of many new fossil vertebrates, significant both in numbers of genera and species as well as complete skeletons, during the last five decades have made them more useful in the understanding of the evolutionary history of vertebrates on the continent. The major Gondwanan basins of India (Fig. 1), the Pranhita-Godavari (P-G), Satpura, Son-Mahanadi (S-M) and Damodar basins (Fig. 2) are now known for their vertebrate assem- blages that occur mostly in fluvial red beds. Among these four, the P-G basin provides the most complete succession of vertebrate faunas rang- ing from Permian to Jurassic. The Satpura basin has a succession from Permian to Middle Triassic. In the S-M basin, a succession from Per- mian to Late Triassic has been identified so far, whereas a succession from Permian to Early Triassic is seen in the Damodar basin. A global terrestrial faunal turnover during the Late Triassic and Early Jurassic epochs has been noted by several workers (Colbert, 1949, 1958; Olsen et al., 1987, 1990). Though initially this was not generally accepted, a scrutiny of the faunal assemblage during this time showed that some major groups, namely phytosaurs, procolophonids, and prolacertiforms, were replaced by crocodylomorphs and/or sphenodontians. Later detailed, end-Triassic faunal analyses in different parts of the world indicated that various mammal-like reptiles, rhynchosaurs, and “thecodontians” also disappeared at the end of Trias- sic and were replaced by early dinosaurs, crocodylomorphs, etc. – in short, there was a faunal shift at the beginning of the Jurassic (Olsen and Galton, 1977; Benton, 1986a). This Late Triassic-Early Jurassic faunal turnover led to a downward shift in the boundary of the Early Jurassic (Olsen and Galton, 1977; Padian, 1986), which prompted Bandyopadhyay and Roy Chowdhury (1996) to examine the Triassic and Jurassic verte- brate faunal assemblage of the P-G basin. Earlier, the Kota Formation had been considered to represent the continental Jurassic in Indian Gondwana. The faunal component of the Kota Formation, however, FIGURE 1. Stratigraphic successions of the major Gondwana basins of India. showed that the upper part of the Dharmaram Formation, which had been previously considered Upper Triassic (Norian), actually contains mainly demarcated the beginning of the Jurassic in India. In the other vertebrates that mark the beginning of the Jurassic in India. Their study Gondwanan basins, a continuous succession from Triassic to Jurassic is 78 faunal succession on the whole, evolving chronologically within a sedi- mentary package that is continuous in nature. The Upper Permian Kundaram Formation is characterized by red mudstone, sandstone, sandstone-mudstone alternations, and ferruginous shale. The sand bodies are laterally persistent, trough cross-stratified exhibiting unimodal palaeocurrent direction, and were formed by the lateral migration and avulsion of channels (Ray, 1997). The Kundaram Fauna An assemblage of Permian reptiles, characterized by abundant dicynodonts, has been recovered from the mudstone unit of the Kundaram Formation. The dicynodonts are dominated by Endothiodon followed by Cistecephalus, Pristerodon, Emydops, Oudenodon and Kingoria (Ray and Bandyopadhyay, 2003). The vertebrate assemblage also contains a gorgonopsian and a captorhinid. Paleontologically, this horizon is im- portant because it is the only horizon in India that produces Permian reptiles. On the basis of similarities of the Kundaram fauna with those of the Tropidostoma Assemblage zone and Cistecephalus Assemblage zone of the Middleton Formation of South Africa, Ray (1999, 2001) dated the Kundaram fauna as late Late Permian (Tatarian). Recently, Ward et al. (2005, supplementary information fig. S4) showed that, except Endothiodon, the other Kundaram fauna ranges either up to the middle or to the end of Dicynodon Zone, of which Emydops is again restricted only in the Dicynodon Zone. Besides, only Pristerodon ranges from the Tapinocephalus Zone to the middle of the Dicynodon Zone. The abun- dance of Endothiodon in the Kundaram Formation, and the stratigraphical ranges of other Kundaram dicynodonts, further strengthen a late Late FIGURE 2. Major Gondwana basins of India. Permian (Tatarian) age for this horizon. The lithology of the overlying Kamthi Formation includes silt- not recorded (Fig. 1); hence, they will not be treated further here. stone and ferruginous sandstone that is pebbly in places. A medium- The present paper describes briefly the geological history of the grained, poorly-sorted, argillaceous quartzose sandstone (quartz wackes P-G basin, along with the faunal associations of important vertebrate containing up to 45% clay matrix) and thin sheets of sandy siltstone horizons. This is followed by a discussion of faunal distribution and characterize the lower part, while the upper part has coarse, poorly- faunal turnover across the Triassic-Jurassic boundary, substantiating the sorted argillaceous yellowish brown sandstone with siltstone clasts and pattern of extinction, origination and diversification of terrestrial verte- quartz and quartzite pebbles (Sengupta, 1970). From the basal siltstone, brates. It may be mentioned that the details of the Gondwanan verte- two as-yet undescribed specimens of Lystrosaurus sp. have been found brates of India, current through 1999, are given in Bandyopadhyay (1999). (S. Ray, personal commun.), while Brachyops laticeps has been collected Below, only the references of subsequent publications on Indian from the upper part of Kamthi Formation (Mangli ‘beds’) (Owen, 1855). Gondwanan vertebrates are mentioned. The formations successively overlying the Kamthi Formation, the Yerrapalli, Bhimaram, Maleri, and Dharmaram, are mostly mud- BRIEF GEOLOGICAL HISTORY OF THE dominated, red bed units rich in vertebrate fossils. The red mudstones PRANHITA-GODAVARI BASIN are considered to have been deposited from suspension in interchannel The Gondwana succession in the Pranhita-Godavari basin occurs floodplain areas (Sengupta, 1970) and indicate good drainage and well- as a narrow, rectilinear outcrop trending NNW-SSE and is bordered on aerated floodplain deposits under a warm, moist climate with seasonally both sides by Proterozoic and/or Archean rocks. The overall dip of the distributed rainfall (Robinson, 1970; Behrensmeyer and Hook, 1992, succession is 5º to 12º N and NW, with a general northward paleocurrent Sheldon, 2005). direction (Sengupta, 1970; Veevers and Tewari, 1995). Glacial, Red to violet mudstone with scattered, thin sheets of quartzose fluvioglacial, fluvial, and lacustrine sediments were deposited in this sandstone and relatively smaller lenticular sand bodies made up of cali- basin during the Permo-Mesozoic period (Robinson, 1970; Read and che-derived calcarenite/calcirudite are characteristic features of the Watson, 1975; Veevers and Tewari, 1995). Yerrapalli Formation (Dasgupta, 1993). Small, lenticular sand bodies Glacial sediments of the Talchir Formation were the first Phanero- enclosing the mudstones (Fig. 3) represent fillings of small ephemeral zoic deposits in this basin. Boulder-pebble conglomerate, pebbly sand- channels that wandered over an extensive floodplain. Parallel laminated, stone, and khaki-green shale of Early Permian age characterize this unit. sheet-like sandstones often displaying parting lineation were deposited The overlying Barakar Formation contains medium to coarse, white to by waning currents of sheet flows associated with episodic overbank yellow sandstone with major coal seams and carbonaceous shale. These flooding of ephemeral streams (Maulik and Chaudhuri, 1983). units do not contain any vertebrate fossils. The succession above the The Yerrapalli Fauna Barakar Formation starts with the Late Permian Kundaram Formation followed by a number of Triassic, Jurassic, and Cretaceous formations. The Yerrapalli vertebrate community comprises a dipnoan (Fig. 1). (Ceratodus), an actinopterygian, (Saurichthys), a capitosaurid It is important to note that the Permian to Jurassic succession of (Stanocephalosaurus) (Schoch and Milner, 2000), and some diapsid and the P-G basin apparently shows no inter- or intraformational synapsid reptiles. Two large dicynodonts, Wadiasaurus and Rechnisaurus unconformity. Hence, the ages of all the faunal assemblages starting from (Roy Chowdhury, 1970), and some undescribed cynodont the Kundaram Formation, where the first occurrence of Permian verte- (trirachodontid) teeth (Chatterjee et al., 1969) represent the Synapsida, brate fossils have been noted, are important to build up the picture of the while diapsids include a rhynchosaur (Mesodapedon) (Chatterjee, 1980), 79
FIGURE 3. Lithologic successions of the Triassic-Jurassic horizons of the Pranhita-Godavari basin along with its vertebrate fauna. Additional lithologs of the Yerrapalli and Maleri formations are provided on the left side to indicate variations of thickness (after Dasgupta, 1993; Bandyopadhyay et al., 2002). a prolacertid (Pamelaria) (Sen, 2003), a rauisuchian (Yarasuchus) (Sen, Namibia, the age of the Yerrapalli Formation had been assigned to the 2005), and an undescribed erythrosuchid (cf. Erythrosuchus) (Fig. 3). early Middle Triassic (Anisian) (Bandyopadhyay, 1988; Cox, 1991; Jain et al. (1964) considered the age of Yerrapalli Formation as late Lower Anderson and Anderson, 1993; Bandyopadhyay and Sengupta, 1999). Triassic or possibly early Middle Triassic. Later, with the recovery of Sen (2005) opined that the presence of Pamelaria and Yarasuchus fur- new material, their assessment and correlation with coeval formations, ther strengthen this correlation between Middle Triassic. namely the Manda Formation of Tanzania, N’tawere Formation of Zam- The overlying Bhimaram Formation is dominated by coarse, peb- bia, the Donguz Series of Russia, and the Omingonde Formation of bly, yellowish brown, feldspathic sandstone intercalated with abundant 80 red mudstone; fragmentary remains of temnospondyl amphibians and dicynodonts have been found from this unit (Kutty et al., 1987). The Maleri Formation, above the Bhimaram Formation, is an- other mud-dominated, richly fossiliferous horizon. It begins with a thick, red mudstone and passes upward into a succession of sandstone-mud- stone alternations. Thick, sheet-like sandstones alternate with relatively thicker mudstones intervals. The mudstones are dominantly red to brown- ish red and occasionally green, structureless, and poorly lithified. The sandstones are quartzose, medium- to coarse-grained, and are cross bed- ded with overlapping channel fills forming multistoried packages with individual channel fills having bottom facies rich in clay galls and trans- ported bone fragments. Lenticular bodies of caliche-derived peloidal calcirudite/calcarenite are common (Sarkar, 1988). These appear either at the bottom of channel sand bodies or as solitary bodies enclosed in the mudstones. The Maleri Fauna The Maleri Formation has been biochronologically divided into a lower part with a Carnian fauna, and an upper part with an Early Norian fauna (Fig. 4). The characteristic Lower Maleri fauna includes a metoposaurid (Buettneria) (Sengupta, 2002), a rhynchosaur (Hyperodapedon), a phytosaur (Parasuchus), a prolacertid (Malerisaurus), a basal saurischian (Alwalkeria) (Langer, 2004), and a cynodont (Exaeretodon) (Figs. 3, 4). In addition, there are a dipnoan (Ceratodus), an undescribed xenacanthid, an unnamed aetosaur resem- bling Typothorax (Kutty et al., 1987), and a prosauropod (cf. FIGURE 4. The main biotic horizons of the of the Pranhita-Godavari basin. Massospondylus) (Kutty et al., 1987). The Upper Maleri fauna consists of two chigutisaurs (Compsocerops and Kuttycephalus), and two The Kota Fauna phytosaurs (Rutiodon and Leptosuchus) (Hungerbühler et al., 2002). A The Lower Kota has produced several reptiles and mammals (Fig. dicynodont and an aetosaur are also found in this horizon. 3, 4). Among the reptiles, there are two sauropod dinosaurs (Barapasaurus A thick, basal sandstone followed by an alternating series of sand- and Kotasaurus) (Yadagiri, 2001), two sphenodontians (Rebbanasaurus stone and mudstone occurs in the overlying Dharmaram Formation. The and Godavarisaurus) (Evans et al., 2001), and two other lepidosaurs sand bodies are comparatively thicker and the sand-mud ratio is higher (Bharatagama and Paikasisaurus) (Yadagiri, 1986; Evans et al., 2002). than the Maleri Formation (Fig. 3). On the basis of isolated teeth, nine mammals have been identified. Among The Dharmaram Fauna the non-therians, there are two triconodonts (Paikasigudodon and Dyskritodon) (Prasad and Manhas, 2002), two docodonts (Gondtherium Two successive vertebrate faunal zones have also been identified and Godavariodon) (Prasad and Manhas, 2001; Prasad, 2003), a in the Dharmaram Formation (Fig. 4). The Lower Dharmaram fauna kuehneotheriid (Kotatherium); the other two non-therians are contains a dipnoan (Ceratodus), a xenacanthid (Xenacanthus), a large Indozostrodon and Indotherium (Datta and Das, 2001). In addition, phytosaur and three different types of aetosaurs. One of the aetosaurs there are a therian (Trishulotherium) and a holotherian incertae sedis has been mentioned as a “Paratypothorax-like form” by Kutty and (Nakunodon) (Averianov, 2002). Sengupta (1989), a characteristic member of the lower Dharmaram fauna. The Upper Kota contains three semionontids (Lepidotes, On the basis of the faunal components, Kutty and Sengupta (1989) have Paradapedium and Tetragonolepis), a pholidophorid (Pholidophorus), suggested a Late Norian age for this zone. The Upper Dharmaram fauna and a coelacanth (Indocoelacanthus). The reptiles of the Upper Kota includes a large plateosaurid and a sphenosuchian. The age of the Upper include a pterosaur (Campylognathoides), a mesosuchian Dharmaram was previously considered Late Triassic (Kutty and crocodylomorph, and a cryptodiran turtle (Indochelys) (Datta et al., Sengupta, 1989). On the basis of faunal analyses, Bandyopadhyay and 2000). An Early Jurassic age has been considered for the Kota Formation Roy Chowdhury (1996) suggested that the beginning of the terrestrial since the nineteenth century. Close scrutiny of the taxa of both the lower Jurassic started in the Upper Dharmaram. Though the details of the and the upper units of the Kota Formation led Bandyopadhyay and Roy fauna are not yet available, stratigraphic disposition, absence of typical Chowdhury (1996) to suggest that the Kota Formation has an age rang- Triassic fauna and preliminary comparison of the upper Dharmaram ing from Hettangian to Toarcian, and it may even extend into the Middle fauna with other Early Jurassic faunas indicates an early Early Jurassic Jurassic. It may be mentioned that among the Kota fishes, Lepidotes age (Hettangian) for this horizon. This will be discussed in detail later. deccanensis is closest to L. elvensis, which is found from the Toarcian of The overlying Kota Formation is also divisible into two parts, western Europe (Jain, 1983). The other Kota fishes, Paradapedium and each with its own distinct biozone (Fig. 4). The lower part of Kota Tetragonolepis, also show strong similarities with the Toarcian forms of includes a thick, hard, compact and coarse sandstone that is pebbly in western Europe (Schaeffer and Patterson, 1984). Presence of such forms places and grades both laterally and vertically into finer siltstone and in the lacustrine Kota limestone was believed to be due to influence of mudstone. On top of siltstone-mudstone beds, the Kota Formation con- extensive Toarcian transgression that according to Patterson and Owen tains marl and limestone, and then mudstone and ferruginous shale (1991), might have been instrumental for the invasion of the European interbedded with sandstone. Rudra and Maulik (1994) opined that a Liassic fishes in the Indian sub-continent as well as other circum-Tethyan meandering river system deposited the lower part of this horizon, while continents. Bandyopadhyay and Roy Chowdhury (1996) thus suggested a braided river system formed the upper part; the limestone facies was a Toarcian age for these Kota fishes. Hence, the age of the Lower Kota interpreted as a lacustrine deposit. with its sauropods and therian and non-therians, overlying the Hettangian 81 Upper Dharmaram, ranges from Sinemurian to Pliensbachian, while the A scheme of land vertebrate faunachrons (LVF) of the Triassic, overlying Upper Kota, with its Toarcian fishes, crocodylomorph, and with the first appearance datum (FAD) of Lystrosaurus followed by the turtles, has a Toarcian to probably ?Aalenian age. FADs of Cynognathus, Shansiodon, Mastodonsaurus, Paleorhinus, The Kota Formation is unconformably overlain by the Gangapur Rutiodon, Pseudopalatus, Redondasaurus, and Protosuchus, respectively, Formation (Kutty, 1969) characterized by coarse ferruginous sandstones has been proposed by Lucas (1998). The LVFs were, however, based on with pebble bands succeeded by a succession of alternations of sand- the FAD with its beginning and an end, whereas a considerable gap stone, mudstones, and siltstones. The mudstone often contains ferrugi- occurs “between the last occurrence of the type assemblage of a LVF and nous concretions. The mudstone of Gangapur yields a floral assemblage the FAD of the fossil taxon that marks its end” (Langer, 2005, p. 220). of Early Cretaceous age, but no vertebrate fossils have yet been identi- Lucas (1998) did not define the LVF on the basis of any type assemblage, fied. but rather named the LVF on the basis of the geographical location from where the type fauna was recovered. Rayfield et al. (2005) commented DISCUSSION that index fossils like Metoposaurus, Paleorhinus, and Rutiodon and few From the above, it can be seen that the P-G basin has a vertebrate of the aetosaur taxa, which are important in the context of P-G basin, record with nine biozones – one Permian, five Triassic, and three Jurassic show taxonomic instability and are not easily correlatable. Lehman and (Fig. 4). It starts with the (1) upper Upper Permian (Tatarian) Kundaram Chatterjee (2005) stated that the Otischalkian and Apachean faunachrons Formation, followed by (2) the Lower Triassic Kamthi. The following of Lucas’ LVF scheme do not support their vertebrate findings from the Middle Triassic (Anisian) Yerrapalli Formation (3) occurs below the Dockum Group. Bhimaram Formation, which can also be tentatively identified as a sepa- In an Indian context, the undescribed phytosaurs, aetosaurs, rate biozone of Anisian to Ladinian age. The Maleri Formation has been sphenodontian, and prosauropods of the P-G basin create some uncer- biochronologically divided into a Carnian Lower Maleri (4) and an early tainty for definite correlation. However, Anderson and Anderson (1993) Norian Upper Maleri (5). The overlying Dharmaram is again had noted that the Yerrapalli and Maleri faunas are important Triassic biochronologically subdivided into late Norian Lower Dharmaram (6) faunas in global aspect. Recently, Langer (2005) equated the Lower and an early Early Jurassic (Hettangian) Upper Dharmaram (7). The Maleri with the lower fauna-bearing zone of the Ischigualasto Forma- Jurassic Kota Formation has an Early Jurassic (Sinemurian to tion, lower part of the Cacheuta and the upper part of lower Makay, Pliensbachian) age Lower Kota (8) and a late Early Jurassic to early Pebbly Arkose, and Molteno formations. The Upper Maleri faunal zone Middle Jurassic (Toarcian to ?Aalenian) age Upper Kota (9). has been correlated with the Lower Elliot, lower part of the upper Makay, It is evident from the above faunal analyses and stratigraphy of and lower parts of the Rio Blanco and Los Colorados. Langer (2005) also the P-G basin that the fauna of the Kota Formation is the youngest fauna indicated a gap between the lower and upper fauna of the Maleri and he present in this basin. The Upper Kota fauna includes semionontids marked the base of the Ischigualasto as 227.8 Ma from the radiometric (Lepidotes, Paradapedium and Tetragonolepis), a pholidophorid, a co- dating of bentonites (Rogers et al., 1993). All these push up the age of elacanth, a pterosaur, a mesosuchian crocodylomorph, and a cryptodiran the Upper Maleri fauna (post Ischigualasto sensu Langer, 2005) beyond turtle. The Lower Kota has sauropods, sphenodontians, a lepidosaur, a what was envisaged by Lucas (1998), who kept the Upper Maleri and lizard, therians and non-therian mammals. The faunal analyses of both the Ischigualasto Formation among some other “principal correlatives” the Upper and Lower Kota have established an Early Jurassic to early of the Adamanian LVF of Upper Carnian age. On the other hand, Lehman Middle Jurassic age for the entire formation. Hence, the Triassic Jurassic and Chatterjee (2005) noted that Redondasaurus and Typothorax (= boundary in the P-G basin has to be placed below the Kota Formation. Redondasuchus), the two major taxa of the Apachean LVF of Lucas Immediately underlying the Kota Formation is the Dharmaram Forma- (1998), are also present in the Cooper Canyon Formation, whose detrital tion that has two faunal zones. The lower faunal zone contains phytosaurs biotites have been dated as 210 Ma (Long and Lehman, 1993). Hence, and aetosaurs and hence, its age is definitely Triassic. The Upper the large phytosaur and “Paratypothorax”-like aetosaur of the Lower Dharmaram contains a plateosaurid and a sphenosuchian, which are not Dharmaram fauna are more likely to indicate a Late Norian age. definite markers of Jurassic. However, the phytosaurs and aetosaurs of Thus, the faunachronology of the P-G basin also suggests a depar- the Lower Dharmaram are completely absent there. Other Triassic ture from the LVF scheme of Lucas (1998). The ages of the Maleri and sphenosuchians of the world are always associated with Triassic faunal Dharmaram faunas, according to the present analysis, should be younger elements. Dromicosuchus from the Newark Supergroup is intimately than was envisaged by Lucas (1998), and the Triassic-Jurassic boundary associated with a rauisuchian archosaur close to Postosuchus (Sues et al., (TJB) should be at the base of the Upper Dharmaram fauna 2003). Hesperosuchus from the Chinle (Clark et al., 2001) and (Bandyopadhyay and Roy Chowdhury, 1996). The Upper Dharmaram Saltoposuchus from the Middle Stubensandstein (Schoch and Wild, 1999) fauna, which occurs between the phytosaur and aetosaur-bearing Lower are all associated with Triassic faunal elements. The Los Colorados For- Dharmaram and mammal- and sauropod-bearing Lower Kota faunas, is mation that is now considered as uppermost Triassic and has a “mixture” the “transitional fauna” between the Triassic and Jurassic in India. of Triassic and Jurassic faunas, also has aetosaurs with advanced theropod As stated above, lithostratigraphically, the Triassic-Jurassic for- dinosaurs (Arcucci et al., 2004). Exploration in the Upper Dharmaram mations of the P-G basin form a continuous, conformable succession zone during the last three decades has not yielded a single piece of any (Figs. 3, 4). Though the P-G basin does not have a complete faunal phytosaur and aetosaur skeletal remains, which are quite abundant in the assemblage of Permian or Early Triassic age, a continuity in the faunal Lower Dharmaram zone. There is incompleteness in the faunal scenario succession from Triassic to early Middle Jurassic can be envisaged. The of the P-G, basin as the phytosaurs, aetosaurs, plateosaurids and presence of Lystrosaurus, Brachyops, and the estheriid ostracode Cyzicus sphenosuchian of the Upper and the Lower Dharmaram are yet to be (Euestheria) mangliensis Jones 1862 (Tasch et al., 1973; Ghosh et al., described. However, the Upper Dharmaram fauna, sandwiched between 1987) indicate an Early Triassic age for the Kamthi Formation. the Late Triassic Lower Dharmaram and the Jurassic Kota fauna, has no Dicynodonts and labyrinthodonts continue through the Yerrapalli and faunal element that goes against the Jurassic age. Besides, no faunal Bhimaram formations and into the Upper Triassic Maleri Formation. element of the Lower Dharmaram or the Lower Kota has been found in The Yerrapalli faunal association also has a rauisuchian, a prolacertid, this zone. Moreover, the Dharmaram Formation is conformable with the and an erythrosuchid. The Upper Maleri and the Lower Dharmaram Kota Formation. Hence, the base of the Upper Dharmaram faunal zone faunas have phytosaurs and aetosaurs, but these abruptly disappear at is a place likely to indicate the Triassic-Jurassic boundary in the P-G the mudstone horizon at the base of the uppermost sandstone unit of the basin, and the scanty Upper Dharmaram fauna is all that is present of an Dharmaram Formation. From this point upward, a new faunal assem- early Early Jurassic terrestrial fauna in India. blage, containing a large plateosaurid and a sphenosuchian, occurs in the 82
FIGURE 5. The Triassic-Jurassic boundary plotted on the geological map of the northern part of the Pranhita-Godavari basin around the villages of Maleri and Dharmaram. The white and black bands in the Maleri indicate sandstone and calcirudites, respectively, within the mudstone. The dark grey bands in the Dharmaram indicate sandstones within mudstone. The amount of dip varies from 10º to 18º. Upper Dharmaram. Lucas (1998) defined the Apachean, a Rhaetian land Thecodontosauridae. However, Lucas (1994) argued that, out of this vertebrate faunachron characterized by the metoposaurid Apachesaurus list, only the Phytosauridae and Procolophonidae have well-established gregorii, a sphenodontian, a procolophonid, the phytosaur Late Triassic records, whereas the other families became extinct prior to Redondasaurus bermani, the aetosaur Redondasuchus reseri, theropods, the Norian. But, in the P-G basin, the phytosaurs and aetosaurs are and a possible cynodont. None of these elements are noted in the Upper present up to the Late Norian in the Lower Dharmaram but are absent in Dharmaram fauna, whereas the metoposaurid-phytosaur-aetosaur com- the Hettangian Upper Dharmaram or Sinemurian to Pliensbachian Lower bination is typical of the horizons occurring below the Upper Dharmaram. Kota Formation. Benton (1986a, b, 1994) further suggested two differ- Metoposaurids, in fact, disappeared after the Lower Maleri, which is ent episodes of tetrapod extinction during the Triassic – one at the end of quite early in comparison to the North American genera (Sengupta, 2003). the Carnian and the other at the end-Triassic. In the P-G basin, So, an abrupt faunal change actually occurs between biozone 6 (i.e., the metoposaurids, rhynchosaurs, prolacertids, and non-mammalian Lower Dharmaram) and biozone 7 (i.e., the Upper Dharmaram); farther cynodonts became extinct after the end-Carnian Lower Maleri. The Early upward, a new faunal association, dominated by sauropods, Norian Upper Maleri is marked by the appearance of chigutisaurids, sphenodontians, lepidosaurs, crocodylomorphs, turtles and mammals, advanced phytosaurs (Rutiodon, Leptosuchus) and aetosaurs. Thus, the appears (Fig. 4). So, the Triassic-Jurassic transition in the Indian sce- P-G basin faunal succession shows two faunal turnovers during the nario appears to commence at the base of the upper part of the Dharmaram Triassic. Stratigraphic positions of these two major faunal turnovers in Formation, close to the uppermost sandstone band of the Dharmaram the P-G basin are indicated in the lithologic map and succession (Figs. 3, Formation (Fig. 5). 5). Benton (1993) listed several terrestrial reptile families that be- The biotic turnover in the Triassic-Jurassic faunal community, came extinct at the end-Triassic; these are the Proganochelyidae, both in the marine and terrestrial realms, is considered to be due to an Kuehneosauridae, Pachystropheidae, Trilophosauridae, Phytosauridae, end-Triassic extinction (Colbert 1949, 1958; Padian, 1986; Olsen et al., Stagonolepididae, Rauisuchidae, Ornithosuchidae, Saltoposuchidae and 1987, 1990). A major end-Triassic extinction was first proposed by 83 Newell (1963) on the basis of the extinction of ammonoid families and “long-term ecological degradation” (Tanner et al., 2004; p. 115). replacement of many groups of amphibians and reptiles by dinosaurs. Several workers have argued, and are still arguing, about the “hazy” However, Lucas and Tanner (2004) considered the “Triassic-Jurassic” record of terrestrial tetrapod extinction during the end-Triassic (Lucas, extinction event as a series of smaller, step-wise extinctions during which 1994; Fraser and Sues, 1994; Tanner et al., 2004). However, it can be about 76% of species became extinct (Raup, 1992). A gradualistic faunal said that a significant faunal turnover did occur in India between the Late replacement due to sea-level change, oceanic anoxia, climatic change, and Norian Lower Dharmaram and the Hettangian Upper Dharmaram. It is widespread aridity during the Late Triassic was proposed by several hoped that more details of Upper Dharmaram fauna will shed new light authors (Newell, 1963; Tucker and Benton, 1982; Hallam, 1990). A on the exact timing and nature of this change. contrasting suggestion, however, involved a sudden change induced by a ACKNOWLEDGMENTS catastrophic bolide impact at the end of the Triassic that caused an increase in atmospheric opacity, outgassing of CO2 and SO2 due to severe The authors take the opportunity to thank Prof. Tapan K. volcanism, and sudden release of methane hydrates from the seafloor, Roy Chowdhury, formerly of Indian Statistical Institute, for fruitful which affected the total milieu of the Earth (Raup and Sepkoski, 1984; discussion on this topic. The authors are extremely thankful to Dr. J.D. Raup, 1986; Olsen et al., 1987, 2002a, b; Retallack, 2001). Tanner et al. Harris for his comments and careful scrutiny that helped to improve the (2004), however, concluded that the Late Triassic-Early Jurassic biotic quality of the manuscript. Comments from other reviewers, Dr. S.G. turnover involved multiple forcing mechanisms, e.g., sea-level or climate Lucas and Dr. M.C. Langer, were also very helpful. The infrastructrual change along with a bolide impact or volcanism, that resulted in the facilities wre provided by ISI.
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