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