BASAL SAUROPODOMORPHS (DINOSAURIA: SAURISCHIA) FROM THE LOWER JURASSIC OF INDIA: THEIR ANATOMY AND RELATIONSHIPS

Journal of Paleontology, Nov 2007 by Kutty, T S, Chatterjee, Sankar, Galton, Peter M, Upchurch, Paul

ABSTRACT-

The Upper Dharmaram Formation (Lower Jurassic, Sinemurian) of India has yielded three sauropodomorph dinosaurs, two new taxa and an indeterminate one. Lamplughsaura dharmaramensis n. gen. and sp., represented by several partial skeletons, is a heavily built quadrupedal form (body length ~10 m). Autapomorphies include teeth with strongly emarginated distal edge; caudal cervical neural spines bearing a vertically oriented ligamentous furrow on cranial and caudal surfaces and a transversely expanded spine table; caudal neural spines bearing a craniodorsally directed spur (proximal caudal vertebrae) or a large process (midcaudal vertebrae); caudal neural spines shorter than transverse processes so former lost first in passing along tail; and a plesiomorphy that is the nontrenchant form of manual ungual I. The Indian dinosaurs were coded for two recent datamatrices for basal sauropodomorphs. The results of this preliminary analysis indicate that Lamplughsaura is either a basal Sauropoda or, less likely, based on Templeton's test, a stem sauropodomorph. The second large form, represented by the proximal half of a femur, is a sauropodomorph that is more derived than Saturnalia (Brazil) and Thecodontosaurus (Great Britain) from the Upper Triassic. This is also true for the smaller (body length ~4 m as adult) Pradhania gracilis n. gen. and sp. which lies outside of the Sauropoda Plateosauria clade, so it is definitely a stem sauropodomorph. Pradhania is known from fragmentary material; an autapomorphy is the very prominent medial longitudinal ridge on the maxilla.

INTRODUCTION

BASAL SAUROPODOMORPHS are small to large (maximum lengths approximately 2.5-15 m), bipedal, facultatively bipedal or quadrupedal saurischian dinosaurs. They were herbivores with small skulls, elongate necks and tails, and strong limbs that pioneered high browsing on the foliage of trees that other contemporary herbivores could not reach (Bakker, 1978; Parrish, 1998). They are usually the most common terrestrial vertebrates in the Upper Triassic and Lower Jurassic beds in which they occur. They have been found on all the major continents, including Antarctica (Hammer and Hickerson, 1994; Rich et al., 1997), but the Australian record of basal sauropodomorphs was based on Agrosaurus Seeley, 1891 that proves to be mislabeled material of Thecodontosaurus Riley and Stutchbury, 1836 from the Upper Triassic of England (Vickers-Rich et al., 1999; Gallon, 2000). Basal sauropodomorphs first appeared in the early Carnian (~230 Ma), became distributed globally in the Norian, endured the terminal Triassic extinction, rebounded in the Sinemurian and Pleisbachian, but finally disappeared at the end of the Toarcian (~178 Ma). For nearly 50 million years, they were the dominant group of herbivorous dinosaurs in the terrestrial ecosystem of the Pangean world. For details on the basal sauropodomorphs traditionally referred to the Prosauropoda, see Gallon and Upchurch (2004), and for basal sauropods see Upchurch et al. (2004) and discussion by Wilson (2005).

Records of basal sauropodomorphs from the Upper Triassic-Lower Jurassic beds of India are sparse. Huene (1940) reported two fragmentary vertebrae of a sauropodomorph from the Upper Triassic Maleri Formation of the Pranhita-Godavari Valley, Andhra Pradesh, but this identification is incorrect (Colbert, 1958; Roy Chowdhury, 1965). Kutty (1969) briefly reported the discovery of a basal sauropodomorph fauna from his newly defined Dharmaram Formation, which lies immediately above the Maleri Formation. He identified two taxa, a large plateosaurid and a small thecodontosaurid, but the material was never described. Later, Kutty and Sengupta (1989) discussed the Triassic faunal succession of the Pranhita-Godavari Valley and considered the sauropodomorph-bearing horizon, the upper Dharmaram Formation, to be coeval with the uppermost Rhaetian stage of the Triassic sequence of Germany. They also listed another small basal sauropodomorph from die upper Maleri Formation (Massospondylus sp. Kutty et al., 1987), which now appears to be a basal saurischian dinosaur similar to Guaibasaurus Bonaparte et al., 1999 from the Upper Triassic of Brazil (see Table 1). The material from the lower Dharmaram Formation listed by Kutty and Sengupta (1989) is fragmentary and nondiagnostic. In this paper, we describe the sauropodomorph material from the upper Dharmaram Formation (Lower Jurassic, Sinemurian) of India as two new taxa and a third indeterminate one, based on the plateosaurid and thecodontosaurid of Kutty (1969).

Institutional abbreviations.-ISI, Geological Studies Unit of the Indian Statistical Institute, Calcutta, India; IVPP, Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, Peoples Republic of China.

GEOLOGIC AND PLATE TECTONIC SETTING

The continental Gondwana formations, which are well exposed in the Pranhita-Godavari Valley of southern India (Fig. 1) and range in age from Permian to Cretaceous, have yielded several vertebrate fauna! successions (Table 1). Among these successions, the Upper Triassic Maleri Formation is probably the best known with a variety of vertebrate remains, including metoposaurs, rhynchosaurs, and phytosaurs, which are comparable to the fauna from the Lowenstein Formation (as Lower and Middle Stubensandstein) of Germany (Roy Chowdhury, 1965; Chatterjee, 1974, 1978). Above the Maleri Formation, Kutty (1969) mapped a new formation, the Dharmaram Formation, an alternating series of sandstones and mudstones, originally thought to be coeval with the Trossingen Formation (as Upper Stubensandstein-Knollenmergel) of the Germanic Upper Triassic. Later, Kutty and Sengupta (1989) recognized four temporally successive vertebrate faunal zones in the Upper Triassic sequences of India: the Lower Maleri, Upper Maleri, Lower Dharmaram, and Upper Dharmaram. Each zone has a distinctive faunal assemblage (Table 1). The sauropodomorph material described here is from the Upper Dharmaram Zone. Recent analysis of the Upper Dharmaram fauna suggests the presence of a sphenosuchian similar to Dibothrosuchus Wu and Chatterjee, 1993 (Lower Jurassic Lufeng Group, China), and a large theropod, mainly represented by isolated teeth and limb fragments, that is similar to Dilophosaurus Welles, 1984 (Lower Jurassic Kayenta Formation, Arizona). Surprisingly, the typical Upper Triassic fauna of the Lower Dharmaram, such as phytosaurs and aetosaurs, are entirely absent in the Upper Dharmaram Formation, indicating that its age is younger than Norian. However, both basal sauropodomorphs and sphenosuchians are known to extend into the Early Jurassic period in several parts of the world. The boundary between the Lower and Upper Dharmaram Formations may coincide with the Triassic-Jurassic extinction event that occurred 208 million years ago. The beginning of the Jurassic age in the Upper Dharmaram horizon is marked by the proliferation of basal sauropodomorphs and the disappearance of the typical Triassic (Norian) fauna. The associated fauna of the Upper Dharmaram Formation is related to faunas from the Upper Elliot and Clarens Formations in southern Africa, the Kayenta Formation of Arizona, and the Lower Lufeng Group of China. Olsen and Gallon (1977) correlated these groups on the basis of vertebrate remains and favored an early Jurassic assignment. The correctness of this assignment has been confirmed by several subsequent studies (e.g., Olson and Gallon, 1984; Olsen and Sues, 1986; Shubin and Sues, 1991; Sues et al., 1994). Accordingly, we agree with Bandopadhyay and Roy Chowdhury (1996) who regard the Upper Dharmaram Formation as early Jurassic (Sinemurian) in age.