CONTINENTAL MICROFAUNA FROM KOTA FORMATION (JURASSIC), PRANHITA-GODAVARI VALLEY (A.P.), INDIA

Abstract

Continental Jurassic sediments are of global significance from the standpoint of vertebrate paleontology, particularly because of their importance in understanding the evolutionary history of early mammals. However, our knowledge of continental vertebrates ofJurassic age in the Gondwanaland continents, still remains inadequate. This is particularly true in the case of Jurassic mammals which at present are mainly documented from the Laurasian continents. The Kota Formation, which is an integral part ofthe upper Gondwana sequence in the Pranhita-Godavari valley in the south Indian State of Andhra Pradesh, provides an excellent opportunity to document a Jurassic terrestrial ecosystem from India. The Kota Formation, which is largely a fluvio-lacustrine sequence has long been known to yield fish and reptilian faunas. However, microvertebrates from the Kota Formation continue to be poorly known and the present investigation was undertaken to fill this major gap, with special emphasis on micromammals. During the three field seasons between 1998-2000, the microbiota was recovered mainly by screenwashing techniques. The five investigated sections which form part of the Godavari subbasin of the Pranhita-Godavari valley, are exposed near Paikasigudem, Kadamba, Metpalli, Manganpalli (District Adilabad) and Kota (District Chandrapur). Microvertebrates recovered during the present investigation comprise over 3500 identifiable elements of fish, rhynchocephalians, lizards, crocodiles, sauropods, ornithischians, theropods and mammals. These elements include isolated teeth, fragmentary dentaries, maxillae, premaxillae, scales, dermal denticles and phalanges. Associated fauna includes ostracods and trace fossils. The vertebrate assemblage comprises 25 genera and 25 species. Fish constitute the most dominant group with 8 genera and 8 species representing holosteans and elasmobranchs. The semonotids are the most common fish. Among the reptiles, theropods and ornithischians are recorded for the first time from this formation. Theropods are known mainly by isolated teeth grouped into 4 morphotypes and ornithischians into two. These finds clearly indicate that the dinosaur fauna was much more diverse than hitherto believed. Another important group in the collection are the rhynchocephalians (sphenodontids) which are known by 2 taxa. Crocodiles and lizards are each represented by a single taxon. Significantly, the acrodont iguanid lizards reported in this work form the first record from the Kota Formation and also the oldest record of this group anywhere in the world. Mammals are the most important component of the presently described Kota vertebrate assemblage. They are represented by 2 orders, Docodonta and Triconodonta. The present docodont material forms the first Gondwanan occurrence of this group. Their discovery from the Indian subcontinent suggests aPangean distribution for this group in the Jurassic. Triconodonts form the most diversified group among the Kota mammals, represented by Dyskritodon indicus sp. nov., Paikasigudodon yadagirii nov. comb., Indotherium pranhitai and Triconodonta indet. Invertebrate fauna is represented by a single ostracod genus with two species. Ichno fossils include Planolites and Monocraterion. Significantly, the new data suggest, based on mammals, that an Upper Jurassic- Lower Cretaceous age is more likely for the Kota Formation than the traditionally held Lower Jurassic (=Liassic) age. The mammalian data is also indicative of a land-locked position for India in the Jurassic, in agreement with geophysical models. Further, the Kota community, overall, represents an admixture ofaquatic, semi-aquatic, terrestrial and aerial elements that thrived in and around a shallow freshwater lake.