Early Paleogene mammal faunas of India: a review of recent advances with implications for the timing of initial India-Asia contact

摘要

In recent years, a series of discoveries of a diverse early Eocene (-54.5 Ma) terrestrial mammal fauna from the open-cast lignite mines of western India (District Surat, Gujarat) have attracted global attention. The principal early Eocene mammal-bearing horizon in India occurs in the Cambay Shale, and is sandwiched between the two main hyperthermal events ETM2 and PETM, making this mammal fauna the oldest known (basal Eocene, at least similar to 54.5 Ma) assemblage of Cenozoic mammals in South Asia. The assemblage comprises over 10 placental mammal orders including artiodactyls, perissodactyls, primates (the APP taxa), creodonts, condylarths, insectivores, apatotherians, proteutherians, rodents, bats, lagomorphs, tillodonts, together with possible marsupials, preserved in a low-energy, coastal marsh-bay complex of mangrove swamps and lagoonal muds. Several of these mammalian orders (the APP taxa) are known to make their first appearance in the Holarctic (northern) continents around the Paleocene-Eocene boundary (56 Ma) during the intense warming interval that coincided with this boundary. A review of this mammal fauna and its implications for faunal exchanges between India and Asia and the timing of India-Asia collision is presented here along with a discussion on taxonomic/nomenclatural issues where necessary. Significantly, data from the Vastan lignite mine supports an Indian origin (i.e. the Out-of-India hypothesis) for the order Perissodactyla whose antiquity can be traced to the primitive Indian genus Cambaytherium. A similar scenario is likely for certain other groups such as the adapoid and possibly omomyid primates, whose early history is traceable to the primitive Indian taxa Marcgodinotius and the poorly known Vastanomys, respectively. The presence of early Eocene Holarctic mammals in India, especially those of medium to large size, points to significant terrestrial faunal exchanges between India and Europe/Asia around the Paleocene-Eocene boundary. Previous paleobiogeographic interpretations have emphasized the European affinities of the Indian early Eocene terrestrial fauna but have not highlighted the component withAsian ties. Taxa with Asian affinities assume particular significance when seen in the context of the timing of India-Asia collision. The discovery of a primitive tapiromorph at Vastan (Cambaylophus) with close relations to the basal Eocene (Bumbanian) Chinese tapiromorph Orientolophus, and the more recent find of a nyctitheriid insectivore (Indonyctia cambayensis) with close affinities to the late Paleocene Voltaia from Kazakhstan, suggest that terrestrial faunal connections between the Indian and Asian landmasses were in place at or slightly before the Paleocene-Eocene boundary at 56 Ma, providing independent evidence that a subareal contact of some kind was established between these landmasses in response to the initiation of India-Asia collision. The exact route of India-Asia faunal exchanges is not yet resolved, but it is likely that faunal migrations between India and As a were facilitated by the Kohistan-LadakhArc which acted as a filter bridge between the two landmasses. Apart from the early Eocene mammal fauna from Cambay Shale, significant advances have been made in recent years in our understanding of early middle Eocene (similar to 48 Ma) anthracobunids and raoellid artiodactyls from the Subathu Formation of NW Himalaya and correlative sequences in Pakistan, and the late middle Eocene (similar to 42 Ma) cetaceans and sirenians from Kutch, western India. Anthracobunids have recently been removed from Tethytheria and reinterpreted as stem perissodactyls along with cambaytheres, whereas, raoellids, particularly Indohyus, are believed to be closely tied to the origin of whales as a sister taxon to the order Cetacea which includes whales. Combined together, the Himalayan and Kutch Eocene faunas of India and the coeval faunas of Pakistan have helped to elucidate in great detail the first steps of