New Brontotheriidae (Mammalia, Perissodactyla) from the early and middle Eocene of Pakistan with implications for mammalian paleobiogeography

Brontotheriids are common in Eocene faunas of North America and Asia but are poorly known from the Indian subcontinent. Here we describe three new late early Eocene brontotheriids from Pakistan, found in the upper part of the upper Ghazij Formation and representing the oldest Asian brontotheres. Eotitanops pakistanensis n. sp. is a small, primitive species, Balochititanops haqi n. gen. n. sp. is slightly larger and more derived, and fragmentary specimens identified as cf. Balochititanops sp. appear to represent a third, larger taxon. Improved knowledge of early brontotheres from North America permits better taxonomic resolution of some middle Eocene brontothere remains from Pakistan. "Eotitanops" dayi from the Kuldana Formation is shown to be closer to Palaeosyops and is renamed Palaeosyops dayi n. comb. A new astragalus from the Baska Formation probably represents Pakotitanops latidentatus. A previously described humerus and a new calcaneum, both from the Subathu Formation, are tentatively referred to Mulkrajanops moghliensis. Phylogenetic interpretation suggests that Eotitanops pakistanensis is as primitive as the North American species of this basal brontothere genus, and also, within the limits of stratigraphic resolution, Eotitanops appeared on both continents at the same time. The origin of brontotheres is therefore equally likely to have been in Asia or in North America. The presence of the primitive brontotheres Eotitanops and Palaeosyops in Indo-Pakistan and North America indicates faunal exchange, almost certainly through Asia, although the direction of dispersal cannot be determined. The postulated high-latitude exchange coincides with a warm interval known as the Early Eocene Climatic Optimum.