The Siwalik deposits of Pakistan yield numerous superposed assemblages that record the small mammal fauna throughout the middle and late Miocene on the Indian subcontinent. A few stratigraphic intervals are poorly represented by microfaunas. Between older, rich Chinji Formation assemblages and fossiliferous levels high in the Nagri Formation, few fossil localities yield abundant rodents between about 11.3 and 10.4 Ma. Locality Y797, predating the first local appearance of hipparionine equids, is notable for presence of a new large diatomyid rodent, Willmus maximus gen. et sp. nov., recorded nowhere else. Willmus has affinity with Diatomys from China and Thailand and with predecessors such as Fallomus from the pre-Siwalik formations of Bugti and the Zinda Pir Dome, Pakistan. Willmus is derived in its large size, absence of accessory cusps, and extreme bilophodonty, and is by far the latest and rarest member of its group. Faunal similarity indices between Y797 and well-sampled older Chinji and younger Nagri rodent faunas are very high at the species level. Large mammal faunas are also similar. Fauna and lithology suggest nothing unusual about this locality and offer no compelling evidence for a unique microhabitat. Willmus is a derived end member in a distinct rodent group of low taxonomic diversity. This unusual family reappears in the Siwalik record at Y797 after a long absence from the fossil record. It is perhaps the derived features of Willmus that contributed to the survival of this group.
KEY WORDS: Pakistan, Siwaliks, Miocene, ctenodactyloid rodents, Diatomyidae, faunal composition
PE Article Number: 8.1.17
Copyright: Society of Vertebrate Paleontology. May 2005
Submission: 25 June 2004. Acceptance: 6 March 2005.