The rare, large diatomyid Willmus maximus occurs only at locality Y797 in the Nagri Formation of the Potwar Plateau. There is no evidence of a precursor to this species in the Chinji Formation and no successor known anywhere. Like Will Downs, this taxon is unique, a fascinating character mixed in with a fairly typical group of suspects. Because the depositional context is common for the Siwaliks, that is, a floodplain setting near major streams, and because the faunal composition is so similar to older Chinji assemblages, we do not attribute the record of Willmus to unusual conditions of preservation. What conditions in the fossil record are required to assert that a taxon is "rare?" If any taxon can be called rare, Willmus can; its fossils are few in number and encountered at a single locality exhibiting usual taphonomic conditions.
Additional circumstances remain a mystery. Willmus maximus is the youngest (by far) and most derived of the diatomyids. It is high crowned, truly bilophodont, and simplified in loss of accessory cusps. This may correlate with specialized diet or habitat. It may have been subterranean (at a time before rhizomyines radiated into this niche), but this is conjecture. Was Willmus preserved in the Siwaliks because the family had evolved to succeed in a niche not previously exploited? Possibly the group survived through the middle Miocene in Southeast Asia and reappeared in the Siwaliks in a transformed state in the late Miocene, or a temporary shift toward preferred habitat allowed its return to the Potwar. Whatever influenced its preservation, Willmus was not long-lived in the Siwaliks; the rich assemblages from upper parts of the Nagri Formation and the U-level of the Dhok Pathan Formation yield no trace of Diatomyidae.