Y797, situated in the lower part of the Nagri Formation, provides an opportunity to evaluate to what extent the facies change between the Chinji and Nagri Formations is associated with faunal change, with particular emphasis on small mammals. There are at least 38 mammal taxa present in the recovered sample of 528 specimens from Y797, comprising 21 rodents, three other small mammals, and 14 larger mammals (Table 1). In addition, ostracods, fish, amphibians, turtles, lizards, snakes, crocodiles, and a bird are documented; Acrochordus snake vertebrae are abundant.
Dense sampling at levels dating from 11.3 to 11.5 Ma and 10 to10.4 Ma, with extensive prospecting in between, have allowed for detailed faunal comparisons across these horizons (e.g., Barry et al. 1995). Among large mammals, there is substantial change in faunal composition, including the appearance of hipparionine equids at 10.7 Ma (Pilbeam et al. 1996). A significant cluster of taxa that persists for nearly four million years has last appearances at 10.3 Ma. These include the giraffid, Giraffokeryx punjabiensis, and two suoids, Listriodon pentapotamiae and Conohyus sindiensis. Notable first appearances between from 10.2 and 10.3 Ma include the larger giraffid, Bramatherium megacephalum, the larger suoid, Hippopotamodon sivalense, and the first large bovid, Selenoportax vexillarius (Barry et al. 2002). In addition, three tragulid species have last appearances between 10.4 and 10.7 Ma, and three other tragulid species have first appearances between 10.3 and 10.4 Ma. The bovids also show marked species change during this 0.9 m.y. interval, with last appearances of several dominant Chinji taxa (Barry, personal commun. June 2004). Most of the large mammal faunal change between 10 and 11.5 Ma appears to occur in the younger half of this interval. The Y797 large mammal fauna, though currently represented by only 14 taxa, supports this interpretation with faunal composition similar to upper Chinji localities. Giraffokeryx punjabiensis, Listriodon pentapotamiae, and Conohyus sindiensis are all recorded at Y797. However, Y797 does record the first appearance of an unnamed tragulid species.
Rodents support the same conclusion of overall close similarity to Chinji faunas and difference from younger Nagri assemblages. The small mammal record, previously nearly unknown between Chinji locality Y76 at 11.3 Ma and Y259 at 10.4 Ma, is enhanced for various rodent groups, e.g., cricetids, murids, ctenodactylids (Lindsay 1988; Jacobs and Downs 1994; Baskin 1996), by data from Y797 and other smaller assemblages (Cheema et al. 2000). Ongoing taxonomic revision of Siwalik microfauna, e.g., Lopez-Antoñanzas and Sen (2003) tends to alter taxon names, but numbers of taxa and lineages are stable, lending support to our conclusions.
The rodent fauna of Y259 differs moderately in species composition from that of Y76, with five first appearances at Y259 and seven last appearances at Y76. Two Democricetodon species are not present at Y76 but extend into rocks younger than Y259. Y797 affords a look at the small mammal record during the lower portion of this interval. Percent similarity comparisons of the rodent taxa at Y259, Y797, Y76, and Y504 range from 63% to 84% (Table 2). This percent similarity is calculated by taking the number of common species, divided by the lesser of the two species richnesses. Y797 is taxonomically close to all; Y259 differs more from the Chinji localities than it does from Y797. Y797 extends downward from Y259 the range of one species, the murid, Karnimata sp. A; it does not record the other four newly appearing taxa present at Y259, Ratufa sylva, Eutamias new sp., Rhizomyides punjabiensis, and an unnamed murid species. Y797 extends the range upward of four of the seven taxa previously last found at Y76, Eutamias urialis, cf. Hylopetes big sp., Dakkamyoides perplexus, and Myocricetodon sp. Two other taxa, Kanisamys indicus, and Megacricetodon daamsi have last appearances at Y76, although M. daamsi is rare enough that this could be a sampling issue. The seventh taxon is a unique undescribed murid currently only known from Y76. Thus, both Y76 and Y259 record unique murid species currently unidentified elsewhere, and Y797 preserves two rodent species not found elsewhere, Willmus maximus and an additional species of Sayimys (sp. B of Baskin 1996). In addition, Y797 extends the known ranges of five taxa in the fossil record, including one first and four last appearances of taxa. The relative abundance of rodent taxa at Y797 also indicates general comparability with most well sampled lower and middle Siwalik localities. Murines and tachyoryctines are dominant. Together these two subfamilies comprise more than 70% of all recovered specimens at Y797 and are distributed among three species. Cricetids, which dominate the lower and middle Chinji Formation, are progressively less common, although interestingly they remain diverse throughout the Chinji and into the lower part of the Nagri Formation, with six taxa represented at Y797. The sciurids and ctenodactylids are stable members of Siwalik faunas at relatively low abundances. Thus, in patterns of abundance as well as species composition, Y797 is similar to upper Chinji Formation microfaunas, except for the presence of the diatomyid, Willmus maximus.
Willmus is represented by two teeth from among the 321 rodent specimens recovered. Willmus is large, with a body mass exceeding 250 g by reference to larger tachyoryctine rhizomyids, and there should be minimal collecting bias against its recovery. Possibly there was taphonomic bias against its preservation if it was not on the menu of preferred prey by predatory concentrators. Willmus is among the largest of the Siwalik rodents, exceeded in size by large Brachyrhizomys and Hystrix of the upper Siwaliks. It is truly a unique, rare taxon amongst a fairly typical "Chinji" fossil assemblage in the lower Nagri Formation that precedes the larger scale faunal and climatic changes of the upper middle Siwaliks.