PHYLOGENY OF SIWALIK MURINE RODENTS: IMPLICATIONS FOR MUS-RATTUS DIVERGENCE TIME

摘要

The freshwater Siwalik deposits of the Indian subcontinent, ranging in age from ~18 Ma to ~.5 Ma, have yielded over 28 murine species belonging to 14 genera, most of them showing an in situ evolution. In the present work, a cladistic analysis (using PAUP) based on 26 derived and generalized dental characters distributed among Siwalik fossil murines of the Indian subcontinent and their extant counterparts has been carried out. The cricetids, Potwarmus and Myocricetodon were used as the out-group taxa to the in-group murine rodents. The results support monophyly of murines. The basal most murine taxon Antemus shares a common ancestry with rest of the murines. Progonomys was next to diverge, followed by the split leading to two clades, one comprising Karnimata and Parapelomys and the other having mostly modern murine taxa, with Mus occupying the basal-most position. Rest of the taxa of the latter clade form another clade comprising Millardia, Cremnomy, Dilatomys, Golunda, Hadromys, Rattus, Bandicota and Nesokia. The cladogram has been calibrated using the Siwalik chronostratigraphical data. Presently, Mus-Rattus divergence date is placed between 10-12 Ma based either on the first occurrence of Progonomys or on the Progonomys-Karnimata split date. The cladogram presented here suggest a rather different scenario indicating that the first appearance datum of Mus in the geological record, which at the moment is 7.3 Ma, appears to denote the divergence date of Mus and several rat genera including the genus Rattus. Further, this younger date of Mus-Rattus split validates younger first appearance dates of fossil Millardia, Cremnomy, Dilatomys, Golunda, Hadromys, Rattus, Bandicota and Nesokia.