Neogene Siwalik mammalian lineages: Species longevities, rates of change, and modes of speciation

A long depositional sequence from northern Pakistan provides a good fossil record of terrestrial vertebrates for the interval of 18-7 Ma. Making allowances for possible range extensions, we use this record as a direct measure of species longevities. There is a correlation between body size and longevity, smaller mammals frequently being short-lived and larger taxa showing durations up to ca. 10 m.y. The distribution of small mammal longevities indicates an exponential decrease in frequency from a high modal value in the smallest increment measured (0–200,000 yrs). The most frequent value for large mammals is also the smallest increment measured (1 m.y.), but that distribution may not be unimodal. Small mammal taxa of short duration are concentrated late in the sequence, after 9 Ma especially. The middle Miocene fauna is more stable, with species showing longer durations. The contrast in longevities corresponds with hypothesized greater environmental stability in the middle Miocene. For comparison, the Paleogene sequence of Wyoming indicates short median species durations, with few surveyed taxa lasting over 2 m.y. Siwalik mammals show diverse modes of evolution, but stasis in at least some features is usual, with species boundaries corresponding to morphological breaks. Up to half of the Siwalik rodent and artiodactyl species surveyed likely immigrated from outside the biogeographic province, and for a few, historical data are sufficient to stipulate when and by what route they came to the Indian subcontinent.