PATTERNS OF FAUNAL TURNOVER AND DIVERSITY IN THE NEOGENE SIWALIKS OF NORTHERN PAKISTAN

The fluvial Neogene Siwalik formations of northern Pakistan contain a long and richly fossiliferous sequence of terrestrial vertebrate fauna s in which patterns of faunal turnover and changes in diversity can be documented and analyzed for intervals having durations of 0.5 m.y. Th e complete sequence extends from circa 18.5 to 1 Ma, but the part betw een 18.5 and 5.5 Ma is best sampled, and most intervals within it are well represented. Thirteen orders of Siwalik mammals have been identif ied, with well-sampled intervals having 50 or more species. Most Siwal ik mammals, however, are either rodents or artiodactyls. Bovids are th e most common and most speciose of the larger mammals, while murid and ''cricetid'' rodents dominate the small mammal assemblages. Between 1 8.5 and 5.5 Ma species diversity varied considerably. Among artiodacty ls and rodents the number of species first increased between 15 and 13 Ma and then fell. Data on stratigraphic ranges of rodents and artioda ctyls shaw that faunal change in the Siwaliks was episodic, occurring during short intervals with high turnover followed by longer periods w ith considerably less change. Maxima of first appearances occurred at approximately 13.5 and 8.5 Ma, while maxima of last occurrences were a t 12.5 and 8.0 Ma. Some of the observed faunal events can be correlate d to climatic and environmental changes. The Middle Miocene diversific ation occurred during a period of global cooling, while the latest Mio cene decline in diversity and increased turnover accompanied oxygen an d carbon isotopic changes that correlate to globally increasing season ality and aridity. Other correlations are ambiguous. The marked decrea se in diversity and the major turnover events between 13 and 8 Ma do n ot correspond to known local or global events. The Neogene Siwaliks an d Paleogene Bighorn-Crazy Mountains sequence in Wyoming and Montana sh are many similarities. They have equivalent levels of temporal resolut ion and similar levels of completeness of their fossil records. Siwali k ordinal abundance and diversity patterns differ markedly from those of the Paleogene, but generic, and probably species, diversity was app roximately the same, although the Siwalik faunas may have been slightl y less diverse. Over time, changes in diversity were of comparable mag nitude, with monotonic trends persisting for more than 5 million years . The magnitude of faunal turnover was also similar, ranging from less than half to 3.5 times that expected. In both sequences faunal change appears to have been episodic, with strong pulses between intervals o f low turnover. The Siwaliks, in contrast to the Paleogene sequence, m ay have had more distinct pulses and longer intervals between pulses. Neither sequence has peaks of first occurrences coinciding with peaks of last occurrences.