Snake paleontology of the Siwalik group (Miocene of Pakistan): Correlation of a rich fossil record to environmental histories

The fossil snake record of the Siwalik Group (Miocene), Pakistan, was examined to determine responses in diversity, ecology, and phyletic history to known environmental change. The Siwalik Group includes a rich fossil record of snake vertebrae spanning a temporal interval of 18–6 Ma, as well as known histories of environmental change including increasing aridity and seasonality and a transition from C3- to C4-dominated floras.; In order to determine the systematic utility of vertebral morphology in diagnosing Siwalik Group snakes, phylogenetic analysis was performed using a data matrix of 39 characters for 45 extant taxa. Analysis of established higher-order snake clades resulted in consensus hypotheses that are generally consistent with results based on alternative data sets. Species-level analysis found little or no support for some higher-order clades and most generic and specific relationships, due to a lack of lower-order synapomorphies.; Applying this analysis to the Siwalik Group snake record resulted in the recognition 13 taxa and morphotypes throughout the section, with an increase in raw taxonomic richness through time that is partially related to increasing sample sizes. However, comparisons of sample size to taphonomic factors do not demonstrate any significant relationship, except for a positive correlation to depositional environment for the record of Acrochordus dehmi, which may reflect habitat preferences of a fully aquatic taxon as opposed to differences in fluvial transport.; Taxonomic richness and equality indices demonstrate an increase between 10.0 and 9.0 Ma. Ecologically, this represents a transition from Acrochordus-dominated faunas to more diverse, balanced faunas including multiple terrestrial and semi-aquatic taxa. This transition is generally concurrent with increases in aridity and seasonality of precipitation inferred to represent the inception of the Asian monsoon system, and may represent a response to increases in environmental heterogeneity. Changing diversity between 8.0 and 6.5 Ma may be related to the C3–C4 floral transition, but more precise relationship is difficult to discern.; Examination of the Acrochordus dehmi fossil record does not corroborate previous hypotheses of phyletic change through time within the lineage. Estimated body lengths based on regression relationships between vertebral length and snout-vent length determined for extant taxa are 1.5 to 4.0 meters indicating that A. dehmi was larger than all living Acrochordus, with a consistent mean body length throughout its fossil record. Morphometric analysis of vertebral shape change through time does not reveal a pattern consistent with hypotheses of phyletic evolution or faunal turnover, but demonstrates that A. dehmi is morphometrically distinct from extant Acrochordus. Homogeneity in A. dehmi may have resulted from a low-energy physiology shared with extant Acrochordus, which could have served as a buffer against environmental change.