THE SIWALIK GROUP SNAKE RECORD

The Siwalik Group provides a unique opportunity to examine snake faunal history due to the large sample sizes, number of localities, and precise chronostratigraphic control. In contrast, the majority of European and North American Neogene snake records consists of assemblages derived from a few large, time-averaged localities with much coarser temporal control (e.g., Szyndlar 1987; Szyndlar and Schleich 1993; Holman, 2000). The Siwalik Group additionally provides data on evolutionary histories of South Asian snakes, which are poorly known relative to European and North American taxa. Thus, examination of the Siwalik record focuses on faunal change within the local section and the implications of Siwalik snakes for understanding the larger evolution of Asian snake faunas, after accounting for changes in record quality and taphonomic biases.

The Siwalik snake assemblage consists of two separate records derived from surface-collection and screen-washing, as with the mammalian record (Badgley et al. 1995, 1998). Each method recovered specimens of different sizes. Surface-collected methods recovered specimens with centrum lengths greater than 1 cm, whereas screen-washed methods recovered specimens with centrum lengths less than 1 cm (Table 1). As a result, the records are treated separately in examining record quality and sample biases. Unlike the mammalian record, however, the two snake records sample the same fauna, because almost half of the described taxa and morphotypes were collected using both methods (Table 1). Taxa represented by single occurrences, Boidae? indeterminate (13.50 Ma), cf. Erycinae indeterminate (13.55 Ma), and Gansophis potwarensis (6.78 Ma), are not included in taphonomic and diversity analyses.

Record Quality, Specimens, and Localities

Figure 11 shows the numbers of specimens collected by both methods at 0.5 Ma intervals, the shortest temporal interval for which localities can be combined to produce consistently large sample sizes of specimens and localities (see Barry et al. 1995). Intervals range from 0 to .5 Ma, with the youngest age used as the reference datum for each. Several intervals between 6.5 and 18.0 Ma are either poorly represented or lack snake fossils. The dataset was culled to exclude those intervals with less than one locality for both the surface-collected and screen-washed records. Two exceptions are the 11.5 Ma and 8.5 Ma intervals, both of which possess high record quality for one of the records.

The datasets after culling are plotted in Figure 11B, D. Culling both data sets results in the absence of data for several important intervals, including 18.0 to 14.5 Ma, 10.5 Ma, 9.5 Ma, 8.0 Ma, and 6.0 Ma. The 18.0 to 14.5 Ma interval spans the Kamlial Formation in the Siwalik Group. Although this interval is crucial for determining the origin of the Siwalik snake fauna, its low record quality renders it uninformative with respect to this analysis. The younger intervals span a history of significant environmental change within the Siwalik Group, and their exclusion limits interpretation of faunal histories (see below).

Both records show a generally poor relationship between sample size (number of individual specimens) and the number of localities throughout the section. The surface-collected record shows an inverse relationship between sample size and number of localities between the14.0 Ma and 10.0 to 9.0 Ma intervals, followed by a more positive relationship between sample and locality sizes from 9.0 to 8.5 Ma. The screen-washed record demonstrates a positive relationship between sample size and number of localities from 14.0 to 12.0 Ma, followed by an inverse relationship between 11.5 and 7.0 Ma. Both records demonstrate a decrease in sample size at 6.5 Ma.

Comparing the records reveals two distinct histories of record quality. The surface-collected record shows a greater concentration of specimens relative to localities between 14.0 to 10.0 Ma, followed by a decrease in both concentration and total number of specimens from 10.0 Ma and younger. The screen-washed record demonstrates the inverse—a sharp increase in the relative concentration and overall number of specimens from 11.0 to 10.0 Ma and from 9.0 to 7.5 Ma.

Figure 12A plots the total number of Siwalik specimens for each taxon or morphotype. The record is dominated by Acrochordus dehmi, which constitutes approximately 80% of the entire sample. This record differs from most Neogene snake assemblages that often possess more even distributions of specimens among taxa or are dominated by several taxa (e.g., Ivanov 2000, p. 587), and likely reflects both depositional and ecological histories within the Siwalik snake record (Head 1998, see below). The relationships between number of taxa and morphotypes and sample size for the screen-washed and surface-collected records are plotted in Figure 12B, C. Taxonomic/morphotypic diversity has a positive, significant relationship to sample size for the screen-washed record, as documented for the screen-washed mammal fauna (Badgley et al. 1998), but no significant relationship exists between diversity and sample size for the surface-collected record.

Taphonomic Comparisons

I examined the relationship of sample size to depositional and collection processes using Analysis of Variance (ANOVA) and least squares linear regression. Biases affecting sizes, distributions, and compositions of fossil assemblages are well documented for the Siwalik Group, and consist of variability in fluvial transport and depositional environment among surface-collected records, and biotic agents, namely predator accumulations, for screen-washed samples (Badgley 1986a, 1986b; Badgley et al. 1998). These factors presumably affect the composition of the snake faunas as well. Collection bias can be examined as sampling intensity for the screen-washed record (see below), but cannot be measured for the surface-collected record.

The extent to which fluvial transport determines sample size was tested using ANOVA between sample size and grain-size lithology for both surface-collected and screen-washed localities. Badgley et al. (1998) constrained the effects of fluvial transport on screen-washed mammal assemblages from the Chinji Formation using ANOVA and demonstrated that sample size was independent of lithology. For the snake record, the analysis demonstrates the absence of a relationship between sample size and grain size for screen-washed samples (Figure 13B), but shows a statistically significant relationship between surface-collected specimens and grain size (Figure 13A). The correlation with grain size can be explained by depositional processes but may also represent habitat specificity of autochthonous taxa.

Least-squares linear regression was used to determine the relationship of sample sizes in both records to locality temporal duration and the relationship of screen-washed sample size to collection intensity and the potential for chemical destruction. Temporal duration refers to the time span within which deposition and incorporation of fossils occurred at individual localities, and ranged from 0.02 to 0.22 m.y. for screen-washed localities, and 0.01 to 0.16 m.y. among surface-collected localities (see Barry et al. 2002 for locality durations and estimation methods). Potential biases affecting the screen-washed record include sampling intensity and productivity, chemical destruction, and biotic agents of assembly. As seen in Figure 12C, D, there is little correlation between sample size and the number of screen-washed localities per interval through the Siwalik section; however, the extent to which individual localities have been sampled could potentially explain the increase in the number of recovered specimens from screen-washed sites. screen-washed specimens were recovered from reduction of bulk matrix, and the measure of sampling intensity was the total mass of processed matrix from each locality (Badgley et al. 1998).

Comparison with the Siwalik rodent record was used to test the influence of chemical destruction because screen-washed vertebrate remains of similar size and composition should show similar patterns of changing abundance if changes in sample size through time are the result of changing chemical preservation modes. Material differences in rodent teeth and snake vertebrae somewhat limit the use of rodents as proxy data for chemical environment, because dense enamel is more resistant to destruction. However, the record provides at least a first-order approximation.

Regression of sample size onto the described taphonomic variables failed to produce significant relationships (Figure 14A-D). ANOVA results indicate that depositional bias affects the surface-collected record in the same general pattern as for other Siwalik vertebrates, with the lowest sample sizes in fine-grained deposits (Badgley et al. 1995, 1998), but no significant bias amongst the screen-washed records. The independence of screen-washed snake records from any of the examined sources of bias suggests an agent of accumulation other than sedimentological, chemical, or collection processes, such as predation, as proposed for Chinji Formation rodents (Badgley et al. 1998).

Taxonomic and Ecological Richness

Figure 15B plots relative frequency of taxa/morphotypes, expressed as number of identified specimens (NISP) from the combined records. Both raw richness and range-through (Cheetham and Deboo 1963) values are plotted. Overall, the number of taxa and morphotypes increases through time in the Siwalik Group: richness is comparatively low through the Chinji Formation, followed by a progressive increase through the Nagri and Dhok Pathan formations, starting between 11.0 and 10.0 Ma, with maximum richness at 7.0 Ma, and a decline at 6.5 Ma. This pattern is consistent within both the screen-washed and surface-collected records. Raw and range-through values track each other, suggesting that the increase in taxa and morphotypes is a historical event as opposed to simply a preservational bias. The pattern of increasing richness is similar to the increasing sample size within the screen-washed record (Figure 11D), reflecting (in part) the positive relationship between taxonomic/morphotypic diversity and sample size in the screen-washed record. The richness increase in the surface-collected record is concurrent with a decrease in sample size (Figure 11B, Table 2), however, indicating that sample size alone does not explain the increase.

Figure 15B plots relative frequency of taxa/morphotypes, expressed as number of individual specimens (NISP) from the combined records. Because sample sizes are extremely small for most taxa (Table 2, Table 3), all colubroid taxa and morphotypes are summed as Colubroidea. Between 14.0 and 10.0 Ma, Acrochordus dehmi represents between 88-97% of all recovered specimens. Between 10.0-8.5 Ma, the relative frequency of A. dehmi decreases sharply to 28%, due to both lower sample sizes for the taxon and the influx of colubroid specimens. The spike in relative frequency of Acrochordus at 7.0 Ma is partially artifactual, because that interval includes a locality (Y-935) containing a large number of elements (84) that likely represent a single, associated, surface-collected skeleton.

Changes in taxonomic richness and relative frequency are coeval with changes in depositional environment, despite a poor correlation of sample size to lithology for the screen-washed record. The three Siwalik formations included between 14.0 and 6.5 Ma represent distinct fluvial systems. The Chinji Formation (14.2 to 11.2 Ma) is primarily a fine-grained sequence representing large upland-sourced river systems, and characteristically includes regularly spaced overbank deposits. The Nagri Formation (11.2 to 9.0 Ma) is characterized by massive sandstones and represents large trunk systems. The Dhok Pathan Formation (10.1 to 3.5 Ma) is fine grained and represents smaller, upland-sourced river systems with increased avulsion and poorly drained floodplains (Willis and Behrensmeyer 1995; Barry et al. 2002). The domination by Acrochordus occurs through the larger river systems of the Chinji and Nagri Formations between 14.0 and 11.0 Ma, and decreases throughout the more heterogeneous Dhok Pathan Formation. The increase of taxonomic and morphotypic richness is approximately concurrent with the transition from the Nagri fluvial system to the Dhok Pathan system at approximately 10.1 Ma (Barry et al. 2002), with the oldest locality at which diversity increases dated to 10.2 Ma (Y-450).

Changes in relation to depositional environment are consistent with inferred ecology. Acrochordus dehmi, Python sp., and Colubroidea can be broadly characterized for habitat and prey preference via phylogenetic constraint and comparison with extant sister taxa. Acrochordus represents a fully-aquatic piscivore (Shine 1986a, 1986b). Python represents a terrestrial predator of large vertebrates (Murphy and Henderson 1997). Colubroidea represent a wide range of semi-aquatic to fully terrestrial small carnivores (e. g., Mushinsky 1987). Within this context, the snake fauna of the Chinji and Nagri formations are predominately aquatic, and the Dhok Pathan record includes increasing representation of both large and small semi-aquatic and terrestrial taxa. Python is present in low abundance as a large vertebrate predator throughout the entire Siwalik sequence, and likely represents an allochthonous component of most depositional environments.