ECOLOGICAL COMPARISON OF SMALL AND LARGE GASTROPOD COMMUNITIES IN THE PENNSYLVANIAN FINIS SHALE OF TEXAS

Marine invertebrate fossils illustrate ecological conditions. Studies are often limited to larger specimens that are easiest to collect, process, and identify. By only analyzing the larger size distribution of a community, the insight that can be gained from smaller, often excluded, fossils may be overlooked. Here, we compare large (> 2mm) and small (0.85 - 2mm) gastropod fossils to determine if the small specimens are juveniles or small adults. We compared the size fractions to explore how and why the communities may differ.

Bulk sediment samples of the Pennsylvanian Finis Shale (Jacksboro Spillway, north-central Texas) were collected from two depths: 1.4 (low) and 2.4 (high) meters above unit base. Fossils were disaggregated then cleaned and sorted by size. The small fraction was split into four equal portions, one of which was compared to the complete large sample. We identified gastropods to the genus level, then calculated sample size, richness, evenness, and relative abundance.

The small gastropods had adult features, including ornament and whorl number, suggesting they are the same taxa as the large fraction. The most drastic difference between the small and large fractions was sample size. The low samplecontained 73 small gastropods (in a single 25% split) compared to 21 large gastropods. The high sample contained 34 small gastropods (25%) and 19 large. In the low sample, the small category had a richness of 15 and the large was 10. In the high sample, small richness was 9 and large richness was 10. Evenness among all samples was high (ranging 0.83-0.92). Relative abundances showed similarities and differences: Meekospira (low) was similarly abundant between small (4.1%) and large (4.7%); in contrast, Straparollus (high) differed (small 1.3%, large 9.5%).

The adult size variability within taxa may be natural variation, or may indicate ecological pressures that influenced adult size, such as resource availability or predation threat. It may also represent preservational differences. Overall, similarities in richness and evenness suggest that the small and large communities were broadly the same. Specific differences in richness and relative abundances may be the result sample size disparity or ecological factors that influenced the small community but not the large community.