PALEOCOMMUNITY AND ONTOGENETIC COMPARISON OF GASTROPOD SIZE VARIATION IN THE PENNSYLVANIAN FINIS SHALE OF TEXAS

In marine paleoecological research, a sampling bias toward larger, more easily accessible fossils may bias perceived ecological interactions, excluding ecological insights gained from including smaller specimens. Here, we compare large (>2 mm) and small (0.85-2mm) fossil gastropod abundance and distribution in the Pennsylvanian Finis Shale of north central Texas. Our goal is to determine whether the two size fractions represent older and younger cohorts of the same taxa, or if the small fraction represents a taxonomically and ecologically distinct portion of the community.

We collected three bulk sediment samples from Jacksboro Spillway, and one from Causeway Road, 1 km apart. Samples were disaggregated, cleaned, and sorted by size into the small or large category. Small samples were split into four equal portions, and then compared to the corresponding large samples. Gastropods were identified to the genus level, and sample size, richness, evenness and relative abundance were calculated. The number of whorls were counted on trochospiral taxa and compared between size fractions to determine whether small specimens were juvenile or fully grown.

All small samples contained 2 to 6 times as many individuals as corresponding large samples. Richness of the Spillway samples differed inconsistently between small and large, while Causeway richness between small and large was equal. Evenness among all samples was high. Relative abundance revealed similarities and differences between genera and between samples. For example, Euphemites had a wide range of abundance within both Spillway and Causeway samples. Some taxa were only present in one size fraction and/or location, such as Phymatopleura and Shansiella. Because small and large gastropods had similar whorl counts, many of the small gastropods may be dwarf forms of larger taxa, not ontogenetic stages of the large communities. The size variation may indicate time- or spatially-averaged environmental fluctuation. Due to higher sample sizes, the smaller specimens may have the potential to yield more detailed information about these fluctuations.