COMPARISONS OF COMMUNITY AND PREDATION BETWEEN LARGE AND SMALL GASTROPODS IN THE MIOCENE ST. MARY’S FORMATION, MARYLAND

Studies of fossil molluscs tend to focus on larger specimens, those easier to collect and identify. However, small mollusks (< 3 mm) are often more abundant. Here, we compare the small and large parts of gastropod assemblages to see how community composition and predation in the small category influence the large category.

The Miocene St. Mary’s Formation, Maryland, has well-preserved, diverse gastropods. Five bulk samples from one bed, spaced over 2 km, were sieved into two sizes: >3mm (Large) and 1-3mm (Small). For the Large fraction, the entire samples were processed (mean sample size 589, median 552); for the Small size, random subsamples were taken (mean sample size 90, median 99). Gastropods were sorted, identified to genus, and counted. Predatory drill frequencies (DF) were determined for one abundant genus, Ilyanassa. Richness and evenness were calculated and the similarities among the assemblages were examined using ordination.

Turritella was overwhelmingly dominant and difficult to count, so it was omitted from the evenness calculations and ordination. For the Large samples, richness ranged 7 - 16; in the Small samples, 8 - 11. In evenness, the Large samples ranged 0.69 - 0.82; the Small, 0.57 - 0.76. There were no spatial patterns in richness or evenness and no relationships between the size categories within samples.

The ordination had a clear separation between the two sizes, likely due to differences in evenness and relative abundances (RA), rather than unique taxa. The RA of naticids was 2-3 times greater for Large samples than Small (15%-27% versus 3%-13%), whereas the RA of Ilyanassa was greater in the Small samples (large: 29%-43%; small: 40%-63%). There was no relationship between these metrics and Ilyanassa DF, which ranged from 0.22-0.36 (Large) and 0.24-0.5 (Small). DF differed significantly between size categories in only one sample (p-test, p = 0.05).

Differences between the sizes in the relative abundances may be due to differential preservation (small naticids are likely to be highly fragmented and hard to identify or count) and to demographic differences (young, small Ilyanassa fall in the 1-3mm fraction). Future work will further explore the ecological pressures (e.g., predation) on the small community and how these might influence the larger, “macro” part of the community.