COMPARING TAXONOMIC COMPOSITION OF LARGE AND SMALL MOLLUSKS IN THE MIOCENE ST. MARYS FORMATION, MARYLAND

Exploring fossil communities can reveal ecological patterns and conditions from throughout Earth’s history. Studies typically focus on larger specimens that are relatively easy to collect and identify, omitting the smaller members of the paleocommunity. However, analysis of these smaller organisms can reveal characteristics in community structure that may be missed when examining only larger taxa. Here, we explore a smaller size fraction and compare taxonomic composition and abundance of two different size categories within a stratigraphic unit.

We collected four 4L bulk sediment samples from four stratigraphic positions in the Miocene St. Marys formation in Maryland, USA. Samples were disaggregated, separated into 1mm-2mm and 2mm+ size fractions, and 2.5 mL subsamples were taken from the 1-2mm fractions. Mollusks were identified and counted from the full 2mm+ samples and the 1-2mm subsamples. There were 20,680 total specimens among the four 2mm+ samples (71% bivalves, 29% gastropods) and 346 in the 1-2mm samples (79% bivalves, 21% gastropods). In the stratigraphically lowest sample #1, gastropods dominated over bivalves (2mm+: 21% bivalves, 79% gastropods, 4,001 total specimens; and 1-2mm: 11% bivalves, 89% gastropods, 44 total specimens). In contrast, bivalves dominated the higher three samples: sample #2 (2mm+: 74% bivalves, 26% gastropods, 7,355 total; and 1-2mm: 83% bivalves, 17% gastropods, 113 total); sample #3 (2mm+: 92% bivalves, 8% gastropods, 8,205 total; and 1-2mm: 92% bivalves, 8% gastropods, 132 total); and sample #4 (2mm+: 78% bivalves, 22% gastropods, 1,119 total; and 1-2mm: 93% bivalves, 7% gastropods, 57 total).

Through the four stratigraphic samples, bivalve-gastropod relative abundance patterns and taxonomic compositions were similar between the 2mm+ and 1-2mm fractions, suggesting that the large and small fractions do not represent distinct portions of the paleocommunity. Does the 1-2mm fraction represent an ontogenetically younger version of the 2mm+ fraction, or are they smaller “adult” individuals? Future examinations will further quantify and statistically compare the community compositions between the two size fractions to determine the relationship between the 2mm+ and 1-2mm communities.