23-5 Effect of the Mid-Piacenzian Warm Period on Spionid Polychaete Presence and Intensity on Molluscan Hosts from the Yorktown Formation

Session: Paleontology of North America (Posters)

Presenting Author:

Authors:

Abstract:

Ecological interactions between marine organisms have been widely studied in both ancient and modern oceans; however, little is known about how these relationships will change as global warming progresses. This is true of the relationship between bivalves and spionid polychaetes, which can range from commensalism to parasitism. The Mid-Piacenzian Warm Period (MPWP), a period of warming 3 Ma that lasted around 300ky, is a uniquely suited time interval to study how climate change affected this relationship. Sea surface temperatures increased by about 3°C and CO₂ levels were around 425 p.p.m.v, which is similar to end-of-the-century estimates. The Yorktown Formation contains well-preserved molluscan assemblages and was deposited across the MPWP. This study seeks to understand how the presence and intensity of spionid polychaete infestation change over the course of the MPWP using the traces they leave on bivalve shells in the form of blisters and borings.

Four bulk samples of Yorktown Formation material were borrowed from the Virginia Museum of Natural History, two from the Sunken Meadow Member (before the MPWP) and two from the Rushmere Member (during the MPWP). To understand how presence and intensity varied based on life habit, four highly abundant bivalve genera were chosen for this study: two epifaunal taxa, Ostrea (Linnaeus, 1758) and Chesapecten (Ward and Blackwelder, 1976), along with two infaunal taxa, Astarte (Sowerby, 1816), and Glycymeris (Da Costa, 1778). Monograph descriptions were used to identify bivalves with discernible hinges to the genus level. The presence or absence of traces was recorded for all valves, and if traces were present, the number of each type was documented. Length and height were measured for each valve using digital calipers. T-tests and logistic regression analyses were performed to assess whether there were significant differences in the presence and intensity of spionid polychaete infection before and during the MPWP for each genus. 

Overall, the preliminary results suggest that the presence of spionid polychaetes did not significantly differ across the MPWP for any of the genera, while intensity significantly increased during the MPWP for two of the four genera: Chesapecten and Glycymeris. These results indicate that spionid polychaete infection intensity could increase in some bivalves as increasing temperatures and acidification contribute to the parasite’s ability to infiltrate and utilize bivalve shells.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1, 2026

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.