HOW METABOLIC RATE AFFECTS SURVIVORSHIP: THE GRIM STORY OF BIVALVES ACROSS A PLIOCENE EXTINCTION

Studies of Cenozoic extinction events among marine bivalves offer important insights into the processes that drive evolution. One such event, part of the multi-phased Plio-Pleistocene extinction, is recorded in the upper Pliocene of the Mid-Atlantic, straddled by the uppermost Yorktown Formation below and Chowan River Formation above.

Previous work hypothesized that escalation affects survivorship of extinction events. Most studies of escalation and extinction have been restricted to morphological antipredatory traits (e.g., spines, coarse ribs, and thick shells) and have yielded little support for this hypothesis. A closer look at physiological escalation was needed, especially because escape tactics are the primary way some species avoid being consumed by predators.

Collections from the Virginia Museum of Natural History representing mid-upper Pliocene strata have been counted to determine relative abundances of bivalve taxa. Activity levels (high, moderate, and low), used as a proxy for metabolic rate, have been assigned to the taxa in question based on primary literature. Metabolic rates of taxa not reported in literature will be determined by oxygen isotope analysis.

Using biogeochemical techniques, metabolic rates of fossil bivalves can be studied. Ratios of stable oxygen isotopes vary seasonally in marine environments; ¹⁸O enrichment occurs during cooler periods and depletion in warmer periods. As bivalves secrete their shells, the isotopic ratios are recorded. Stable isotope analysis of representative shells from taxa of unresolved activity level will be performed serially from umbo to commissure to produce sinusoidal oxygen isotope curves. From these curves, annual growth rate can be determined, which will allow interpretation of metabolic rate for taxa for which activity level data are unavailable.

Preliminary results suggest that most taxa did not change significantly in relative abundance, and those that did show no obvious trends of increase or decrease based on activity levels. However, the activity levels of many groups remain unresolved, and oxygen isotope analysis of representatives from each should decipher that. Other factors must also be considered, such as water depth and climate at time of deposition as well as the possibility of increased predation pressures.