ISOTOPIC RECONSTRUCTION OF GROWTH RATES: METABOLISM, ESCALATION AND EXTINCTION SUSCEPTIBILITY OF MIDDLE MIOCENE BIVALVES

Studies that have tested and failed to support the hypothesis that escalated species (e.g., those with predation-resistant adaptations) are more susceptible to elimination during mass extinctions have concentrated on the distribution and degree of morphological defenses in molluscan species. Previous work classified coarsely ornamented, thick, non-gaping bivalve species as escalated and those lacking these traits as non-escalated. This morphological approach to determining level of escalation in bivalves may be oversimplified. For instance, many morphologically non-escalated burrowing bivalves with thin, weakly ornamented shells or gapes exhibit a highly energetic mode of life (an escalated characteristic). Metabolic rate is an important measure of escalation not incorporated into recent tests of extinction susceptibility. Growth rates in living bivalves are dependent on metabolic rate and thus are potential indicators of level of escalation. To evaluate this approach, we used oxygen isotopes to reconstruct growth rates (and thus metabolic rates) for two unornamented bivalve species (Macrocallista marylandica and Glossus markoei) from the Calvert Formation of Maryland that did not survive the middle Miocene mass extinction. Although both species are classified as non-escalated based on morphology, the isotopic data indicate that M. marylandica was a fast-growing species with high metabolic rates and G. markoei was a slower-growing species with lower metabolic rates. Based on these results, we predict that some morphologically non-escalated species in previous tests of extinction susceptibility should be reclassified as escalated because of their fast growth rates (i.e., high metabolic rates). Studies that evaluate the level of escalation of a fauna should take into account the energetic physiology of taxa as well as morphology to avoid misleading results.