BECOMING YOUNGER WHILE GETTING COLDER: EXPLORING THE EVOLUTIONARY ROLE OF HETEROCHRONY IN LONG-LIVED BIVALVES FROM THE EOCENE OF SEYMOUR ISLAND, ANTARCTICA

Evolutionary change within the bivalve genus Cucullaea is well recorded in the nearshore sediments of the Eocene La Meseta Formation of Seymour Island, Antarctica, preserving the succession from the large, robust C. raea to the consistently smaller taxon C. donaldi. Previous work using high-resolution stable isotope profiles has demonstrated that the pronounced growth bands of Cucullaea are annual, allowing for precise age determinations and documenting ex-treme longevity in C. raea (upwards of 130 years). Geochemical studies have constructed a record of declining mean annual temperature on the Antarctic shelf through the Eocene, with the most rapid and severe episode of cooling associated with the transition from C. raea to C. donaldi. Following on this work, the current research explores the potential role of heterochrony during this evolutionary transition by quantifying changes in the size, shape, and lifespan of individuals. This record of evolutionary change together with isotopically-derived climate data provides a multifaceted, quantitative look into how an evolving lineage responds to the environmental stress of regional cooling.

Morphometric data are obtained by rotating a camera around the specimen to photograph and trace the external outlines of progressively older ontogenetic stages in each individual. Counting and measuring the growth bands of cross-sectioned individuals, followed by fitting the size-age data using a growth function, then provides direct age assignments to each digitized outline. Sclerochronology, in combination with isotopic and morphometric analyses, provides an excellent means of filling in the missing component of many studies of heterochronic change by standardizing specimens differing in both size and shape to a common age or developmental stage. This allows the diagnosis of how (or if) selection is favoring specific evolutionary processes through changes in the timing of development, the rate of development, and/or the age at the termination of growth. To date, we have documented a significant decrease in lifespan from C. raea to C. donaldi, suggesting the heterochronic process of progenesis, combined with a decrease in growth rate, as responsible for the smaller size and paedomorphic shape of C. donaldi.