CREEPING INTO THE ICEHOUSE: MORPHOLOGICAL CHANGE IN GASTROPODS OF THE EOCENE LA MESETA FORMATION (SEYMOUR ISLAND, ANTARCTIC PENINSULA) IN RESPONSE TO THE ONSET OF CENOZOIC COOLING

Understanding community-level response to temperature change has become increasingly important with the recognition that Earth's climate is less stable than was once thought. A critical episode of global cooling occurs at the Eocene/Oligocene transition, associated with the buildup of permanent ice on Antarctica. The Eocene La Meseta Formation (Seymour Island, Antarctic Peninsula) provides the only abundant antarctic fossils from this critical time. The La Meseta fossil record suggests that cooling began during the late Eocene; stable isotope studies have confirmed this. Molluscan diversity decreases upsection, whereas dense assemblages of ophiuroids and crinoids occur only in the upper part of the La Meseta. Dense echinoderm beds suggest a paucity of durophagous predators. Because cold water is likely a factor in the absence of durophagous crabs and fishes in the modern antarctic, it is expected that late Eocene cooling would have reduced their abundance upsection. Some La Meseta gastropods bear the mark of peeling by crabs, providing an index of the frequency of durophagous attacks. Preliminary data confirm a decrease in durophagous predation upsection between the middle and upper La Meseta.

Because of the metabolic cost of calcification, a reduction in durophagous predation is expected to bring a decline in the defensive architecture of gastropod shells. Morphological response by the gastropod community to this reduction in predation is assessed through semiquantitative analysis of eight morphological traits known to have antipredatory value. Multidimensional scaling analysis shows that most of the gastropod genera have shell morphologies that lie within a cluster of relatively poorly defended forms, in contrast to three well-defended outliers. These outlying genera are absent from the upper part of the La Meseta, though present in the middle part. This extirpation of well-armored gastropods is thus concurrent with the observed decline in peeling predation.