DEVELOPMENTAL BASIS OF SIZE DECREASE IN TURRITELLA GASTROPODS DURING A REGIONAL MASS EXTINCTION IN THE PLIO-PLEISTOCENE OF FLORIDA

Small body size is a common feature among survivors of mass extinctions, but, thus, far, researchers have only speculated about how and why this phenomenon occurs. One possibility is that catastrophic declines in population size during the extinction event itself select for an opportunistic (r-selected) life history strategy, in which individuals are short-lived and reproduce early (i.e., at small size). In contrast, small size could reflect the effects of environmental stress (e.g., low productivity, harsh conditions) or reduced pressure from competitors and predators on individual growth rates. A third scenario is that both selection filters are influential, and small survivors both mature early and grow slowly.

Here, we use stable isotope sclerochronology to deconvolve growth rate and age – the developmental components of size – for pre- and post-extinction species of the gastropod genus Turritella in the Plio-Pleistocene of Florida, which was the geographic center of a late Pliocene regional mass extinction. Shell lengths of Turritella decreased from a maximum of over 140 mm in the middle Pliocene to less than 40 mm in the Pleistocene and Recent. This decrease in maximum size appears to have occurred in at least two steps – the first occurred after the middle Pliocene Pinecrest Beds, and the second occurred after the latest Pliocene Caloosahatchee Formation. By the early Pleistocene Bermont Formation, maximum size in Turritella had stabilized. Our analysis brackets this step-wise decline by including representative shells from middle Pliocene, latest Pliocene, and early Pleistocene shell beds.

Preliminary isotopic reconstructions of Turritella development suggest that size decrease was due to reduced growth rates with no change evident in age at maturation. Thus, small size in post-Pleistocene Turritella does not reflect a double selection filter but rather a singular phase of selection (or an extended period of phenotypic plasticity) in an environmentally stressed or enemy-free environment. This is consistent with the consensus view that the late Pliocene extinctions were the result of steady and permanent nutrient decline in the western Atlantic rather than a sudden, catastrophic event such as a bolide impact.