TRENDS IN BODY SIZE ACROSS THE END-CRETACEOUS AND EARLY CENOZOIC EXTINCTIONS IN VENEROID BIVALVES

Body size is widely considered one of the most important attributes of organisms. Although the question of whether extinctions are size-selective has received considerable attention, the influence of extinction events on large-scale macroevolutionary trends in body size is rarely investigated. The goal of this study is to quantify the effects of a series of extinctions on long-term patterns in veneroid body size during the late Mesozoic and Paleogene. Centroid size measures were calculated for 101 subgenera across the end-Cretaceous (K/T), mid-Eocene (mid-E), and end-Eocene (E/O) extinction events and global stratigraphic ranges were used to assess extinction selectivity and preferential recovery. Systematic data compiled for 140 subgenera of veneroids from the Late Cretaceous through Oligocene of North America and Europe document substantial extinction across the K/T (81% of subgenera) and minor extinction across the mid-E (8.5%) and E/O (18.6%) events. No evidence of size-selective extinction exists across any of these events; however, the recovery intervals that follow each event were strongly size-biased. The direction of this selectivity varies according to the event, with smaller veneroids radiating after the K/T and larger veneroids diversifying after the two Eocene events. The protracted nature of the size decrease after the K/T and the fact that the subgenera responsible do not appear to be opportunists or ecological generalists makes it unlikely that the patterns documented in this study are an example of the “Lilliput” effect. While the K/T recovery reinforced a previously-established decrease in veneroid size, the Eocene recoveries produced a short-lived increase in size. Minimum and subclade tests suggest that trends in veneroid size across the K/T and Eocene events were most likely driven and passive, respectively. Shifts in size do correlate with early Cenozoic changes in temperature, although the timing suggests that shifts in productivity may be a more likely explanation.