Body size, extinction events, and the early Cenozoic record of veneroid bivalves: a new role for recoveries?

—Mass extinctions can play a role in shaping macroevolutionary trends through time, but the contribution of recoveries to this process has yet to be examined in detail. This study focuses on the effects of three extinction events, the end-Cretaceous (K/T), mid-Eocene (mid-E), and endEocene (E/O), on long-term patterns of body size in veneroid bivalves. Systematic data were collected for 719 species and 140 subgenera of veneroids from the Late Cretaceous through Oligocene of North America and Europe. Centroid size measures were calculated for 101 subgenera and global stratigraphic ranges were used to assess extinction selectivity and preferential recovery. Veneroids underwent a substantial extinction at the K/T boundary, although diversity recovered to preextinction levels by the early Eocene. The mid-E and E/O events were considerably smaller and their recovery intervals much shorter. None of these events were characterized by significant extinction selectivity according to body size at the subgenus level; however, all three recoveries were strongly size biased. The K/T recovery was biased toward smaller veneroids, whereas both the mid-E and E/O recoveries were biased toward larger ones. The decrease in veneroid size across the K/T recovery actually reinforced a Late Cretaceous trend toward smaller sizes, whereas the increase in size resulting from the Eocene recoveries was relatively short-lived. Early Cenozoic changes in predation, temperature, and/or productivity may explain these shifts. Rowan Lockwood. Department of Geology, The College of William and Mary, Post Office Box 8795, Williamsburg, Virginia 23187. E-mail: [email protected] Accepted: 17 January 2005