Body size trends and recovery amongst bivalves following the end-Triassic mass extinction

Fossils in the immediate aftermath of mass extinctions are often of small size, a phenomenon attributed to the Lilliput Effect (temporary, size reduction of surviving species). There has been little attempt to study size trends during subsequent recovery intervals nor has the relationship between size, diversity and environmental controls been evaluated. Here we examine the recovery following the end-Triassic mass extinction amongst bivalves of the British Lower and Middle Lias. Three distinct phases of size change are seen that are independent of other recovery metrics: initially bivalves are small but the Lilliput Effect is a minor factor, the majority of small taxa belong to new species that undergo a later within-species size increase (the Brobdingnag Effect) throughout the subsequent Hettangian Stage. New species that appeared during the Hettangian were also progressively larger and Cope's Rule (size increase between successive species) is seen – notably amongst ammonites. The size increase was reversed during the Sinemurian Stage, when bivalves once again exhibited small body sizes. During the Pliensbachian Stage another phase of size increase occurred with further evidence of the Brobdingnag Effect. These three phases of size change are seen across all suspension feeding ecological guilds of bivalve but are not expressed among deposit feeders. Local environmental conditions explain some aspects of size patterns, but factors such as temperature, marine oxygenation and sea level, do not correlate with the long-term size trends. The Brobdingnag Effect may reflect increased availability/quality of food during the recovery interval: a factor that controlled bivalve size but not evolution.