PERMIAN-JURASSIC TRENDS IN BODY SIZE, SHAPE, AND GEOGRAPHIC RANGE IN BRACHIOPODS AND MOLLUSKS

The Paleobiology Database contains size measurements for nearly 60,000 brachiopod and mollusk specimens (representing more than 9,750 species) from the Permian, Triassic, and Jurassic. Because these sizes are linked to occurrences in specific fossil collections, the data can be used to test geographic, stratigraphic, or taphonomic effects on body size. I will present two case studies. The first will examine temporal trends in body size during the end-Permian and end-Triassic extinctions. Mean body sizes are fairly constant in all groups (brachiopods, bivalves, gastropods, and ammonoids) through the Permian but clades exhibit highly variable size decreases during the end-Permian mass extinction. The tempo of size recovery following the extinction closely tracked the rate of diversification of each clade, with ammonoids exhibiting the quickest recovery and brachiopods the most sluggish. Size reduction is not apparent after the end-Triassic extinction, either because the extinction had different size-selectivity from the Permian crisis or because the recovery was more rapid. The second case study will test the relationship between body size and geographic range in brachiopods and between body shape and range in ammonoids. Unlike geographic range, which reaches its maximum in the middle of a taxon’s stratigraphic range, body size does not exhibit any trend during a genus’s range. Although body sizes are not yet as comprehensive as occurrence data, the Paleobiology Database is a valuable resource for answering macroevolutionary questions that require size data.