WHY ARE EXTINCTION PROBABILITIES OF PALEOZOIC TEREBRATULIDES HIGHER IN THE TROPICS?

Most, but not all, clades are more diverse in the tropics. Does extinction play a role in structuring latitudinal diversity profiles? Are the tropics primarily a source of diversity (higher speciation rate) or a sink for diversity (lower extinction rate)? Examining biogeographic extinction selectivity in a clade with an anti-tropical distribution may provide insight into the relationship between extinction and biogeographic distribution through time. Our study investigates relative extinction probabilities between tropical and pandemic terebratulide brachiopods in order to test hypotheses of biogeographic selectivity. The order Terebratulida is ideal for testing hypotheses concerning extinction selectivity, because they have survived every mass extinction since the clade's origin in the Early Devonian.

A database of Paleozoic terebratulidine and terebratellidine brachiopods comprising 164 genus-temporal interval-locality combinations was constructed. Locality data were collected via online databases and the primary literature, and supplemented by museum collections and field studies. Paleolatitude information was determined from the Scotese Paleogeographic Map Archive. Genus-temporal interval (GTI) combinations were placed into one of three categories: tropics only, temperate only, and pandemic. Temporal intervals include: Lower, Middle, and Upper Devonian, Lower, Middle, and Upper Carboniferous, and Lower and Upper Permian. Of the 16 tropics-only Devonian and Carboniferous GTIs, 13 fail to appear in the subsequent interval. Of the 10 Devonian and Carboniferous GTIs that are either temperate-only or pandemic, all 10 survive into the next temporal interval, demonstrating a statistically significant difference (McNemar's Chi-Square) in extinction bias between tropics-only and all other taxa.

Two hypotheses could account for these patterns: 1) extinction selectivity may be based on geographic range size; Rapoport's rule holds that tropical species tend to have smaller ranges, or 2) tropical terebratulides are more prone to extinction independent of range size; other selective factors are operating on the tropics-only genera. Extinction bias against tropics-only terebratulides may have led to the high-latitude distribution of extant terebratulide genera.