GLOBAL BIOGEOGRAPHIC PATTERNS OF BRACHIOPOD GENERA DURING THE LATE PALEOZOIC ICE AGE

The late Paleozoic ice age (LPIA) was a time of uniquely low rates of net diversification and turnover for marine animal genera. To investigate the taxonomic, ecologic, geographic, and temporal patterns during the LPIA at a global scale, a biogeographic database of brachiopod genera for the Mississippian, Pennsylvanian, and Permian was assembled.

Patterns of diversity and turnover differed among latitudinal zones. Brachiopods were grouped into four large paleogeographic regions representing four broad latitudinal zones: polar Siberia, northern temperate Europe, equatorial southern Laurentia, and polar Australia. In each region, the origination and extinction rates were distinctly low during the LPIA, indicating that the average duration of genera was longer during the LPIA than before or after. Strikingly, there was no significant difference between regions in the magnitude of origination or extinction rates. Diversity patterns differ considerably among latitudinal regions: nonequatorial regions recovered diversity following the Devonian extinction, then fell to low levels at the onset of the LPIA, while the equatorial region had little recovery of diversity following the Devonian extinction, and little decrease in diversity at the onset of the LPIA.

Patterns of diversity and turnover also differed between endemic and cosmopolitan genera. Turnover rates were highest for endemic brachiopod genera (those occupying only one region); for more broadly distributed genera, rates were approximately the same regardless of their biogeographic coverage. Turnover rates were low during the LPIA for both endemic and cosmopolitan genera relative to before or after. Endemic genera recovered diversity quickly following the Devonian extinction, suffered a heavy loss of diversity at glacial onset, expanded slowly through the LPIA, and increased in diversity abruptly after the LPIA ended. In contrast, cosmopolitan genera suffered less loss of diversity at glacial onset, and recovered slowly through the LPIA with no abrupt increase in diversity at its end.

Future research will increase the resolution of the data and incorporate them into a Geographic Information System, to allow changes in the ecology, biogeography, and rates of evolutionary turnover to be related to the climatic changes of the LPIA.