BIOGEOGRAPHIC PATTERNS OF BRACHIOPOD FAMILIES AS AN EXPLANATION FOR PHYLOGENETIC CLUMPING FOLLOWING THE LATE ORDOVICIAN MASS EXTINCTION

The Late Ordovician mass extinction was the second largest mass extinction during the Phanerozoic in terms of number of species eliminated, but resulted in very little ecological restructuring. Potential for explaining this discrepancy lies in illuminating the manner by which evolutionary history is impacted by extinction and origination. Our previous work indicates the pattern of extinction and origination of brachiopod genera shifts from randomly dispersed among families during the Ordovician to clumped within families during the Silurian. An underlying shift in the geographic distributions of families is a possible explanation for this observed switch to phylogenetically clumped extinction and origination patterns.

We used global paleolatitude data from the Paleobiology Database to calculate average geographic ranges for genera within 185 brachiopod families spanning the Late Ordovician to Late Silurian. Preliminary results indicate an increase in average latitudinal range from the Ordovician to early Silurian. This reflects the elimination of genera with narrow latitudinal ranges as a result of the mass extinction. The average geographic ranges of genera then decrease during the Silurian for both families that survive the extinction event and new families. These results suggest that families preferentially added narrow ranging genera through the Silurian.

This study provides an explanation for transition from random to clumped origination and extinction following the Late Ordovician mass extinction, which in turn begins to address the underlying causes of origination patterns during intervals of high diversification rates.