DYNAMICS OF EXTINCTION AND RECOVERY: A STUDY OF MORPHOSPACE SHIFTS DURING AND AFTER THE LATE ORDOVICIAN MASS EXTINCTION

Mass extinctions profoundly affect the history of life by both destroying existing diversity and ecological structure and creating new evolutionary and ecological pathways. The Late Ordovician mass extinction, although the second largest in terms of taxonomic impact, has been characterized by little morphological and ecological restructuring. However, there is a major turnover of brachiopod faunas at the ordinal level, and diversification in the recovery is clumped within families. This suggests that morphological selectivity might play a role in driving patterns of brachiopod origination in the Silurian. Morphometrics can help resolve this conflict by illuminating how brachiopod morphology changes throughout the extinction and recovery. This can provide insight into the severity of this extinction and the extent to which it altered the trajectory of brachiopod evolution.

We used principal coordinate analysis to ordinate character data from a previously published phylogenetic analysis of the brachiopod order Strophomenida containing 61 taxa. Character data for the 45 ancestral nodes in the phylogeny were also included and were generated using parsimony character state reconstructions from Mesquite. A bootstrap of the centroid of PCO values indicates that (1) each of the three major clades in the phylogenetic analysis occupy a different region of morphospace, (2) extinct taxa are randomly removed from morphospace, and (3) new Silurian taxa are clustered within a smaller previously unexploited region of morphospace. Diversification throughout the Silurian remains morphologically constrained, which suggests that the Late Ordovician mass extinction had a long-term impact on strophomenid morphological diversity.

These results provide evidence for morphological selectivity during the Late Ordovician mass extinction and cast doubt on the idea that it had little evolutionary significance. This implies that focusing on only one metric of diversity leads to an incomplete picture of the long-term evolutionary effects of mass extinctions. Examining multiple components of diversity is important to understanding the broader impact of mass extinctions on morphology and ecology.