Examining the Morphology of Brachiopods to determine how the Late Ordovician Mass Extinction and Silurian Recovery affected long-term evolutionary trends

Effects of mass extinction and recovery dynamics on long-term evolutionary trends: a morphological study of Strophomenida (Brachiopoda) across the Late Ordovician mass extinction

by Judith A. Sclafani, Curtis R. Congreve, Andrew Z. Krug, and Mark E. Patzkowsky

Summarized by Soraya Alfred. Soraya Alfred is currently pursuing an undergraduate degree in Geology with a minor in Geographic Information Systems. She is a senior and intends to further her education by attending graduate school and then working in a Geology-related field. In her free time, she enjoys hanging out with friends and doing yoga.

What data were used? The distinct morphology of the shells of 61 species of Strophomenida (a type of extinct brachiopods) and 45 ancestor nodes, obtained from an evolutionary (phylogenetic) analysis.

Methods: Morphometric (shape differences) analysis was done through the use of principal coordinate analysis (PCO), which was used to plot the character data from the time- scaled phylogeny in morphospace. Morphospace is a type of graph used to represent the different morphologies of organisms, with different axes representing variables that define the characteristics of an organism. Twenty morphospace plots were made for the twenty set time-intervals between the early Ordovician and Devonian.

Results: When the morphospace at the time of the Ordovician mass extinction was examined, the data showed that the geometric center of the taxa that survived the extinction is similar to that of the genera that went extinct during the mass extinction. This implied that there were no specific morphologic characteristics that were targeted during the extinction event and, hence, was random. On the other hand, examination of the morphospace of the survivors of the Ordovician extinction, compared to the morphospace of the new genera that appeared in the Silurian showed that the center of mass shifted. This meant that the new taxa that emerged after the extinction filled a different region in morphospace, suggesting that origination was selective towards certain features. 

Figure showing the 20 morphospace plots for different time intervals. Members of Strophomenida occupy little morphospace in the lower Ordovician, but increase their area in morphospace during the Great Ordovician Biodiversification Event in the Darriwilian. After the mass extinction, new taxa that emerge in the Sillurian occupy the upper left half of morphospace which was previously unoccupied. The taxa that originated in the Devonian slowly become extinct into the Devonian.

Why is this study important? The Strophomenida order of brachiopods had a large geographic range, as well as a long geologic existence, making it ideal to study the repercussions of a mass extinction. As such, the results of this study can be applied to different lineages that were affected during the extinction in order to see how such events affect evolutionary history.

The big picture: Due to the fact that extinction happened randomly to taxa, a large amount of phylogenetic diversity was maintained, which made it possible for a great amount of diversification during the Silurian recovery. This diversification, however, resulted in less variability of morphology, which caused a morphological bottleneck. It is not possible to tell whether these changes were advantageous in an evolutionary sense or not, and so more has to be done to examine the true ecological effect of the Ordovician mass extinction. It was only through the examination of the characteristics of both the extinction and recovery period that we can begin to fully understand the evolutionary history of Strophomenida and similar patterns in other invertebrate taxa point to understand if this pattern was isolated or happened across multiple groups.

Citation: Sclafani, J. A., Congreve, C. R., Krug, A. Z., & Patzkowsky, M. E. (2018). Effects of mass extinction and recovery dynamics on long-term evolutionary trends: A morphological study of strophomenida (brachiopoda) across the late ordovician mass extinction. Paleobiology, 44(4), 603. Retrieved from http://ezproxy.lib.usf.edu/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=edo&AN=133394289&site=eds-live

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