EXTINCTION SELECTIVITY FOR PALEOZOIC AMMONOID SUTURE MORPHOLOGY

Analyses of temporal changes in morphological disparity can yield significant insight into potential selectivity during extinction events. However, most studies compare the disparity of pre-existing taxa to that of the taxa occurring during a similar length of time after the event. This makes discrimination of selective and non-selective events difficult as disparity loss due to taxonomic extinction can be distorted or concealed by gains during the rebound. Furthermore, the focus on disparity informs only on the relative density and diversity of morphospace occupation, giving no consideration to the distribution of taxa within that space.

Here, I account for these concerns by comparing the suture morphologies of pre-existing and surviving Paleozoic and basal-Triassic ammonoid genera during nine severe extinctions (> 50% generic loss). Simple analysis of disparity change through time provides evidence for selectivity during only two of the events considered. In contrast, direct comparison of the relative position and distribution of pre-existing and surviving faunas in morphospace shows that eight of the nine severe extinctions were in fact selective with respect to suture morphology.

During the early evolutionary history of the ammonoids, mass extinctions systematically removed the dominant subclade, resulting in the preferential survivorship of taxa possessing localized subsets of suture morphologies. Recoveries from these events were gradual and characterized by diversification within previously established morphological limits. In contrast, events during the late Carboniferous selectively eliminated outlying morphotypes and conserved modal morphotypes, while those in the Permian did the opposite: i.e. selectively eliminated the more common morphologies, conserving a variety of unrelated extremes. Morphologic recovery from these extinctions was rapid and characterized by exploration of new regions of morphospace; many innovations in suture morphology occurred during these post-extinction rebounds. Thus, while extinctions consistently influenced the evolution of the Paleozoic ammonoids, the dynamic of extinction selectivity and recovery varied significantly during the history of the group.