THE EFFECTS OF PHYLOGENETIC AUGMENTATION IN ANALYSES OF PALEODIVERSITY: A CASE STUDY ON PERMO-TRIASSIC EUTHERIODONTS AMNIOTA: THERAPSIDA

Analyses of taxonomic richness and morphologic diversity (disparity) have been widely used to study evolutionary radiations during major biotic transitions. Some workers have argued that phylogeny should be used in such studies for a more complete picture than offered by a strict reading of the fossil record alone. Cladistic methods have been used to supplement studies of paleodiversity by allowing for the recognition of unsampled ghost lineages prior to the first appearances of taxa. We empirically assess the effects of such augmentation in analyses of both taxonomic richness and ecomorphological disparity of eutheriodont therapsids (therocephalians and cynodonts) across the Permo-Triassic boundary, to determine if phylogenetic trees alter our understanding of their diversity.

We performed traditional analyses of taxonomic and ecomorphologic diversity. We then performed phylogenetic analysis of all adequately known therocephalian and nonmammalian cynodont genera using both cladistic and stratocladistic methods, using the resulting trees to recalculate richness, disparity and rates of evolution.

Phylogenetic augmentation using cladistic methods raised the taxonomic richness at most intervals due to the presence of ghost lineages when compared to observed biostratigraphic ranges, whereas stratocladistic augmentation provided intermediate estimates of richness. Significantly, overall patterns were consistent regardless of the method, suggesting that in at least some groups, nonphylogenetic estimates of diversity are sufficient. This is particularly important for taxa for which phylogeny is poorly understood.

In general, taxonomic richness of Paleozoic eutheriodonts peaked at the Permo-Triassic boundary, followed by a steep decline into the Early Triassic. Conversely, ecomorphological disparity remained stable throughout the Late Permian, but dramatically increased into the Early Triassic, suggesting the extinction of intermediate forms. Rates of generic origination and extinction both increased into the Triassic, indicating higher turnover rates. If consistent among other taxa, higher turnover rates may have allowed for restructuring of Triassic communities and the consequent expansion of ecospace.