Abstract

Temnospondyls—a major component of Permian and Carboniferous terrestrial ecosystems—display great diversity in
skull shapes and proportions. To quantify and interpret this diversity, we conducted a geometric morphometric analysis
using 45 landmarks on the dorsal skull surface of 90 species with well-represented cranial material. Results show a
correlation between morphospace occupation and phylogenetic proximity of taxa for trees in which dvinosaurs and
dissorophoids are sister groups and join an edopoid-–eryopoid–basal archegosauriform clade. Most large groups of
Carboniferous and Permian temnospondyls occupy specific areas of morphospace. Nearest-neighbor analyses reveal
significantly greater taxon clustering than expected under either uniform or Gaussian null models. Size correlates
strongly with shape across the whole data set, highlighting the association of some features (short, broad snout; large
orbits) with small size. A significant relationship between size and shape is not observed in clades such as
branchiosaurids, dvinosaurs, and olsoniforms (the clade encompassing trematopids plus dissorophids). This suggests
that evolutionary allometry patterns vary across temnospondyls. In the case of branchiosaurids, this pattern may be
explained by the similar sizes and relatively conservative morphologies of the constituent species. Distance-based
disparity measures indicate that edopoids, eryopoids, and basal archegosauriforms make the largest contributions to
total disparity (reflecting the peripheral locations of some of the constituent taxa in morphospace), whereas
amphibamids, dissorophids, and trematopids make the smallest contributions. Disparity correlates strongly with
diversity within groups, suggesting that skull shape was not subject to character state exhaustion (decrease or cessation in
the acquisition of novel morphological conditions during clade evolution). The Kasimovian, Roadian, Wordian, and
Changhsingian are time intervals of high disparity despite their low diversity. We hypothesize that this pattern stems
from the fact that times of high diversity are characterized by larger areas of morphospace, which results in mean shapes
being located relatively close to the grand mean; in contrast, mean shapes for low-diversity stages are based on
incomplete morphospace samples. Many of these patterns are similar to those observed in previous analyses of
stereospondyls, suggesting that similar controls on skull shape may have operated throughout the history of
temnospondyls.