DID END-PERMIAN TETRAPODS GASP THEIR LAST BREATHS? TESTING HYPOXIA AS AN EXTINCTION MECHANISM WITH DICYNODONT THERAPSIDS (SYNAPSIDA)
Dicynodont therapsids are abundant and well-studied Late Permian and Early Triassic tetrapods. Because at least two major Permian lineages survived into the Triassic, the clade provides an ideal natural test of whether selection for increased respiratory efficiency occurred. We measured the area of the internal nares and the length of the secondary palate in over 250 Permian and Triassic dicynodont specimens to test Retallack et al.s hypothesis that the internal nares of Triassic dicynodonts were relatively smaller than those of Permian forms (creating a less obstructed airway), as well as the alternative hypothesis that there was no significant difference in nares size between Permian and Triassic taxa. Statistical comparisons were carried out in a phylogenetic context using independent contrasts to account for similarities caused by common ancestry. Our preliminary results imply that nares size is influenced by phylogeny, and may have increased across the boundary. These findings may be consistent with a hypoxia kill mechanism, but also could be correlated with other functional changes in the dicynodont skull.