KAMMERER, Christian F.; Univ. of Chicago: Temporal patterns in non-mammalian synapsid morphospace

Non-mammalian synapsids (commonly called �mammal-like reptiles�) were the dominant group of terrestrial vertebrates during the Permian and much of the Triassic Period (~280-200 million years ago). Although patterns of taxonomic distribution are well studied in the group, comparatively little attention has been given to quantifying changes in morphological disparity during their history. I undertook a geometric morphometric analysis of non-mammalian synapsids using 16 cranial landmarks. Digitized images of synapsid skulls were analyzed using Procrustes methods in the TPS suite of programs, with relative warps analysis used to determine the major components of shape-based variance in the data. The majority of variance was related to facial elongation and relative size of the sensory capsules. Synapsid evolution in the Permo-Triassic is characterized by three major pulses of morphospace exploration: the initial radiation of the �pelycosaurs� (non-therapsid synapsids), the radiation of the therapsids in the late Permian, and the radiation of two therapsid subgroups, the cynodonts and kannemeyeriiform dicynodonts, following the Permo-Triassic extinction. The Permian pulses follow a �boom and bust� pattern in which the maximal area of occupied morphospace is achieved very early in clade history, followed by extinction-mediated contraction, and subsequent reexploration of the initial morphospace by a survivor taxon. The Triassic radiation breaks from this in that major areas of morphospace lost in the Permo-Triassic extinction are never reexplored by the survivor taxa. Furthermore, the cynodonts gradually increase in cranial disparity throughout their history, rather than establishing an early range of morphotypes that is subsequently �filled in� by increases in taxonomic diversity.