A critical analysis of skull osteology in Australian Agamidae with implications for the fossil record
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The Australian agamid radiation was long noted for homogeneity of morphological characters and remains one of the most poorly studied major clades of squamates. Anatomical studies of the various lineages of endemic Australian agamids lag far behind genetic studies and would greatly enhance our knowledge of the current status and evolution of this complicated and dynamic clade. Australian agamids are an ideal group with which to test hypotheses of squamate speciation, diversity, and disparity because of their relatively recent diversification into Australia, about 30 Million Years Ago and geographical constraints. The two chapters of this thesis address two aspects of my research. In Chapter 1, I compile and evaluate morphological characters previously suggested to be useful for identifying Agamidae. I evaluate these characters for intraspecific variation, ontogenetic influence, and sexual dimorphism, for which sample sizes approached 20: Ctenophorus caudicinctus, Ctenophorus isolepis, and Ctenophorus reticulatus. Within the invariant characters, six were invariant only in Ctenophorus caudincinctus, five were invariant only for Ctenophorus isolepis, and two were invariant only in Ctenophorus reticulatus. Morphological characters that varied within taxa (thus, excluding all invariant morphological characters) were statistically tested for covariance with ontogeny (using skull length as a proxy) and sexual dimorphism. Sixteen of the 60 characters measured varied for at least one taxon with ontogeny and nine varied with sex. In Chapter 2, I use X-ray computed tomography (CT) of two specimens to describe the skull of the Australian agamid Cryptagama aurita, a species known only from only four alcohol-preserved specimens. Cryptagama aurita appears to share a great number of skull characters with other desert-dwelling Australian agamids. I conclude that the information currently available for Australian Agamidae is inadequate to interpret the fossil record of Australian Agamidae. Any identifications and phylogenetic analyses are likely to be inaccurate because most of the characters proposed by previous authors are not demonstrably apomorphic. Ontogenetic age is an important source of variation for the three species of Ctenophorous I examined. Morphological variation of extant taxa must be understood in order to compare extant and extinct species to study the recent radiation of Australian Agamidae.