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The evolutionary origin of groups of tetrapods that we recognize today are often shrouded in mystery. Animals alive today are distinctive in their anatomy, but this differing anatomy can make it difficult to infer relationships, because adaptation, especially of the skull, can overprint evidence of shared common ancestry. The fossil record can help solve this problem, by preserving animals that have fewer adaptations and more of the generalized features present in the common ancestor between two higher groups of animals, but the fossil record is itself patchy in its preservation of different groups of animals. Add to this the fact that the skeletal system (from which evidence of common ancestry is preserved in fossil tetrapods) is finite, and given enough time two different groups of animals can come to superficially resemble one another purely by chance.

The origin of modern amphibians (frogs, salamanders, and the elongate limbless caecilians) is an excellent case study of these difficulties. They have been evolutionary distinct groups for at least 270 million years, but because their skeletons are small and fragile, they have exceedingly poor fossil records. When they appear, they are nearly recognizable members of modern families. Complicating this story further, archaic fossil tetrapods suggested to be ancestral to modern amphibians have numerous very primitive features not seen in any living group (or at least not easily recognizable as such). As a result, the question of modern amphibian origins is very contentious, with three different hypotheses currently being offered by workers in the field.

One way out of this conundrum is by finding new areas of evidence for common ancestry. New fossils are excellent sources for new data, of course, but finding them is not regularly done from field season to field season. Where we in my lab have had great success, and what the present Discovery Grant will support us continuing, is by digging deeper into the evidence we already have. By using CT scans, we can pull new information out of the internal skull anatomy of modern and fossil amphibians. We can even study the impressions of the brains and inner ears, which tell us about the past life of fossil animals, and whether they are good candidates for relatives of modern amphibians. In parallel, we can also study the development of living animals. By looking at ontogeny, and what growth is coordinated through metamorphosis, we can extrapolate back to what we can see in the fossil record. Finally, we can test the patterns of evolution from studying amphibians to other groups to seee how well they generalize: early fin-to-limb tetrapods, other archaic fossil groups, and even more advanced groups. By comparison, we can see what factors vary or are the same in the origin of higher groups of, say, early reptiles, to see how their evolution parallels or differs in response to similar selective pressures.