Evolutionary origin of higher taxa

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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.