Evolutionary origin of higher taxa

我们今天所认识的四足动物群体的进化起源常常笼罩在神秘之中。今天活着的动物在解剖结构上是不同的，但这种不同的解剖结构使得推断它们之间的关系变得困难，因为适应，尤其是头骨的适应，可以重叠共同祖先的证据。化石记录可以帮助解决这个问题，因为它保存了一些适应能力较弱的动物，而在两个高级动物群体之间的共同祖先中存在着更多的普遍特征，但化石记录本身在保存不同动物群体方面是不完整的。除此之外，骨骼系统（四足动物化石中保存了共同祖先的证据）是有限的，如果有足够的时间，两个不同的动物群体完全可以偶然地在表面上彼此相似。

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.