Evolutionary origin of higher taxa

我们今天认识的四足动物群体的进化起源往往笼罩在神秘之中。今天活着的动物在解剖学上是独特的，但这种不同的解剖学可能会使推断关系变得困难，因为适应，特别是头骨的适应，可能会重叠共同祖先的证据。化石记录可以帮助解决这个问题，方法是保存适应较少的动物，以及更多存在于两个较高动物群体的共同祖先中的普遍特征，但化石记录本身在保存不同动物群体方面是参差不齐的。除此之外，骨骼系统(四足化石中保存着共同祖先的证据)是有限的，如果有足够的时间，两种不同的动物群体可能纯粹是偶然的从表面上看彼此相似。

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.