Evolutionary origin of higher taxa

我们今天认识到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，因为适应性，特别是头骨，可以叠加共享共同祖先的证据。化石记录可以帮助解决这个问题，通过保存具有较少适应性和更多一般特征的动物，这些特征存在于两个更高动物群体之间的共同祖先中，但化石记录本身在保存不同动物群体方面是不完整的。再加上骨骼系统（在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，两个不同的动物群可以纯粹偶然地在表面上彼此相似。 现代两栖动物（青蛙、蝾螈和细长无肢的无足目）的起源是研究这些困难的一个很好的案例。它们作为进化上不同的群体至少有2.7亿年的历史，但由于它们的骨骼又小又脆弱，化石记录非常少。当他们出现时，他们几乎是现代家庭的成员。使这个故事进一步复杂化的是，被认为是现代两栖动物祖先的古代四足动物化石具有许多在任何现存群体中都看不到的非常原始的特征（或者至少不容易识别）。因此，现代两栖动物起源的问题是非常有争议的，该领域的工作人员目前提出了三种不同的假设。 解决这个难题的一个方法是找到新的证据来证明共同祖先。当然，新的化石是新数据的极好来源，但在一个又一个的野外季节里，发现它们并不是经常性的。我们在实验室取得了巨大的成功，目前的发现补助金将支持我们继续下去，是通过深入挖掘我们已经拥有的证据。通过使用CT扫描，我们可以从现代和化石两栖动物的内部头骨解剖中提取新的信息。我们甚至可以研究大脑和内耳的印象，这告诉我们化石动物的过去生活，以及它们是否是现代两栖动物亲戚的好候选人。同时，我们也可以研究活体动物的发育。通过观察个体发育，以及通过变态协调的生长，我们可以推断出我们在化石记录中所看到的。最后，我们可以测试从两栖动物到其他类群的进化模式，看看它们的概括能力如何：早期的鳍到肢的四足动物，其他古老的化石类群，甚至更高级的类群。通过比较，我们可以看到，在更高级的群体（比如早期爬行动物）的起源中，哪些因素是不同的或相同的，从而了解它们在类似的选择压力下的进化是如何相似或不同的。

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.