Müller Gerd B, Alberch Pere
Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138.
J Morphol. 1990 Feb;203(2):151-164. doi: 10.1002/jmor.1052030204.
Crocodilians and birds are the extant representatives of a monophyletic taxon known as archosaurs. Their limbs are highly derived in terms of reduction in number of skeletal elements in both the carpus and the tarsus. It is necessary to have a detailed description of crocodilian limb ontogeny to address the evolutionary issue dealing with the origin and organization of the avian limb. In this paper, we present an analysis of the early development of the crocodilian limb skeleton. Contrasting with earlier observations, we redefine the number and composition of carpal, tarsal, and phalangeal elements. This ontogenetic information is then used to introduce a revision of the homologies of the skeletal elements in the crocodilian limb. Some invariances are pointed out in the developmental organization of tetrapod limbs and this evidence serves to readdress several issues concerning the evolution of the avian limb. We present further embryological data in support of the hypothesis that digits 2-3-4 are the components of the wing skeleton in birds. In general, our comparative survey indicates that the elements that appear late in ontogeny are the ones lost in phylogeny. By comparing turtle (primitive) limb development with crocodilian and bird development, we propose a hypothesis in which the derived skeletal patterns found in crocodilians and birds have originated by a heterochronic process of paedomorphosis.
鳄鱼和鸟类是被称为主龙类的一个单系类群的现存代表。它们的四肢在腕骨和跗骨的骨骼元素数量减少方面高度特化。有必要对鳄鱼肢体个体发育进行详细描述，以解决与鸟类肢体起源和组织相关的进化问题。在本文中，我们对鳄鱼肢体骨骼的早期发育进行了分析。与早期观察结果不同，我们重新定义了腕骨、跗骨和指骨的数量和组成。然后，这些个体发育信息被用于对鳄鱼肢体骨骼元素的同源性进行修订。我们指出了四足动物肢体发育组织中的一些不变性，这些证据有助于重新审视一些与鸟类肢体进化有关的问题。我们提供了进一步的胚胎学数据，以支持2-3-4指是鸟类翅膀骨骼组成部分的假说。总的来说，我们的比较研究表明，在个体发育后期出现的元素是在系统发育中丢失的元素。通过将龟（原始）肢体发育与鳄鱼和鸟类发育进行比较，我们提出了一个假说，即鳄鱼和鸟类中发现的特化骨骼模式是通过幼态持续的异时过程起源的。
