Burke Ann C, Alberch Pere
Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 01238.
J Morphol. 1985 Oct;186(1):119-131. doi: 10.1002/jmor.1051860111.
The long-standing controversies involving the number and homologies of the elements of the carpus and tarsus of turtles are reviewed from a developmental perspective. The analysis is based on a detailed description of the chondrogenesis of the carpus and tarsus in the species Chelydra serpentina and Chrysemys picta. The first stage described is the differentiation of a Y-shaped chondrogenetic condensation involving the humerus (femur)-radius/ ulna (tibia/fibula). This stage is followed by the early formation of a series of connected condensations off the distal end of the postaxial element (ulna or fibula). This linear array, which we refer to as the primary axis, comprises the ulnare-distal carpal 4-metacarpal 4 in the carpus and the fibulare-distal tarsal 4-metatarsal 4 in the tarsus. There are two precondensations that branch off the primary axis. The proximal one will soon form the intermedium while the distal one will generate a digital arch that will give rise sequentially to digits 3-2-1, in this order. Digit 5 is not part of the digital arch and forms as an independent condensation. We emphasize that chondrogenetic foci often form as "branches" off existing condensations. This well-defined pattern of connectivity is used to establish unambiguous homologies and allows comparisons with other vertebrates. We conclude that preaxial elements such as the radiale and tibiale are absent in the turtles examined and probably in all turtles. The observed proximal elements that form in the anterior region of the limb and that are often homologized as radiale or tibiale have clear connections to the postaxial elements. For this reason we argue that these elements should be homologized as centralia. Therefore, we find two chondrogenetic condensations in the tarsus and three in the carpus, which we consider centralia. They subsequently fuse with neighboring elements in a complex pattern. We also describe the variable presence of a late-developing element in the tarsus of Chelydra, which, to our knowledge, has never been described. We propose this element to be an atavistic pretarsale. Comparison of the chondrogenetic pattern in turtles with those described in the literature for other vertebrates indicates that there are two invariant patterns in all tetrapods with the exception of the urodeles. These are (1) the primary axis and (2) the digital arch.
从发育的角度对长期以来关于龟类腕骨和跗骨的元素数量及同源性的争议进行了综述。该分析基于对滑龟和彩龟腕骨和跗骨软骨形成的详细描述。所描述的第一阶段是涉及肱骨（股骨）-桡骨/尺骨（胫骨/腓骨）的Y形软骨形成凝聚的分化。此阶段之后是在轴后元素（尺骨或腓骨）远端形成一系列相连的凝聚物。这个线性排列，我们称之为主轴，在腕骨中包括尺侧腕骨远端4-第4掌骨，在跗骨中包括腓侧跗骨远端4-第4跖骨。有两个预凝聚物从主轴分支出来。近端的那个很快会形成间介骨，而远端的那个会产生一个指弓，该指弓将依次产生第3-2-1指，按此顺序。第5指不是指弓的一部分，而是作为一个独立的凝聚物形成。我们强调软骨形成灶通常作为现有凝聚物的“分支”形成。这种明确的连接模式用于确定明确的同源性，并允许与其他脊椎动物进行比较。我们得出结论，在所研究的龟类以及可能所有龟类中，不存在桡侧腕骨和胫侧跗骨等轴前元素。在肢体前部区域形成的、常被同源化为桡侧腕骨或胫侧跗骨的观察到的近端元素与轴后元素有明确的连接。因此，我们认为这些元素应同源化为中央骨。因此，我们在跗骨中发现两个软骨形成凝聚物，在腕骨中发现三个，我们认为它们是中央骨。它们随后以复杂的模式与相邻元素融合。我们还描述了滑龟跗骨中一个后期发育元素的可变存在情况，据我们所知，此前从未有过描述。我们认为这个元素是一个返祖的跗前骨。将龟类的软骨形成模式与文献中描述的其他脊椎动物的模式进行比较表明，除了有尾目动物外，所有四足动物都有两种不变的模式。这两种模式是：（1）主轴和（2）指弓。
