Department of Geology and Geophysics, Yale University, New Haven, CT 06511, USA.
Biol Lett. 2012 Dec 23;8(6):1028-31. doi: 10.1098/rsbl.2012.0462. Epub 2012 Jul 18.
The turtle shell and the relationship of the shoulder girdle inside or 'deep' to the ribcage have puzzled neontologists and developmental biologists for more than a century. Recent developmental and fossil data indicate that the shoulder girdle indeed lies inside the shell, but anterior to the ribcage. Developmental biologists compare this orientation to that found in the model organisms mice and chickens, whose scapula lies laterally on top of the ribcage. We analyse the topological relationship of the shoulder girdle relative to the ribcage within a broader phylogenetic context and determine that the condition found in turtles is also found in amphibians, monotreme mammals and lepidosaurs. A vertical scapula anterior to the thoracic ribcage is therefore inferred to be the basal amniote condition and indicates that the condition found in therian mammals and archosaurs (which includes both developmental model organisms: chickens and mice) is derived and not appropriate for studying the developmental origin of the turtle shell. Instead, among amniotes, either monotreme mammals or lepidosaurs should be used.
龟壳以及肩带与胸腔内部或“深层”的关系，让现代动物学家和发育生物学家困惑了一个多世纪。最近的发育和化石数据表明，肩带确实位于壳内，但在胸腔之前。发育生物学家将这种方位与在老鼠和鸡等模式生物中发现的方位进行了比较，它们的肩胛骨位于胸腔的侧面。我们在更广泛的系统发育背景下分析了肩带相对于胸腔的拓扑关系，并确定在龟鳖类中发现的情况也存在于两栖动物、单孔目哺乳动物和有鳞目爬行动物中。因此，可以推断出位于胸腔前的垂直肩胛骨是基础羊膜动物的条件，这表明在合弓类哺乳动物和蜥形类动物（包括发育模型生物：鸡和老鼠）中发现的情况是衍生的，不适合研究龟壳的发育起源。相反，在羊膜动物中，应该使用单孔目哺乳动物或有鳞目爬行动物。
