Combing Paleontology and Developmental Biology to Understand Skull Evolution in Turtles

陆地脊椎动物的头骨具有多样性的特点。特别是眼睛后面的区域，即颞区，在哺乳动物、蜥蜴、鸟类、鳄鱼和青蛙等主要群体之间显示出相当大的差异。这种多样性的起源-包括不同的颅骨开口和边缘骨减少-很少被理解。特别令人感兴趣的是，海龟没有重新采样任何活着的陆地脊椎动物的头骨形状，尽管它们最近被假设为与现存的爬行动物群体密切相关。为了了解海龟的颞部头骨区域和起源，提出了一种研究古生物和发育完整性的形态发生方法。颞区的解剖结构将在分支系统学意义上被编码，以重建该区域的进化。因此，将观察化石和胚胎学数据。骨骼的进化和发育中的几何形态变化将被定量分析，以说明导致任何一种形态型的途径，并推断系统发育和功能的相关性。显微计算机断层扫描和组织学技术将被应用于表征骨骼发育的进展。基因表达将被分析，以说明胚胎头骨解剖骨骼开始形成之前。从发展的角度来看，共同的祖先和衍生功能将被检测到，导致任何一个形态型。通过研究化石群体，它们在时间区域的多样性甚至比现存的群体更大，将发现进化变化的详细情况。通过对龟鳖类形态型的描述，可以对龟鳖类的系统发育位置做出准确的结论，这对从整体上理解陆生脊椎动物的系统发育具有重要意义。此外，遗传和骨骼发育的整合将使人们对头部发育有一个复杂的认识。在这种情况下，胚胎畸形-如在人类中看到的-将从发育的角度解释，并将推导出它们对进化变化的意义。

The skulls of land vertebrates are characterized by a large diversity. Particularly the region behind the eye, the temporal region, shows considerable differences between major groups, such as mammals, lizards, birds, crocodiles, and frogs. The origin of this diversity - including diverse skull openings and marginal bone reductions - is rarely understood. Of particular interest are turtles that do not resample the skull shape of any living land vertebrate, although they were recently hypothesized to be closely related to extant reptilian groups.In order to understand the temporal skull region and the origin of turtles, a morphogenetic approach to study paleontological and developmental integrity is proposed.Primarily, the anatomy of the temporal region will be coded in a cladistic sense to reconstruct the evolution of this region. Therefore, fossil as well as embryological data will be observed. Geometric morphometric changes in evolution and development of the bones will be analyzed quantitatively to illustrate the pathways leading to either one morphotype and to infer phylogenetic and functional correlations.Micro-computed tomography and histological techniques will be applied to characterize the progress of bone development. Gene expression will be analyzed to illustrate embryonic skull anatomy before the bones begin to form.From a developmental perspective, shared ancestral and derived features will be detected that lead to either one morphotype. By studying fossil groups, which had an even larger diversity of the temporal region than extant groups do, detailed scenarios for the evolutionary changes will be detected. By characterizing the turtle morphotype, a precise phylogenetic conclusion will be drawn for the phylogenetic position of this group that has important impact to understand land vertebrate phylogeny as a whole. Moreover, the integration of genetic and bone development will allow a complex insight to head development in general. In this context, embryological malformations - as seen in humans - will be interpretable from a developmental perspective and their meaning for evolutionary changes will be derived.