Combing Paleontology and Developmental Biology to Understand Skull Evolution in Turtles

陆生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，显示出主要群体之间的巨大差异，如哺乳动物、蜥蜴、鸟类、鳄鱼和青蛙。这种多样性的起源--包括不同的头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，它们不会重新采样任何现存陆地脊椎动物的头骨形状，尽管最近被假设为与现存的爬行动物群体关系密切。为了了解海龟的颞骨区域和起源，提出了一种形态发生学方法来研究古生物和发育的完整性。首先，将以分支的意义对颞骨区域的解剖进行编码，以重建该区域的进化。因此，化石和胚胎学数据都将被观察到。将定量分析骨骼进化和发育过程中的几何形态变化，以阐明导致一种形态类型的途径，并推断系统发育和功能相关性。微型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。在骨骼开始形成之前，将对基因表达进行分析，以阐明胚胎头骨的解剖结构。从发育角度来看，将检测到导致其中一种形态类型的共同祖先和派生特征。通过研究化石群体，他们的时间区域的多样性甚至比现存的群体更大，进化变化的详细场景将被检测到。通过对龟类形态特征的研究，将对该类群的系统发育地位作出准确的系统发育结论，这对从整体上理解陆地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能够对总体发育有一个复杂的洞察。在此背景下，胚胎畸形--如在人类中看到的--将从发育角度得到解释，它们对进化变化的意义将得到推导。

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.