Data from: Shell geometry and habitat determination in extinct and extant turtles (Reptilia: Testudinata)

A number of means, including forelimb proportions and shell bone histology have been used to infer the paleoecology of extinct turtles. However, the height to width ratio of the shell (as a one-parameter shell model) has been dismissed because of its unreliability, and more complex aspects of shell geometry have generally been overlooked. Here we employ a more reliable, three-parameter geometric model of the shell outline in anterior view as a means to assess turtle paleoecology. The accuracy of predictions of extant turtle ecology based on our three-parameter shell model is comparable to that derived from forelimb proportions when distinguishing between three ecological classes (terrestrial, semi-aquatic and aquatic). Higher accuracy is obtained when distinguishing between two classes (terrestrial and non-terrestrial) since the contours of aquatic and semi-aquatic turtles are often very similar. Our model classifies Proterochersis robusta, a stem turtle from the Late Triassic of Germany, as non-terrestrial, and likely semi-aquatic. Our method, combined with inferences based on limb proportions, indicates a diverse range of ecotypes represented by Late Triassic stem turtles. This implies that the ecological diversification of stem-group turtles may have been rapid, or a substantial period of currently cryptic diversification preceded the first fossil appearance of the turtle stem lineage during the Late Triassic.已有多种手段被用于推断已灭绝龟类的古生态学（paleoecology），包括前肢比例与壳骨组织学（shell bone histology）分析。然而，作为单参数壳模型（one-parameter shell model）的壳高宽比因可靠性不足而被摒弃，而壳几何结构（shell geometry）中更为复杂的特征则普遍遭到忽视。本研究采用一种更可靠的、基于前视图（anterior view）壳轮廓的三参数几何模型（three-parameter geometric model），以此作为评估龟类古生态学的方法。当区分三类生态类群（陆生（terrestrial）、半水生（semi-aquatic）与水生（aquatic））时，基于本三参数壳模型的现生龟类（extant turtle）生态预测准确率，可媲美由前肢比例推导得到的准确率。当仅区分两类（陆生与非陆生（non-terrestrial））时，模型准确率更高——这是由于水生与半水生龟类的外形轮廓通常极为相似。我们的模型将产自德国晚三叠世（Late Triassic）的基干龟类（stem turtle）粗壮原颌龟（Proterochersis robusta）归类为非陆生类群，且大概率为半水生。结合基于肢骨比例的推断，本研究方法表明，晚三叠世的基干龟类已呈现出多样的生态型（ecotypes）。这意味着，基干龟类支系（stem-group turtles）的生态分化可能十分迅速，抑或是在晚三叠世龟类基干群谱系（turtle stem lineage）首次出现化石记录之前，该支系已经经历了一段相当长的、当前尚不明确的隐秘分化（cryptic diversification）时期。