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.诸多研究手段已被用于推断已灭绝龟类的古生态学（paleoecology），其中包括前肢比例（forelimb proportions）与壳骨组织学（shell bone histology）分析。然而，壳体的高宽比作为单参数壳体模型（one-parameter shell model）因可靠性不足而被弃用，而壳体几何形状的更复杂特征则长期被忽视。本研究采用一种更可靠的三参数几何模型（three-parameter geometric model），基于壳体的前视图轮廓，以此评估龟类的古生态学。当区分三类生态类群，即陆生（terrestrial）、半水生（semi-aquatic）与水生（aquatic）时，基于我们的三参数壳体模型所得到的现生龟类（extant turtle）生态预测精度，与通过前肢比例推导的结果相当。而当仅区分两类（陆生与非陆生）时，模型预测精度更高——这是因为水生与半水生龟类的外形轮廓往往极为相似。我们的模型将产自德国晚三叠世的基干龟类（stem turtle）粗壮原驰龟（Proterochersis robusta）归类为非陆生类群，且大概率属于半水生。结合基于肢体比例的推断，我们的方法表明，晚三叠世的基干龟类已呈现出多样的生态型（ecotype）。这意味着基干龟类的生态分化可能十分迅速，或者在晚三叠世龟类基干支系（stem lineage）的首个化石记录出现之前，就已经经历了一段长期且目前尚不明确的隐蔽分化期。