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.学界已采用诸多手段推断已灭绝龟类的古生态学（paleoecology），其中包括前肢比例与壳骨组织学（shell bone histology）。不过，作为单参数壳体模型（one-parameter shell model）的壳体高宽比，因可靠性欠佳而被弃用，而壳体几何形态的更为复杂的特征则长期被忽视。本文采用一种更可靠的前视图壳体轮廓三参数几何模型（three-parameter geometric model），作为评估龟类古生态学的方法。基于该三参数壳体模型对现生龟类生态的预测精度，在区分陆生、半水生与水生三类生态类群时，与通过前肢比例得到的结果相当。而在区分两类（陆生与非陆生）时，预测准确率更高——这是因为水生与半水生龟类的轮廓往往极为相似。我们的模型将产自德国晚三叠世的基干龟类（stem turtle）强壮原龟（*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，该方法表明晚三叠世基干龟类涵盖了多样的生态型（ecotypes）。这意味着基干龟类（stem-group turtles）的生态分化可能极为迅速，或是在晚三叠世龟类基干谱系首次以化石形式出现之前，就已经经历了一段长期且目前尚不明确的隐秘分化过程。