Foth Christian, Hedrick Brandon P, Ezcurra Martin D
SNSB, Bayerische Staatssammlung für Paläontologie und Geologie, München, Germany; Department of Earth and Environmental Sciences, Ludwig-Maximilians-Universität, München, Germany; Department of Geosciences, University of Fribourg/Freiburg, Fribourg, Switzerland.
Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA, United States; Department of Biology, University of Massachusetts, Amherst, MA, United States.
PeerJ. 2016 Jan 18;4:e1589. doi: 10.7717/peerj.1589. eCollection 2016.
Non-avian saurischian skulls underwent at least 165 million years of evolution and shapes varied from elongated skulls, such as in the theropod Coelophysis, to short and box-shaped skulls, such as in the sauropod Camarasaurus. A number of factors have long been considered to drive skull shape, including phylogeny, dietary preferences and functional constraints. However, heterochrony is increasingly being recognized as an important factor in dinosaur evolution. In order to quantitatively analyse the impact of heterochrony on saurischian skull shape, we analysed five ontogenetic trajectories using two-dimensional geometric morphometrics in a phylogenetic framework. This allowed for the comparative investigation of main ontogenetic shape changes and the evaluation of how heterochrony affected skull shape through both ontogenetic and phylogenetic trajectories. Using principal component analyses and multivariate regressions, it was possible to quantify different ontogenetic trajectories and evaluate them for evidence of heterochronic events allowing testing of previous hypotheses on cranial heterochrony in saurischians. We found that the skull shape of the hypothetical ancestor of Saurischia likely led to basal Sauropodomorpha through paedomorphosis, and to basal Theropoda mainly through peramorphosis. Paedomorphosis then led from Orionides to Avetheropoda, indicating that the paedomorphic trend found by previous authors in advanced coelurosaurs may extend back into the early evolution of Avetheropoda. Not only are changes in saurischian skull shape complex due to the large number of factors that affected it, but heterochrony itself is complex, with a number of possible reversals throughout non-avian saurischian evolution. In general, the sampling of complete ontogenetic trajectories including early juveniles is considerably lower than the sampling of single adult or subadult individuals, which is a major impediment to the study of heterochrony on non-avian dinosaurs. Thus, the current work represents an exploratory analysis. To better understand the cranial ontogeny and the impact of heterochrony on skull evolution in saurischians, the data set that we present here must be expanded and complemented with further sampling from future fossil discoveries, especially of juvenile individuals.
非鸟恐龙的头骨经历了至少1.65亿年的演化，其形状各异，从细长的头骨（如兽脚亚目的腔骨龙）到短而呈盒状的头骨（如蜥脚亚目的圆顶龙）。长期以来，人们认为有许多因素驱动着头骨形状的变化，包括系统发育、饮食偏好和功能限制。然而，异时性越来越被认为是恐龙演化中的一个重要因素。为了定量分析异时性对恐龙头骨形状的影响，我们在系统发育框架下使用二维几何形态测量学分析了五条个体发育轨迹。这使得我们能够对主要的个体发育形状变化进行比较研究，并评估异时性如何通过个体发育和系统发育轨迹影响头骨形状。通过主成分分析和多元回归，可以量化不同的个体发育轨迹，并评估它们是否存在异时事件的证据，从而检验之前关于恐龙异时性的假说。我们发现，蜥臀目的假想祖先的头骨形状可能通过幼态持续导致了基础蜥脚形亚目，主要通过超形态发生导致了基础兽脚亚目。幼态持续随后导致了弱角龙类到鸟兽脚亚目，这表明先前作者在进步虚骨龙类中发现的幼态趋势可能追溯到鸟兽脚亚目的早期演化。不仅由于影响恐龙头骨形状的因素众多，其形状变化很复杂，而且异时性本身也很复杂，在非鸟恐龙的演化过程中有许多可能的逆转。一般来说，包括早期幼体在内的完整个体发育轨迹的采样远低于单个成年或亚成年个体的采样，这是研究非鸟恐龙异时性的一个主要障碍。因此，目前的工作是一项探索性分析。为了更好地理解恐龙的颅骨个体发育以及异时性对恐龙头骨演化的影响，我们这里展示的数据集必须扩大，并通过未来化石发现的进一步采样，特别是幼年个体的采样来补充。
