The phylogenetic relationships and evolutionary history of the armoured dinosaurs (Ornithischia: Thyreophora)

The armoured dinosaurs (Thyreophora) were a significant component of Mesozoic terrestrial ecosystems, appearing in the earliest Jurassic and surviving until the latest Cretaceous, and fossils of the group have been found on all continents, including Antarctica. However, a patchy fossil record and highly modified anatomy has hindered reconstruction of their evolutionary history. For example, the relationships of many early-diverging taxa are labile and the degree of convergence between the two major clades, Ankylosauria and Stegosauria, has been difficult to assess. There has never been a species-level phylogenetic analysis of the thyreophoran dinosaurs; until recently, the computational ability to analyse such a dataset did not exist and, consequently, the interrelationships of taxa within the group are debated. Here, we address these issues with a new phylogenetic dataset that includes the majority of named thyreophoran taxa (340 characters, 91 taxa). This dataset was analysed using equal- and implied-weights parsimony and Bayesian inference, and further explored using constraint trees and partitioned datasets. Stratigraphical congruence was used to identify a ‘preferred tree’ and these analyses reveal a novel hypothesis for thyreophoran relationships. The traditional ankylosaurian dichotomy is not supported: instead, four distinct ankylosaur clades are identified, with the long-standing ‘traditional’ clade Nodosauridae rendered paraphyletic. Ankylosauridae, Panoplosauridae, Polacanthidae and Struthiosauridae have distinct morphotypes, typified by <i>Euoplocephalus</i>, <i>Edmontonia/Panoplosaurus</i>, <i>Gastonia</i> and <i>Struthiosaurus</i>, respectively. <i>Isaberrysaura</i> is an early stegosaur and <i>Scelidosaurus</i> is a non-eurypodan. Many characters related to feeding and quadrupedality are coincident with the diversification of Eurypoda. Unstable taxa in the analyses are generally highly incomplete but other better-known taxa are also unstable, suggesting the need for taxonomic revisions. Partitioned datasets show a high degree of convergence in thyreophoran postcrania and that osteoderm characters do not contain a strong phylogenetic signal.

装甲恐龙（覆盾甲龙亚目，Thyreophora）是中生代陆地生态系统的重要组成类群，起源于侏罗纪早期，存续至白垩纪末期，该类群的化石已在包括南极洲在内的所有大陆被发现。然而，零散破碎的化石记录与高度特化的解剖结构，阻碍了其演化历史的重建。例如，诸多早期分化类群的系统发育关系不稳定，且甲龙下目（Ankylosauria）与剑龙下目（Stegosauria）这两个主要演化支之间的趋同程度难以评估。此前从未开展过针对覆盾甲龙亚目恐龙的物种级系统发育分析——直至近年，分析此类数据集所需的计算能力才得以实现，因此该类群内部类群的相互关系至今仍存在诸多争议。本研究依托一套涵盖绝大多数已命名覆盾甲龙类类群（含340个性状、91个分类单元）的全新系统发育数据集，对上述问题展开探讨。本数据集分别采用等权简约分析、隐含权重简约分析与贝叶斯推断开展分析，并通过约束树与分区数据集进一步探索类群间的演化关系。借助地层一致性指标筛选出“最优树”后，相关分析揭示了覆盾甲龙类系统发育关系的全新假说。传统甲龙类的二分法并不成立：研究共识别出四个独立的甲龙演化支，长期沿用的传统类群结节龙科（Nodosauridae）被恢复为并系群。甲龙科（Ankylosauridae）、潘氏甲龙科（Panoplosauridae）、多刺甲龙科（Polacanthidae）与突喙龙科（Struthiosauridae）各具有独特的形态型，分别以包头龙（Euoplocephalus）、埃德蒙顿甲龙/潘氏龙（Edmontonia/Panoplosaurus）、加斯顿龙（Gastonia）与突喙龙（Struthiosaurus）为代表。伊莎贝里龙（Isaberrysaura）属于早期剑龙类，肢龙（Scelidosaurus）不属于真足甲龙类（Eurypoda）。诸多与取食行为和四足行走相关的性状，与真足甲龙类的辐射演化高度契合。分析中系统发育位置不稳定的类群普遍保存度较差，但部分认知较为全面的类群同样存在位置不稳定的情况，提示亟需开展分类学修订。分区数据集分析结果显示，覆盾甲龙类的后躯干骨骼存在高度趋同演化，且皮内成骨（osteoderm）性状并未携带较强的系统发育信号。