Data from: Revision of the early crocodylomorph Trialestes romeri (Archosauria, Suchia) from the lower Upper Triassic Ischigualasto Formation of Argentina: one of the oldest-known crocodylomorphs

Trialestes romeri (Reig) is an early crocodylomorph from the Ischigualasto Formation (late Carnian – early Norian; Ischigualasto – Villa Unión Basin, Argentina) and one of the oldest-known members of this clade. Two specimens of this species are known, the holotype (PVL 2561) and a referred specimen (PVL 3889), both consisting of associated cranial and postcranial remains. These specimens are incomplete and poorly preserved thus leading previous authors to propose different phylogenetic hypotheses for the species. Trialestes romeri was originally interpreted as a basal dinosaur, subsequently considered to be a crocodylomorph with some dinosaurian characters (e.g. mesotarsal ankle joint, functionally tridactyl pes), and even proposed as a chimera representing two different genera. Some recent workers have mentioned and discussed Trialestes, but none have described it in detail or included it in a quantitative phylogenetic analysis. Here, we describe in detail all the material assignable to the species and test its phylogenetic relationships using a comprehensive data matrix focused on early archosaurs. We support the referral of PVL 3889 to Trialestes and reject the presence of a mesotarsal ankle joint in this specimen. We recovered Trialestes within Crocodylomorpha, closer to Crocodyliformes than Pseudhesperosuchus, Hesperosuchus, Dromicosuchus and Sphenosuchus. Several stratigraphical units worldwide approximately coeval with the Ischigualasto Formation, have also yielded early crocodylomorph remains, such as the upper Santa María (Brazil), Pekin (USA), and lower Maleri (India) formations. However, the crocodylomorph specimens recovered from those units are more incomplete, possess uncertain phylogenetic affinities, or are part of chimaerae. Therefore, Trialestes represents the most completely known of the earliest non-crocodyliform crocodylomorph taxa known to date.

罗梅里试喙鳄（Trialestes romeri，Reig）是产自阿根廷伊斯基瓜拉斯托—维拉乌尼翁盆地的伊斯基瓜拉斯托组（Ischigualasto Formation，晚卡尼期—早诺利期）的早期鳄形类（crocodylomorph），亦是该演化支已知最古老的成员之一。目前该物种共有两件已知标本：正模标本（PVL 2561）以及一件归入标本（PVL 3889），二者均保存有关联的头骨与颅后骨骼遗骸。这些标本均不完整且保存状态不佳，因此此前的研究者针对该物种提出了多种不同的系统发育假说（phylogenetic hypothesis）。罗梅里试喙鳄最初被认为是基干恐龙类群，后续被归入具有部分恐龙类特征的鳄形类（例如中跗关节（mesotarsal ankle joint）、功能性三趾型足部（functionally tridactyl pes）），甚至有学者提出其为代表两个不同属的嵌合体（chimera）。近期虽有学者提及并讨论过罗梅里试喙鳄，但尚无研究者对其进行详细描述，亦未将其纳入定量系统发育分析（quantitative phylogenetic analysis）中。本文中，我们对所有可归入该物种的标本材料进行了详细描述，并基于一套针对早期主龙类（early archosaurs）的综合数据矩阵，对其系统发育关系进行了检验。我们支持将PVL 3889归入罗梅里试喙鳄的观点，并否定该标本存在中跗关节的结论。我们的分析结果显示罗梅里试喙鳄属于鳄形超目（Crocodylomorpha），相较于伪黄昏鳄（Pseudhesperosuchus）、黄昏鳄（Hesperosuchus）、奔鳄（Dromicosuchus）以及楔齿鳄（Sphenosuchus），其与鳄形目（Crocodyliformes）的亲缘关系更近。全球范围内与伊斯基瓜拉斯托组时代大致相当的多个地层单元也产出了早期鳄形类遗骸，例如巴西的圣玛丽亚组上段（upper Santa María Formation）、美国的佩金组（Pekin Formation）以及印度的马勒里组下段（lower Maleri Formation）。但从这些地层中发现的鳄形类标本均更为破碎，其系统发育亲缘关系尚不明确，或是嵌合体的组成部分。因此，罗梅里试喙鳄是目前已知最完整的早期非鳄形目鳄形类类群。