Data from: Interrelationships of basal synapsids: cranial and postcranial morphological partitions suggest different topologies

Basal synapsids (‘pelycosaurs’) form the basalmost portion of the mammalian stem lineage and document the transition from primitive ‘reptile-like’ basal amniotes to derived, mammal-like therapsids. They dominated terrestrial ecosystems of the latest Carboniferous and Early Permian (∼300–271 million years ago), producing large-bodied terrestrial animals (3–6.5 metres long), high-fibre herbivores, and macropredators for the first time in vertebrate history, alongside an array of smaller-bodied forms. Despite numerous recent discoveries and reassessments of fossils collected over the past 250 years, and despite their importance for understanding the early diversification of terrestrial vertebrates, a comprehensive assessment of global relationships among basal synapsids has not been undertaken. A new phylogenetic dataset comprising 45 taxa (plus four outgroups and four therapsids) and 239 characters (147 cranial; 92 postcranial) reveals considerable uncertainty in the relationships of higher clades of basal synapsids. Although cranial data support the current consensus that Caseasauria is the most basal clade, postcranial data and the full dataset suggest that a clade of Ophiacodontidae + Varanopidae occupies this position. Although relationships within higher clades are well supported, relationships among those clades are poorly supported. The likely source of this uncertainty lies in the exceptionally poor early record of the group, which renders determinations of the plesiomorphic condition of higher clades speculative, although cranial data are generally represented by shorter ghost lineages and should perhaps be favoured. The new dataset suggests well-supported phylogenetic placements for several taxa of historically uncertain affinities: Trichasaurus is a caseid; Lupeosaurus is an edaphosaurid; and Basicranodon and Ruthiromia are varanopids.基干合弓类（synapsids，即盘龙类pelycosaurs）构成哺乳动物茎干谱系的最基部类群，记录了从原始“类爬行动物”基干羊膜动物向特化的似哺乳兽孔类（therapsids）的演化过渡。该类群曾称霸晚石炭世至早二叠世（约3亿至2.71亿年前）的陆地生态系统，演化出脊椎动物历史上首批大型陆地动物（体长3至6.5米）、高纤维植食性类群与大型顶级捕食者，同时伴随一系列小型体型类群。尽管近年间有诸多新发现，且对过去250年间采集的化石进行了多次重新厘定与评估，同时基干合弓类对于理解陆地脊椎动物的早期辐射演化至关重要，但目前尚未有针对基干合弓类全球演化关系的全面系统性研究。本研究构建的全新系统发育数据集包含45个分类单元（外加4个外类群与4个兽孔类therapsids）以及239个形态特征（147个头骨特征、92个头后骨骼特征），结果显示基干合弓类高阶类群间的演化关系存在显著不确定性。尽管头骨数据支持当前主流观点，即卡色龙类（Caseasauria）是最基部的演化支，但头后骨骼数据与完整数据集则显示，由蛇齿龙科（Ophiacodontidae）与蜥代龙科（Varanopidae）构成的演化支占据了这一系统位置。尽管高阶类群内部的演化关系得到了较好的支持，但这些类群之间的亲缘关系支撑度极低。这种不确定性的潜在根源在于该类群早期化石记录极度匮乏，导致高阶类群的祖征状态推断带有推测性；不过头骨数据通常对应更短的幽灵谱系，因此或许更具参考价值。本全新数据集为多个历史上亲缘关系不明的分类单元提供了得到较好支持的系统发育位置：三尖颌龙属（Trichasaurus）隶属于卡色龙科（Caseidae）；卢佩龙属（Lupeosaurus）隶属于异齿龙科（Edaphosauridae）；而颅基龙属（Basicranodon）与露丝龙属（Ruthiromia）则隶属于蜥代龙科（Varanopidae）。