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（Basal synapsids，即盘龙类pelycosaurs）构成哺乳动物干群谱系（mammalian stem lineage）的最基部类群，记录了从原始“似爬行类”基干羊膜动物（basal amniotes）向特化的似哺乳类兽孔类（therapsids）的演化过渡。该类群在石炭纪晚期与二叠纪早期（约3亿至2.71亿年前）的陆地生态系统中占据主导地位，演化出脊椎动物演化史上首次出现的大型陆地动物（体长3至6.5米）、高纤维植食性类群与大型掠食者（macropredators），同时伴随一系列体型较小的类群。尽管近年来已有诸多新发现，且对过去250年间采集的化石开展了重新评估，同时该类群对于理解陆地脊椎动物的早期辐射演化具有重要意义，但目前尚未针对基干合弓类的全球演化关系开展全面研究。本次构建的全新系统发育数据集包含45个分类单元（taxa，外加4个外类群outgroups与4个兽孔类类群）以及239个形态性状：其中147个为颅骨（cranial）相关性状，92个为颅后骨骼（postcranial）相关性状，分析结果显示基干合弓类高级演化支（clades）的亲缘关系存在显著不确定性。尽管颅骨数据支持当前学界的主流观点，即卡色龙类（Caseasauria）是最为基干的演化支，但颅后骨骼数据与完整数据集则显示，由蛇齿龙科（Ophiacodontidae）与巨蜥龙科（Varanopidae）构成的演化支占据这一位置。尽管高级演化支内部的亲缘关系得到了较好的统计支持，但这些演化支之间的亲缘关系支撑度却较低。这种不确定性的潜在来源在于该类群早期化石记录极度匮乏，导致确定高级演化支的近祖性状状态（plesiomorphic condition）带有较强的推测性；不过颅骨数据通常对应更短的幽灵支系（ghost lineages），或许应予以优先考量。本次全新数据集为多个历史上亲缘关系存疑的分类单元提供了得到较好支持的系统发育定位：三脊兽（Trichasaurus）属于卡色龙科类群（caseid）；卢佩龙（Lupeosaurus）属于基龙科类群（edaphosaurid）；基本颅龙（Basicranodon）与鲁蒂罗龙（Ruthiromia）则属于巨蜥龙科类群（varanopid）。