Data from: A new taxon of cynodont from the Tropidostoma Assemblage Zone (upper Permian) of South Africa, and the early evolution of Cynodontia

A new taxon of early cynodont, Abdalodon diastematicus gen. et sp. nov., is described. The new taxon is represented by a single skull from the upper Permian Tropidostoma Assemblage Zone (AZ) of the Beaufort Group (Karoo Basin, South Africa), which was previously referred to Procynosuchus delaharpeae. Abdalodon is distinguished from Procynosuchus by the presence of only four upper and three lower incisors, fewer postcanines, a well-developed depression on the lateral surface of the maxilla posterior to the canine root, a tall mandibular symphysis forming a distinct ‘chin’, a relatively short snout, and weak or absent interdigitation of sutures in the interorbital region. These features are shared with the only other Tropidostoma AZ cynodont, Charassognathus gracilis. Abdalodon is distinguished from Charassognathus by its postcanine morphology, the presence of an expanded masseteric fossa, inset postcanine tooth rows, a broader interorbital region, and proportionally shorter temporal region. Most strikingly, Abdalodon has a lengthy diastema between the canines and postcanines on both the maxilla and the dentary. Charassognathus has a short diastema between the upper canine and postcanines, but not the lowers, and diastemata are absent from the tooth rows of Procynosuchus. Phylogenetic analysis recovers Abdalodon as the sister-taxon of Charassognathus, forming a clade (Charassognathidae fam. nov.) at the base of Cynodontia. These taxa represent a previously unrecognized radiation of small-bodied Permian cynodonts. Despite their small size, the holotypes of Abdalodon and Charassognathus probably represent adults and indicate that early evolution of cynodonts may have occurred at small body size, explaining the poor Permian fossil record of the group.

本文记述了一类早期犬颌兽类（cynodont）的新分类单元——齿隙颌兽（Abdalodon diastematicus）gen. et sp. nov.。该新分类单元仅以南非卡鲁盆地（Karoo Basin）博福特群（Beaufort Group）上二叠统Tropidostoma组合带（Assemblage Zone，AZ）中的一件头骨标本为代表，该标本此前曾被归入德氏前犬颌兽（Procynosuchus delaharpeae）。齿隙颌兽与前犬颌兽属（Procynosuchus）的鉴别特征包括：仅具4枚上门齿与3枚下门齿，后犬齿（postcanine）数量更少；犬齿后方的上颌骨（maxilla）侧面发育有明显凹陷；下颌联合（mandibular symphysis）部高耸，形成清晰的“颏部”；吻部相对较短；眶间区（interorbital region）的骨缝交错叠覆较弱甚至缺失。上述特征与仅见于Tropidostoma组合带的另一类犬颌兽——纤细饰颌兽（Charassognathus gracilis）共享。齿隙颌兽与饰颌兽属（Charassognathus）的区别则在于：后犬齿形态存在差异；具扩大的咬肌窝（masseteric fossa）；后犬齿齿列内收；眶间区更宽；颞区比例更短。最为显著的是，齿隙颌兽的上颌骨与齿骨（dentary）的犬齿与后犬齿之间均存在修长的齿隙（diastema）。纤细饰颌兽仅在上犬齿与后犬齿间具短齿隙，下颌齿列则无此结构，而前犬颌兽的齿列完全不存在齿隙。系统发育分析（phylogenetic analysis）显示，齿隙颌兽与纤细饰颌兽构成姊妹群（sister-taxon），二者共同组成一个新的演化支——饰颌兽科（Charassognathidae fam. nov.），位于犬颌兽类的基部。这些类群代表了一类此前未被认知的小型二叠纪犬颌兽类辐射演化事件。尽管体型小巧，齿隙颌兽与纤细饰颌兽的正模标本（holotype）大概率代表成年个体，这表明犬颌兽类的早期演化可能发生在小型体型阶段，这也解释了该类群二叠纪化石记录较为匮乏的原因。