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.

本文记述了一种早期犬齿兽类（cynodont）的新类群：Abdalodon diastematicus gen. et sp. nov.（新属新种）。该新类群仅以一件产自南非卡鲁盆地（Karoo Basin）博福特群（Beaufort Group）上二叠统托皮多斯托姆组合带（Tropidostoma Assemblage Zone, AZ）的头骨标本为代表，此前该标本曾被归入Delaharpe原犬齿兽（Procynosuchus delaharpeae）。Abdalodon 与 Procynosuchus delaharpeae 的区别在于：仅具4枚上门齿与3枚下门齿、后犬齿数量更少、犬齿齿根后方的上颌骨外侧表面发育有一处显著凹陷、下颌联合（mandibular symphysis）较高并形成明显的“颏部”、吻部相对较短，以及眶间区域的骨缝交互镶嵌程度较弱甚至完全缺失。上述特征与仅分布于托皮多斯托姆组合带的另一犬齿兽类——纤细颌兽（Charassognathus gracilis）的特征高度吻合。Abdalodon 与 Charassognathus gracilis 的区别则在于：其后犬齿形态、扩大的咬肌窝（masseteric fossa）、内缩的后犬齿齿列、更宽阔的眶间区域，以及比例更短的颞区。最为显著的是，Abdalodon 的上颌骨与齿骨的犬齿与后犬齿之间均发育有较长的齿隙（diastema）；而 Charassognathus gracilis 仅在上颌犬齿与后犬齿间存在短齿隙，下颌则无此结构，Procynosuchus delaharpeae 的齿列则完全不存在齿隙。系统发育分析（phylogenetic analysis）显示，Abdalodon 与 Charassognathus gracilis 互为姊妹群（sister-taxon），二者共同构成一个新演化支——颌兽科（Charassognathidae fam. nov.），位于犬齿兽类的基部位置。该类群代表了此前未被认知的小型二叠纪犬齿兽类辐射演化事件。尽管体型小巧，Abdalodon 与 Charassognathus gracilis 的正模标本（holotype）大概率代表成年个体，这表明犬齿兽类的早期演化可能以小型体型为框架，这也解释了该类群二叠纪化石记录较为匮乏的原因。