Data from: The cranial morphology, phylogenetic position and biogeography of the upper Permian dicynodont Compsodon helmoedi van Hoepen (Therapsida, Anomodontia)

Compsodon helmoedi is an obscure dicynodont originally described based on a single specimen from the upper Permian of the Karoo Basin. The discovery of three new specimens of Compsodon from the Luangwa Basin of Zambia and two additional specimens from South African museum collections facilitates a reassessment of its cranial morphology and phylogenetic position. Compsodon is diagnosed by an autapomorphic secondary palate morphology: medial depression at anterior end of premaxillary secondary palate; medial anterior palatal ridges absent; lateral anterior palatal ridges prominent and extend to posterior end of secondary palate; Y-shaped anterior end of posterior median palatal ridge; embayment of palatal rim anterior to caniniform process divided into two depressions by a posteromedially-trending ridge. Other important characters include the presence of maxillary ‘postcanines’ and a postcaniniform keel; long interpterygoid vacuity; palatine pad smooth and pierced by a foramen; pineal foramen flanked by swollen eminences of parietals; parietals fused and narrowly exposed between broad postorbitals on dorsal surface of skull; and pocket-like depression on lateral surface of maxilla. Phylogenetic analysis demonstrates that Compsodon is a member of Emydopoidea, but underscores major outstanding problems in our understanding of Permian dicynodont phylogeny that require further attention. The cranial morphology of Compsodon converges on that of cryptodonts like Tropidostoma, and as such represents a unique emydopoid morphotype. The stratigraphic range of Compsodon probably spans the upper Cistecephalus and lower Daptocephalus assemblage zones, and its presence in South Africa and Zambia reinforces a pattern of small Permian dicynodonts with wide geographical ranges in southern Gondwana.

海尔莫德美齿兽（Compsodon helmoedi）是一种鲜为人知的双齿兽（dicynodont），最初仅依据产自卡鲁盆地（Karoo Basin）上二叠统的单件标本被正式描述。此次在赞比亚卢安瓜盆地（Luangwa Basin）发现的3件美齿兽新标本，以及南非博物馆馆藏的另外2件标本，为重新评估其颅部形态与系统发育位置提供了重要支撑。美齿兽属的鉴别特征依托一处自衍征性的次生腭（secondary palate）形态：前颌骨次生腭前端存在内侧凹陷；内侧前腭脊缺失；外侧前腭脊显著且延伸至次生腭后端；后正中腭脊的前端呈Y形；犬齿突前方的腭缘凹陷被一条向后内侧延伸的脊分割为两个独立凹陷。其他关键鉴别特征还包括：上颌骨具“后犬齿”结构与后犬齿状龙骨；翼间孔（interpterygoid vacuity）狭长；腭垫（palatine pad）光滑且带有穿孔；松果孔（pineal foramen）两侧被顶骨（parietals）的隆起隆脊环绕；顶骨愈合，在颅骨背侧的宽大眶后骨（postorbitals）之间呈狭窄暴露状态；上颌骨（maxilla）外侧表面带有口袋状凹陷。系统发育分析表明，美齿兽属属于龟甲兽超科（Emydopoidea），同时也凸显了当前学界对二叠纪双齿兽系统发育认知中存在的诸多亟待深入研究的核心问题。美齿兽的颅部形态与窄颌兽（Tropidostoma）这类隐齿兽类（cryptodonts）高度趋同，因此代表了一种独特的龟甲兽超科形态类型。美齿兽的地层延限大致覆盖上希斯兽组合带（Cistecephalus assemblage zone）与下达普头兽组合带（Daptocephalus assemblage zone），其在南非与赞比亚的分布进一步印证了冈瓦纳（Gondwana）南部小型二叠纪双齿兽具有广泛地理分布的分布模式。