Early synapsids from the Cisuralian (lower Permian) Pedra de Fogo Formation, Parnaíba Basin, Brazil: the first definitive South American “pelycosaurs”

The paleotropics of Euramerica provide nearly our entire picture of Permo–Carboniferous terrestrial tetrapod evolution. The geographic sampling bias inherent in this record obscures important events, such as the turnover between “pelycosaur”- and therapsid-dominated assemblages. The Cisuralian Pedra de Fogo Formation (PdF) of Brazil is a unique window into Gondwanan tropical to subtropical vertebrate assemblages of this time, including abundant chondrichthyans, actinopterygians, sarcopterygians, and temnospondyls, but the terrestrial tetrapod assemblage reported to date is limited to acleistorhinid stem reptiles and captorhinids. Here we describe the first two specimens of Synapsida from the PdF. One specimen is a natural mold of the cranial face of a posterior dorsal or anterior caudal vertebra. Its large size, amphicoelous centrum, and narrow neural arch with closely set, dorsolaterally angled anterior zygapophyses confirm that it is a synapsid. The second specimen is a natural mold of the medial surface of a partial maxilla. The large caniniform tooth, presence of a supracaniniform buttress lacking a dorsal process, and ventral convexity of the element permit its identification as a member of Sphenacodontia, but the straight, dagger-like caniniform is unusual within the clade. Phylogenetic analysis of the maxilla corroborates its sphenacodont affinities. Besides expanding the phylogenetic diversity of the PdF tetrapod assemblage, the specimens add a new size class (very large terrestrial amniote) and ecological guild (medium-size terrestrial faunivore) to the PdF terrestrial fauna. Together, these fossils hint at the presence of a complex terrestrial ecosystem between expansive, perennial water bodies of sufficient depth to support large aquatic predators.

欧美古大陆的古热带区几乎涵盖了我们对石炭-二叠纪陆生四足动物演化的全部认知。该记录固有的地理采样偏差，掩盖了诸如“盘龙类”与兽孔类占优势的动物群更替这类关键演化事件。巴西乌拉尔阶佩德拉杜福戈组（Pedra de Fogo Formation，简称PdF）是窥探该时期冈瓦纳热带至亚热带脊椎动物群组合的独特窗口，其产出丰富的软骨鱼类、辐鳍鱼类、肉鳍鱼类及离片椎类两栖动物，但目前已报道的陆生四足动物群组合仅局限于无孔螈科基干爬形类与大鼻龙科成员。本文首次报道了PdF产出的两件合弓纲（Synapsida）标本：第一件标本为一枚后段背椎或前段尾椎的椎体颅侧面自然铸型，其体型硕大、椎体双凹型，神经弓狭窄且前关节突间距紧凑、向背外侧倾斜，这些特征均可确认其属于合弓纲；第二件标本为一块部分上颌骨的内侧面自然铸型，该标本具有硕大的犬齿状齿，存在一处无背突的犬齿后隆脊，且骨块腹面凸起，这些特征可将其归入楔齿龙类（Sphenacodontia），但该演化支内直如匕首状的犬齿状齿较为罕见。对上颌骨的系统发育分析证实了其与楔齿龙类的亲缘关系。除了丰富PdF四足动物群组合的系统发育多样性外，这两件标本还为PdF陆生动物群新增了一个全新的体型等级（超大型陆生羊膜动物）与生态类群（中型陆生食肉动物类群）。综上，这些化石暗示，在足以支撑大型水生捕食者的广阔常年性水体之间，存在着复杂的陆生生态系统。