摘要:根据一保存基本完整的骨架建立了始鳍龙类的一新属、种——纤细滇美龙(Dianmeisaurus gracilis gen. et sp. nov.)。标本采自云南省罗平县中三叠统关岭组II段,为体态细长的小型物种。新属种与同产于该地区的利齿滇东龙(Diandongosaurus acutidentatus Shang et al., 2011)和丁氏滇肿龙(Dianopachysaurus dingi Liu et al., 2011a)身体各部位比例和个体大小接近,头骨均具有吻部两侧不收缩、眼眶显著大于上颞孔的特征。但新种眼眶间距要明显小于上颞孔间距,下颌关节与枕髁位于同一水平位置,锁骨前外侧缘的锁骨前突粗大而厚实,尺骨近端明显宽于远端,有比后两种更多的荐前椎(三属种荐前椎数分别为41、38、39枚)。此外,与滇东龙相对比,新种眶前区短于眶后区,前额骨和后额骨不相交。同时,新种下颌联合短,夹板骨参与下颌联合构成。新种前颌和下颌前部齿虽然也为獠齿(犬齿状齿),但未呈显著增大。头后骨骼对比,虽然新种和滇东龙均具有标志性的锁骨前外侧突,但后者的锁骨前外侧突尖细。与滇肿龙相对比,新种尾前部尾肋细长且未见明显加宽肿胀、仅具一个骨化的远侧腕骨、距骨为圆形且不具有近端凹入。去除数据不全的清镇龙、广西龙和三桥龙后的鳍龙类分支系统学分析表明,滇美龙与滇东龙互为姊妹群,同时它们与贵州龙、马家山龙和滇肿龙一起构成了一仅由在中国发现的属种组成的单系类群。这一单系类群与幻龙类的亲缘关系近于它们与欧洲肿肋龙类(Dactylosaurus, Anarosaurus, Serpianosaurus和Neusticosaurus)的亲缘关系。三种小型始鳍龙类的发现,指示云南罗平地区在三叠纪安尼期中晚期时鳍龙类是一类有较高分异度的海生爬行动物。
A new small-sized eosauropterygian (Diapsida: Sauropterygia) from the Middle Triassic of Luoping, Yunnan, southwestern China
SHANG Qing-Hua, LI Chun
Abstract A new eosauropterygian, Dianmeisaurus gracilis gen. et sp. nov. is described based on a nearly complete skeleton from the Member II of Guangling Formation (Anisian) of Luoping County, Yunnan Province. It is a small-sized (with a total length of less than 50 cm) sauropterygian with a slender body. This new species is similar superficially to two other smallsized eosauropterygians, Diandongosaurus Shang et al., 2011 and Dianopachysaurus Liu et al., 2011a, which were also collected from the same stratum of Luoping, in the body proportion, the skull with no contracted snout, and an oval orbit extremely larger than supratemporal fenestra. However, the new species is characterized by an extremely narrowed interorbital septum, the mandibular articulation at the level of occipital condyle, the big and stout anterolateral process of the clavicle, the proximal part of the ulna much wider than the distal end, and the presence of 41 presacral vertebrae. Furthermore, the new species differs from Diandongosaurus in having the preorbital region shorter than the postorbital region, the prefrontal with no contact of the postfrontal along the dorsal margin of the orbit, the short mandibular symphysis with the entrance of the splenial, and the premaxillary and anterior dentary teeth fang like but not king-sized. Additionally, the anterolateral process of the clavicle is very sharp and slender in Diandongosaurus. Compared with the Dianopachysaurus, the new species is different in having no pachyostosis of the caudal ribs, no more than three carpal ossifications, and the rounded astragalus. Our phylogenetic analyses suggest that Dianmeisaurus is probably the sister group of Diandongosaurus. The two genera, together with the Majiashanosaurus, Keichousaurus, and Dianopachysaurus are grouped in a monophyletic clade and phylogenetically more closely related to the Nothosauroidea than the European pachypleurosaurs (Dactylosaurus, Anarosaurus, Serpianosaurus, and Neusticosaurus). The coexist of three small-sized eosauropterygians indicates that sauropterygians were a highly diversified group of marine reptiles in the Luoping fauna, Yunnan Province during the early Middle Triassic.
Key words Luoping, Yunnan; Middle Triassic, Anisian; eosauropterygian, small sized marine reptile