新疆东北部晚侏罗世一新的柱齿兽

描述了哺乳纲柱齿兽目柱齿兽科(Docodontidae,Docodonta,Mammalia)一新属种——孙氏尖钝齿兽(Acuodulodon sunae gen.et sp.nov.)。标本产于新疆东北部准噶尔盆地五彩湾地区上侏罗统牛津阶石树沟组上部(159～161 Ma),为一不完整左下颌骨及齿列。新属下臼齿具柱齿兽类典型特征:齿尖b位于齿尖a前方;齿尖c位于齿尖a后舌侧;齿尖a前舌侧发育有齿尖g。不同于其他柱齿兽,新属下臼齿无齿尖e和齿脊b-e。齿尖g和齿脊b-g很快被磨蚀掉而齿尖a和c却能保持尖锐状态,表明该动物的臼齿在生活中具备并保持切割和碾压双重功能。基于下臼齿性状特征的系统发育分析表明,柱齿兽目作为一单系类群具有显著的鉴定特征。其中尖钝齿兽和Itatodon+(Simpsonodon,Castorocauda+(Tegotherium+Sibirotherium))形成一单系子类群;但tegotheriids各分子未形成独立于柱齿兽科的单系类群。尖钝齿兽的下颌齿骨亦为典型的柱齿兽类型。齿骨内侧下部近腹缘有浅的齿后骨槽和宽大的内侧脊,但两者未延伸到下颌关节髁的基柄部。这表明尖钝齿兽的齿后骨与齿骨的连接比摩根齿兽类更为松散,其中耳在进化上更接近真正意义上的哺乳动物中耳。

A NEW LATE JURASSIC DOCODONT (MAMMALIA) FROM NORTHEASTERN XINJIANG,CHINA

A new genus and species of docodontid(Docodonta,Mammalia),Acuodulodon sunae,represented by a partial left lower jaw and dentition,is described.It is from the upper part of the Shishugou Formation in the Wucaiwan area of the Junggar Basin in northeastern Xinjiang,China,with an estimated age of 159～161 Ma(Oxfordian,early Late Jurassic).The new mammal is typical of docodonts in having a cusp b in front of cusp a,a cusp c distolingual to cusp a and a cusp g mesiolingual to cusp a on lower molariforms.Differing from other docodonts,it has no cusp e or crest b-e developed on lower molariforms.Unique among docodonts.cusps a and c of the new animal maintained their sharpness while cusp g and crest b-g wore away fast,indicating that both shearing and crushing/grinding occurred in the chewing cycle and probably last for most of the life span of the animal.Phylogenetic analysis of a data matrix with 24 lower molariform characters across 15 taxa recovers a monophyletic Docodonta,which has distinct diagnostic characters in lower molariforms.Within docodonts,Docodon and Borealestes are successively basal to other docodonts;Acuodulodon and Itatodon +(Simpsonodon,Castorocauda+(Tegotherium+Sibirotherium))form a monophyletic clade.Tegotheriid genera are nested within Docodontidae,but a monophyletic tegotheriid clade composed of Tegotherium,Sibirotherium,hatodon,and Tashkumyrodon is not recovered.The dentary of Acuodulodon is typical of docodonts.It has a shallow postdentary trough and a wide and sharp medial ridge,both of which do not extend onto the medial side of the condylar peduncle,indicating looser contact between postdentary bones and the dentary than in morganucodontids,a more derived condition in the evolution toward the definitive mammalian middle ear.