Shuotheriids是侏罗纪哺乳动物,具有假蝶齿,其中假齿在下磨牙的三角之前,与Therian哺乳动物的摩擦模式(胎盘,有袋动物和亲属),其中距骨在三角1-4后面。假蝶齿的起源尚不清楚,模糊了我们对Shuotheriid亲和力和哺乳动物早期进化的认识1,5-9。在这里,我们报告了由两个骨骼标本代表的新侏罗纪shuotheriid。它们完整的假三七牙列允许使用序列同源性和牙齿咬合关系重新鉴定牙齿结构。与传统观点1,2,6,10,11相反,我们的发现表明,shuotheriids的牙齿结构可以与docodontans的牙齿结构同源,并部分支持docodontans和其他哺乳动物之间某些牙齿结构的同源陈述6,12。基于新证据的系统发育分析从三叶虫Ausktribosphenids(包括单叶树)中删除了shuotheriids,并将它们与docodontans聚集在一起,形成了一个新的进化枝,Docodontiformes,其特征是假三七细胞的特征。在系统发育中,docodontiforms和“holotherians”(Kuehneotherium,monotremes和therians)13通过舌舌扩大后牙以更有效的食品加工,独立于具有三象牙磨牙的Morganucodon样祖先进化。假三七模式经过了尖点半三角剖分阶段9,而三七模式及其前体经过了尖点三角剖分阶段。这两个不同的过程导致了复杂的牙齿结构和咬合模式,从而阐明了哺乳动物形态的最早多样化。
Shuotheriids are Jurassic mammaliaforms that possess pseudotribosphenic teeth in which a pseudotalonid is anterior to the trigonid in the lower molar, contrasting with the tribosphenic pattern of therian mammals (placentals, marsupials and kin) in which the talonid is posterior to the trigonid1-4. The origin of the pseudotribosphenic teeth remains unclear, obscuring our perception of shuotheriid affinities and the early evolution of mammaliaforms1,5-9. Here we report a new Jurassic shuotheriid represented by two skeletal specimens. Their complete pseudotribosphenic dentitions allow reidentification of dental structures using serial homology and the tooth occlusal relationship. Contrary to the conventional view1,2,6,10,11, our findings show that dental structures of shuotheriids can be homologized to those of docodontans and partly support homologous statements for some dental structures between docodontans and other mammaliaforms6,12. The phylogenetic analysis based on new evidence removes shuotheriids from the tribosphenic ausktribosphenids (including monotremes) and clusters them with docodontans to form a new clade, Docodontiformes, that is characterized by pseudotribosphenic features. In the phylogeny, docodontiforms and \'holotherians\' (Kuehneotherium, monotremes and therians)13 evolve independently from a Morganucodon-like ancestor with triconodont molars by labio-lingual widening their posterior teeth for more efficient food processing. The pseudotribosphenic pattern passed a cusp semitriangulation stage9, whereas the tribosphenic pattern and its precursor went through a stage of cusp triangulation. The two different processes resulted in complex tooth structures and occlusal patterns that elucidate the earliest diversification of mammaliaforms.