Paleontological Museum of Liaoning, Shenyang Normal University, Shenyang, Liaoning 110034, China.
Nature. 2013 Aug 8;500(7461):163-7. doi: 10.1038/nature12429.
The earliest evolution of mammals and origins of mammalian features can be traced to the mammaliaforms of the Triassic and Jurassic periods that are extinct relatives to living mammals. Here we describe a new fossil from the Middle Jurassic that has a mandibular middle ear, a gradational transition of thoracolumbar vertebrae and primitive ankle features, but highly derived molars with a high crown and multiple roots that are partially fused. The upper molars have longitudinal cusp rows that occlude alternately with those of the lower molars. This specialization for masticating plants indicates that herbivory evolved among mammaliaforms, before the rise of crown mammals. The new species shares the distinctive dental features of the eleutherodontid clade, previously represented only by isolated teeth despite its extensive geographic distribution during the Jurassic. This eleutherodontid was terrestrial and had ambulatory gaits, analogous to extant terrestrial mammals such as armadillos or rock hyrax. Its fur corroborates that mammalian integument had originated well before the common ancestor of living mammals.
哺乳动物的最早进化和哺乳动物特征的起源可以追溯到三叠纪和侏罗纪时期已经灭绝的哺乳动物的近亲哺乳形类。在这里，我们描述了一种来自中侏罗世的新化石，它具有中耳下颌骨、胸腰椎的渐进过渡和原始的踝关节特征，但具有高度衍生的臼齿，具有高冠和多个部分融合的根。上臼齿具有与下臼齿交替咬合的纵向尖峰行。这种对植物的咀嚼专门化表明，在有袋类哺乳动物兴起之前，食草动物就在哺乳形类中进化了。尽管该新物种在侏罗纪期间的地理分布广泛，但它与 Eleutherodontid 分支共享独特的牙齿特征，该分支以前仅代表孤立的牙齿。这种 Eleutherodontid 是陆生的，具有可移动的步态，类似于现存的陆生哺乳动物，如犰狳或岩蹄兔。它的皮毛证实了哺乳动物的表皮早在现存哺乳动物的共同祖先之前就已经出现了。
