Jacobs L L, Winkler D A, Murry P A
Shuler Museum of Paleontology, Southern Methodist University, Dallas, TX 75275.
Proc Natl Acad Sci U S A. 1989 Jul;86(13):4992-5. doi: 10.1073/pnas.86.13.4992.
Major groups of modern mammals have their origins in the Mesozoic Era, yet the mammalian fossil record is generally poor for that time interval. Fundamental morphological changes that led to modern mammals are often represented by small samples of isolated teeth. Fortunately, functional wear facets on teeth allow prediction of the morphology of occluding teeth that may be unrepresented by fossils. A major step in mammalian evolution occurred in the Early Cretaceous with the evolution of tribosphenic molars, which characterize marsupials and placentals, the two most abundant and diverse extant groups of mammals. A tooth from the Early Cretaceous (110 million years before present) of Texas tests previous predictions (based on lower molars) of the morphology of upper molars in early tribosphenic dentitions. The lingual cusp (protocone) is primitively without shear facets, as expected, but the cheek side of the tooth is derived (advanced) in having distinctive cusps along the margin. The tooth, although distressingly inadequate to define many features of the organism, demonstrates unexpected morphological diversity at a strategic stage of mammalian evolution and falsifies previous claims of the earliest occurrence of true marsupials.
现代哺乳动物的主要类群起源于中生代，然而在那个时间间隔内，哺乳动物的化石记录总体上很匮乏。导致现代哺乳动物出现的基本形态变化通常由孤立牙齿的小样本所代表。幸运的是，牙齿上的功能性磨损面能够预测可能未被化石所呈现的咬合牙齿的形态。哺乳动物进化的一个重要阶段发生在白垩纪早期，三楔齿臼齿的进化出现，这是有袋类动物和胎盘类动物（现存数量最多、种类最丰富的两类哺乳动物）的特征。来自德克萨斯州白垩纪早期（距今1.1亿年前）的一颗牙齿检验了此前基于下臼齿对早期三楔齿牙列中上臼齿形态的预测。正如预期的那样，舌侧尖（原尖）最初没有剪切面，但牙齿的颊侧在边缘处有独特的齿尖，属于衍生（进化）形态。这颗牙齿虽然在界定该生物体的许多特征方面令人遗憾地不够充分，但它在哺乳动物进化的一个关键阶段展示了意想不到的形态多样性，并推翻了之前关于最早出现真正有袋类动物的说法。
