Sahni A
Centre of Advanced Study in Geology, Panjab University, Chandigarh, India.
Scanning Microsc. 1987 Dec;1(4):1903-12.
The evolution of enamel structure is dealt with here on the basis of fossil reptiles and mammals ranging from the Triassic to the present. The evidence suggests that prismatic enamel had developed in some therapsid reptiles and the mammal, Eozostrodon about 180 million years ago. For the next 100 million years, mammalian evolutionary history is sparingly documented and this is reflected in the poor record of enamel evolution during this period. The few Jurassic reptiles and mammals studied suggest a preprismatic structure. In the Late Cretaceous (80 to 65 million years ago) when the fossil record improves, mammalian enamel investigated from North American localities, are found to be prismatic; allotherian (multituberculate) and metatherian (marsupial) enamels are usually tubular, while eutherian (placental) ones are not. Prism structure in Tertiary mammals in general, conforms to that of their present day descendants, but there are discernible exceptions. The record of evolutionary change in Tertiary mammals is obscured by functional modifications related to biomechanical stresses. Enamel structure may be secondarily modified; similar in phylogenetically unrelated groups (eg., pauciserial enamel of early rodents) or dissimilar at the intra-familial level (eg., rodent families Ctenodactylidae and Ischryomyicae). Prismatic enamel is recorded from the tooth of a hatchling of the gavial, Gavialis gangeticus.
本文依据从三叠纪至今的化石爬行动物和哺乳动物，探讨了牙釉质结构的演化。证据表明，棱柱形牙釉质在约1.8亿年前的一些兽孔目爬行动物和哺乳动物——始祖兽中已经形成。在接下来的1亿年里，哺乳动物的进化史记录稀少，这反映在这一时期牙釉质进化的记录不佳。少数被研究的侏罗纪爬行动物和哺乳动物显示出一种棱柱前结构。在白垩纪晚期（8000万至6500万年前），当化石记录有所改善时，从北美地区研究的哺乳动物牙釉质被发现是棱柱形的；异兽亚纲（多瘤齿兽类）和后兽亚纲（有袋类）的牙釉质通常是管状的，而真兽亚纲（有胎盘类）的则不是。一般来说，第三纪哺乳动物的棱柱结构与其现代后代的结构一致，但也有明显的例外。第三纪哺乳动物进化变化的记录因与生物力学应力相关的功能改变而变得模糊。牙釉质结构可能会发生次生改变；在系统发育上不相关的类群中相似（例如，早期啮齿动物的少列棱柱形牙釉质），或者在科内水平上不同（例如，梳趾鼠科和异鼠科的啮齿动物）。在恒河鳄幼崽的牙齿中记录到了棱柱形牙釉质。
