尖牙或骨甲，哪一个是最先出现的呢？多年来，古生物学家认为牙齿是历史上出现的第一块骨头，它是早期鱼类身上起保护作用的盔甲状覆盖物，这些物质与牙齿的材料组成相似。然而，一项新研究显示：事实恰恰相反。
研究工作将重点放在了远古时代的一群没有下颌的动物——牙形虫上，这种生物消失于距今约2亿年的晚三叠世时期。这些鳗类动物的身体结构中缺乏骨架支撑，但是嘴里有极易石化的硬刺，质地与牙本质和搪瓷相似。长期以来人们认为：鱼类、狗、恐龙以及人类的牙齿都是由这样的质地组成的，基于由内而外的假说，牙形虫嘴里的坚硬结构就是早期的牙齿雏形。
过去的研究工作证实，牙形虫长出这些硬刺的过程与现代动物生长牙齿的过程惊人类似。然而英国布里斯托大学古生物学家Philip Donoghue却认为，它只是进化给人的错觉。
利用X射线层析显微镜技术，研究人员揭示了化石的内部结构和组成。Donoghue和他的团队分析了早期牙形虫，并揭示出这种动物的牙齿样的硬刺是如何进化的。近日，他们在《自然》杂志上报道称：在早期牙形虫嘴里发现的结构，是单独进化的脊椎动物牙齿。
研究人员发现，现代动物牙齿的生长过程为牙釉质分层覆盖在牙本质上，后期的牙形虫也是通过类似的方法形成硬刺的，而早期的牙形虫根本就没有牙釉质样的覆盖物。这就意味着牙齿——正如今天人们所知道的那样——在牙形虫从最终形成人类的古生物中消失之前尚未进化出来。(生物谷Bioon.com)
生物谷推荐的英文摘要
Nature doi:10.1038/nature12645
The origin of conodonts and of vertebrate mineralized skeletons
Duncan J. E. Murdock,Xi-Ping Dong,John E. Repetski,Federica Marone, Marco Stampanoni& Philip C. J. Donoghue
Conodonts are an extinct group of jawless vertebrates whose tooth-like elements are the earliest instance of a mineralized skeleton in the vertebrate lineage inspiring the ‘inside-out’ hypothesis that teeth evolved independently of the vertebrate dermal skeleton and before the origin of jaws3, 4, 5, 6. However, these propositions have been based on evidence from derived euconodonts. Here we test hypotheses of a paraconodont ancestry of euconodonts using synchrotron radiation X-ray tomographic microscopy to characterize and compare the microstructure of morphologically similar euconodont and paraconodont elements. Paraconodonts exhibit a range of grades of structural differentiation, including tissues and a pattern of growth common to euconodont basal bodies. The different grades of structural differentiation exhibited by paraconodonts demonstrate the stepwise acquisition of euconodont characters, resolving debate over the relationship between these two groups. By implication, the putative homology of euconodont crown tissue and vertebrate enamel must be rejected as these tissues have evolved independently and convergently. Thus, the precise ontogenetic, structural and topological similarities between conodont elements and vertebrate odontodes appear to be a remarkable instance of convergence. The last common ancestor of conodonts and jawed vertebrates probably lacked mineralized skeletal tissues. The hypothesis that teeth evolved before jaws and the inside-out hypothesis of dental evolution must be rejected; teeth seem to have evolved through the extension of odontogenic competence from the external dermis to internal epithelium soon after the origin of jaws.
