沉积岩经常含有牙齿状的小型微化石，被称为“牙形石元素”，其丰度是如此之大，以至于它们被普遍用来确定地层年代。
它们是咽部的构成部分，存在于被分类为“牙形石动物门”的像鳗鱼一样的软体动物的喉咙中。它们的组织机构很像脊椎动物牙齿，说明存在一个所谓的“inside out”演化模型，在其中牙齿起源于口腔内。
利用同步辐射X-射线断层扫描显微镜来比较在形态上相似的“真牙形石”元素和“拟牙形石”元素的微结构的一项新的研究，对这种解读提出了质疑。
该研究获得的数据表明，“牙形石动物门”和有颌脊椎动物的最后共同祖先没有矿化的骨骼组织。牙齿似乎是通过“生牙能力”在颌骨起源后不久从外部真皮向内部上皮的延伸而演化形成的。在这一模型中，“牙形石元素”与牙齿之间的相似性是平行演化的一个经典例子。（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 lineage1， 2， 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 euconodonts7， 8， 9， 10， 11 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.
