沉積巖經(jīng)常含有牙齒狀的小型微化石,，被稱為“牙形石元素”，其豐度是如此之大,，以至于它們被普遍用來確定地層年代。
它們是咽部的構(gòu)成部分,，存在于被分類為“牙形石動物門”的像鰻魚一樣的軟體動物的喉嚨中,。它們的組織機構(gòu)很像脊椎動物牙齒，說明存在一個所謂的“inside out”演化模型,，在其中牙齒起源于口腔內(nèi),。
利用同步輻射X-射線斷層掃描顯微鏡來比較在形態(tài)上相似的“真牙形石”元素和“擬牙形石”元素的微結(jié)構(gòu)的一項新的研究，對這種解讀提出了質(zhì)疑,。
該研究獲得的數(shù)據(jù)表明,，“牙形石動物門”和有頜脊椎動物的最后共同祖先沒有礦化的骨骼組織。牙齒似乎是通過“生牙能力”在頜骨起源后不久從外部真皮向內(nèi)部上皮的延伸而演化形成的,。在這一模型中,，“牙形石元素”與牙齒之間的相似性是平行演化的一個經(jīng)典例子。（生物谷Bioon.com）
生物谷推薦的英文摘要
Nature doi:10.1038/nature12645
The origin of conodonts and of vertebrate mineralized skeletons
Duncan J. E. Murdock,，Xi-Ping Dong,，John E. Repetski，F(xiàn)ederica 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.
