尖牙或骨甲,，哪一個(gè)是最先出現(xiàn)的呢？多年來(lái),，古生物學(xué)家認(rèn)為牙齒是歷史上出現(xiàn)的第一塊骨頭,，它是早期魚(yú)類(lèi)身上起保護(hù)作用的盔甲狀覆蓋物，這些物質(zhì)與牙齒的材料組成相似,。然而,，一項(xiàng)新研究顯示：事實(shí)恰恰相反。
研究工作將重點(diǎn)放在了遠(yuǎn)古時(shí)代的一群沒(méi)有下頜的動(dòng)物——牙形蟲(chóng)上,，這種生物消失于距今約2億年的晚三疊世時(shí)期,。這些鰻類(lèi)動(dòng)物的身體結(jié)構(gòu)中缺乏骨架支撐,，但是嘴里有極易石化的硬刺，質(zhì)地與牙本質(zhì)和搪瓷相似,。長(zhǎng)期以來(lái)人們認(rèn)為：魚(yú)類(lèi),、狗、恐龍以及人類(lèi)的牙齒都是由這樣的質(zhì)地組成的,，基于由內(nèi)而外的假說(shuō),，牙形蟲(chóng)嘴里的堅(jiān)硬結(jié)構(gòu)就是早期的牙齒雛形。
過(guò)去的研究工作證實(shí),，牙形蟲(chóng)長(zhǎng)出這些硬刺的過(guò)程與現(xiàn)代動(dòng)物生長(zhǎng)牙齒的過(guò)程驚人類(lèi)似,。然而英國(guó)布里斯托大學(xué)古生物學(xué)家Philip Donoghue卻認(rèn)為，它只是進(jìn)化給人的錯(cuò)覺(jué),。
利用X射線層析顯微鏡技術(shù),，研究人員揭示了化石的內(nèi)部結(jié)構(gòu)和組成。Donoghue和他的團(tuán)隊(duì)分析了早期牙形蟲(chóng),，并揭示出這種動(dòng)物的牙齒樣的硬刺是如何進(jìn)化的,。近日，他們?cè)凇蹲匀弧冯s志上報(bào)道稱(chēng)：在早期牙形蟲(chóng)嘴里發(fā)現(xiàn)的結(jié)構(gòu),，是單獨(dú)進(jìn)化的脊椎動(dòng)物牙齒,。
研究人員發(fā)現(xiàn)，現(xiàn)代動(dòng)物牙齒的生長(zhǎng)過(guò)程為牙釉質(zhì)分層覆蓋在牙本質(zhì)上,，后期的牙形蟲(chóng)也是通過(guò)類(lèi)似的方法形成硬刺的,，而早期的牙形蟲(chóng)根本就沒(méi)有牙釉質(zhì)樣的覆蓋物。這就意味著牙齒——正如今天人們所知道的那樣——在牙形蟲(chóng)從最終形成人類(lèi)的古生物中消失之前尚未進(jìn)化出來(lái),。(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,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.
