我們關(guān)于脊椎動物演化中最早階段的知識,，僅限于我們能從早期脊索動物留下的化石中收集到什么。由于這些動物是軟體的,，沒有骨頭和甲殼,，所以它們留下的東西非常少，主要是相對較少的,、保存極為完好的標(biāo)本,。對這些化石進(jìn)行解讀是有困難的。
現(xiàn)在,，僅有這些化石似乎還不夠,，對文昌魚和幼七鰓鰻（早期軟體脊索動物親緣關(guān)系最近的現(xiàn)存物種）的現(xiàn)代標(biāo)本的腐爛所做的一項研究表明，腐爛過程中典型脊索特征的丟失不是隨機(jī)的,，在種系發(fā)生方面更有信息價值的特征最易丟失,。已經(jīng)固化到化石記錄中的這種異常情況，會導(dǎo)致人們偏向于將脊索動物化石在演化樹上放錯位置。如果這種由腐爛導(dǎo)致的偏見廣泛存在,，我們從保存極為完好的軟組織的化石記錄所了解到的很多重要演化事件將需要認(rèn)真的重新評估,。（生物谷Bioon.com）
生物谷推薦原始出處：
Nature 463, 797-800 (11 February 2010) | doi:10.1038/nature08745
Non-random decay of chordate characters causes bias in fossil interpretation
Robert S. Sansom1, Sarah E. Gabbott1 & Mark A. Purnell1
1 Department of Geology, University of Leicester, Leicester LE1 7RH, UK
Correspondence to: Mark A. Purnell1 Correspondence and requests for materials should be addressed to M.A.P.
Exceptional preservation of soft-bodied Cambrian chordates provides our only direct information on the origin of vertebrates1, 2. Fossil chordates from this interval offer crucial insights into how the distinctive body plan of vertebrates evolved, but reading this pre-biomineralization fossil record is fraught with difficulties, leading to controversial and contradictory interpretations3, 4. The cause of these difficulties is taphonomic: we lack data on when and how important characters change as they decompose, resulting in a lack of constraint on anatomical interpretation and a failure to distinguish phylogenetic absence of characters from loss through decay3. Here we show, from experimental decay of amphioxus and ammocoetes, that loss of chordate characters during decay is non-random: the more phylogenetically informative are the most labile, whereas plesiomorphic characters are decay resistant. The taphonomic loss of synapomorphies and relatively higher preservation potential of chordate plesiomorphies will thus result in bias towards wrongly placing fossils on the chordate stem. Application of these data to Cathaymyrus (Cambrian period of China) and Metaspriggina (Cambrian period of Canada) highlights the difficulties: these fossils cannot be placed reliably in the chordate or vertebrate stem because they could represent the decayed remains of any non-biomineralized, total-group chordate. Preliminary data suggest that this decay filter also affects other groups of organisms and that ‘stem-ward slippage’ may be a widespread but currently unrecognized bias in our understanding of the early evolution of a number of phyla.
