Gess Robert W, Coates Michael I, Rubidge Bruce S
Bernard Price Institute (Palaeontology), School for Geosciences, University of Witwatersrand, Johannesburg 2050, South Africa.
Nature. 2006 Oct 26;443(7114):981-4. doi: 10.1038/nature05150.
Lampreys are the most scientifically accessible of the remaining jawless vertebrates, but their evolutionary history is obscure. In contrast to the rich fossil record of armoured jawless fishes, all of which date from the Devonian period and earlier, only two Palaeozoic lampreys have been recorded, both from the Carboniferous period. In addition to these, the recent report of an exquisitely preserved Lower Cretaceous example demonstrates that anatomically modern lampreys were present by the late Mesozoic era. Here we report a marine/estuarine fossil lamprey from the Famennian (Late Devonian) of South Africa, the identity of which is established easily because many of the key specializations of modern forms are already in place. These specializations include the first evidence of a large oral disc, the first direct evidence of circumoral teeth and a well preserved branchial basket. This small agnathan, Priscomyzon riniensis gen. et sp. nov., is not only more conventionally lamprey-like than other Palaeozoic examples, but is also some 35 million years older. This finding is evidence that agnathans close to modern lampreys had evolved before the end of the Devonian period. In this light, lampreys as a whole appear all the more remarkable: ancient specialists that have persisted as such and survived a subsequent 360 million years.
七鳃鳗是现存的无颌脊椎动物中在科学研究上最容易获取的，但它们的进化史却很模糊。与有甲胄的无颌鱼类丰富的化石记录形成对比（所有这些化石都可追溯到泥盆纪时期及更早），仅记录到两种古生代七鳃鳗，且都来自石炭纪时期。除此之外，最近一份关于保存精美的下白垩统七鳃鳗样本的报告表明，在中生代晚期就已存在解剖学上现代的七鳃鳗。在此，我们报告一种来自南非法门阶（晚泥盆世）的海洋/河口化石七鳃鳗，因其现代形态的许多关键特化特征已然具备，所以很容易确定其身份。这些特化特征包括首次发现的大型口盘证据、首次直接发现的围口齿证据以及保存完好的鳃篮。这种小型无颌类动物，里氏原七鳃鳗（Priscomyzon riniensis gen. et sp. nov.），不仅比其他古生代七鳃鳗样本更具传统意义上的七鳃鳗特征，而且还要早约3500万年。这一发现证明，与现代七鳃鳗亲缘关系接近的无颌类动物在泥盆纪末期之前就已进化出来。据此看来，七鳃鳗作为一个整体显得更加非凡：作为古老的特化物种，它们一直存续至今，并在随后的3.6亿年中存活了下来。
