美发现2.9亿年前两栖动物化石
生物谷报道：《自然》杂志刊载了一项最新考古化石研究显示，美国德克萨斯州发现的一具2.9亿年前两栖新物种化石，一种类似青蛙，长着短而粗壮尾部的动物，它们可以在陆地行走、池塘里游动，捕食蜉蝣类昆虫，还可通过独特的鼓膜耳倾听配偶的呼叫。据悉，这种两栖物种是青蛙与蝾螈进化首次分裂时出现的。它是一种早期类似青蛙和蝾螈的远古动物。
1995年，德克萨斯州贝勒郡一处远古鱼类沉积采石场发现了这种两栖物种化石，虽然化石样本发现较早，但直到近期古生物学家才对此鉴别并列为一种远古新物种，并将其命名为“Gerobatrachus hottoni”，发现者是史密森尼博物馆古生物学家尼古拉斯 奥托，他指出这种远古物种是一种过渡型两栖动物，兼具了青蛙和蝾螈的特征。这项发现填补了青蛙与蝾螈进化史上的缺口。像现代蝾螈一样，这种远古两栖物种脚踝有两块融合的骨骼，同时，它又兼具了青蛙的特征，长着较大的鼓膜耳。所以，这种远古两栖动物很可能用鼓膜耳倾听配偶的呼叫。
研究人员表示，这种远古两栖动物并不像现代青蛙一样经常跳跃，它们很可能在陆地上爬行，在水中游动。事实上，纵观两栖物种进化历史，早期青蛙并没有跳跃，直到侏罗纪或三叠纪。最具权威性具有跳跃能力青蛙的化石可追溯至距今2.48—2.06亿年前的三叠纪时期。这个两栖动物化石发现于存在大量鱼骨骼的池塘沉积层中，但我猜测它在陆地上和水中生活都非常适应。通过对骨骼化石分析显示，它长着许多小牙齿，与现代处于不同发育时期两栖动物的特殊诱捕特征十分相似。当它们用牙齿咬住猎物时，牙齿会像绞链一样将猎物拉入口中，猎物很难逃脱。
这种远古新物种从鼻尖到尾部末端不足5英寸（12厘米），它标志着远古青蛙和蝾螈在进化历程中向现代物种进化发育的分支。依据这项最新发现，我们评估青蛙与蝾螈彼此进化分离是在2.4—2.75亿年前，比之前的预测年代更晚一些。（生物谷www.bioon.com）
生物谷推荐原始出处：
Nature，453, 515-518 (22 May 2008)，Jason S. Anderson, Stuart S. Sumida
A stem batrachian from the Early Permian of Texas and the origin of frogs and salamanders
Jason S. Anderson1, Robert R. Reisz2, Diane Scott2, Nadia B. Fröbisch3 & Stuart S. Sumida4
Department of Comparative Biology and Experimental Medicine, Faculty of Veterinary Medicine, University of Calgary, 3330 Hospital Drive, Calgary, Alberta T2N 4N1, Canada
Department of Biology, University of Toronto at Mississauga, 3359 Mississauga Road, Mississauga, Ontario L5L 1C6, Canada
Redpath Museum, McGill University, 859 Sherbrooke Street West, Montréal, Québec H3A 2K6, Canada
Department of Biology, California State University at San Bernardino, 5500 University Parkway, San Bernardino, California 92407-2307, USA
Correspondence to: Jason S. Anderson1 Correspondence and requests for materials should be addressed to J.S.A. (Email: janders@ucalgary.ca).
The origin of extant amphibians (Lissamphibia: frogs, salamanders and caecilians) is one of the most controversial questions in vertebrate evolution, owing to large morphological and temporal gaps in the fossil record1, 2, 3. Current discussions focus on three competing hypotheses: a monophyletic origin within either Temnospondyli4, 5, 6, 7 or Lepospondyli8, 9, 10, or a polyphyletic origin with frogs and salamanders arising among temnospondyls and caecilians among the lepospondyls11, 12, 13, 14, 15, 16. Recent molecular analyses are also controversial, with estimations for the batrachian (frog–salamander) divergence significantly older than the palaeontological evidence supports17, 18. Here we report the discovery of an amphibamid temnospondyl from the Early Permian of Texas that bridges the gap between other Palaeozoic amphibians and the earliest known salientians19, 20 and caudatans21 from the Mesozoic. The presence of a mosaic of salientian and caudatan characters in this small fossil makes it a key taxon close to the batrachian (frog and salamander) divergence. Phylogenetic analysis suggests that the batrachian divergence occurred in the Middle Permian, rather than the late Carboniferous as recently estimated using molecular clocks18, 22, but the divergence with caecilians corresponds to the deep split between temnospondyls and lepospondyls, which is congruent with the molecular estimates.
