美發(fā)現(xiàn)2.9億年前兩棲動(dòng)物化石
生物谷報(bào)道：《自然》雜志刊載了一項(xiàng)最新考古化石研究顯示,，美國(guó)德克薩斯州發(fā)現(xiàn)的一具2.9億年前兩棲新物種化石，一種類似青蛙,，長(zhǎng)著短而粗壯尾部的動(dòng)物,，它們可以在陸地行走,、池塘里游動(dòng)，捕食蜉蝣類昆蟲,，還可通過(guò)獨(dú)特的鼓膜耳傾聽(tīng)配偶的呼叫,。據(jù)悉,，這種兩棲物種是青蛙與蠑螈進(jìn)化首次分裂時(shí)出現(xiàn)的。它是一種早期類似青蛙和蠑螈的遠(yuǎn)古動(dòng)物,。
1995年,，德克薩斯州貝勒郡一處遠(yuǎn)古魚類沉積采石場(chǎng)發(fā)現(xiàn)了這種兩棲物種化石，雖然化石樣本發(fā)現(xiàn)較早,，但直到近期古生物學(xué)家才對(duì)此鑒別并列為一種遠(yuǎn)古新物種,，并將其命名為“Gerobatrachus hottoni”，發(fā)現(xiàn)者是史密森尼博物館古生物學(xué)家尼古拉斯 奧托,，他指出這種遠(yuǎn)古物種是一種過(guò)渡型兩棲動(dòng)物,，兼具了青蛙和蠑螈的特征。這項(xiàng)發(fā)現(xiàn)填補(bǔ)了青蛙與蠑螈進(jìn)化史上的缺口,。像現(xiàn)代蠑螈一樣,，這種遠(yuǎn)古兩棲物種腳踝有兩塊融合的骨骼，同時(shí),，它又兼具了青蛙的特征,，長(zhǎng)著較大的鼓膜耳。所以,，這種遠(yuǎn)古兩棲動(dòng)物很可能用鼓膜耳傾聽(tīng)配偶的呼叫,。
研究人員表示，這種遠(yuǎn)古兩棲動(dòng)物并不像現(xiàn)代青蛙一樣經(jīng)常跳躍,，它們很可能在陸地上爬行,，在水中游動(dòng)。事實(shí)上,，縱觀兩棲物種進(jìn)化歷史,，早期青蛙并沒(méi)有跳躍，直到侏羅紀(jì)或三疊紀(jì),。最具權(quán)威性具有跳躍能力青蛙的化石可追溯至距今2.48—2.06億年前的三疊紀(jì)時(shí)期,。這個(gè)兩棲動(dòng)物化石發(fā)現(xiàn)于存在大量魚骨骼的池塘沉積層中，但我猜測(cè)它在陸地上和水中生活都非常適應(yīng),。通過(guò)對(duì)骨骼化石分析顯示,，它長(zhǎng)著許多小牙齒，與現(xiàn)代處于不同發(fā)育時(shí)期兩棲動(dòng)物的特殊誘捕特征十分相似,。當(dāng)它們用牙齒咬住獵物時(shí),，牙齒會(huì)像絞鏈一樣將獵物拉入口中，獵物很難逃脫,。
這種遠(yuǎn)古新物種從鼻尖到尾部末端不足5英寸（12厘米）,，它標(biāo)志著遠(yuǎn)古青蛙和蠑螈在進(jìn)化歷程中向現(xiàn)代物種進(jìn)化發(fā)育的分支。依據(jù)這項(xiàng)最新發(fā)現(xiàn),，我們?cè)u(píng)估青蛙與蠑螈彼此進(jìn)化分離是在2.4—2.75億年前,，比之前的預(yù)測(cè)年代更晚一些,。（生物谷www.bioon.com）
生物谷推薦原始出處：
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: [email protected]).
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.
