德國(guó)科學(xué)家近日通過對(duì)遠(yuǎn)古爬行動(dòng)物化石的研究，發(fā)現(xiàn)耳在大約2.6億年前就已出現(xiàn),。他們由此提出一種新的理論認(rèn)為,，耳是脊椎動(dòng)物為適應(yīng)黑暗環(huán)境而進(jìn)化產(chǎn)生的。相關(guān)論文發(fā)表于9月12日的《公共科學(xué)圖書館·綜合》（PLoS One）上,。
哺乳動(dòng)物等陸地脊椎動(dòng)物的耳能聽到空氣載聲（airborne sounds）,，在歷史上大約經(jīng)歷了六次獨(dú)立的進(jìn)化。這些動(dòng)物的耳雖然各有不同,，但大體上都具有以下結(jié)構(gòu)：一個(gè)似鼓膜的隔膜用來捕獲聲波振動(dòng),；細(xì)小的骨骼，比如鐙骨,，用來將聲音傳輸至內(nèi)耳,。很多科學(xué)家根據(jù)化石記錄認(rèn)為，這種類型的耳最早可追溯到大約2億年前,。
為了更進(jìn)一步探索陸地脊椎動(dòng)物聽覺器官的進(jìn)化歷史,，德國(guó)洪堡大學(xué)的古生物學(xué)家Johannes Müller和Linda Tsuji對(duì)幾十塊早期爬行動(dòng)物化石標(biāo)本展開了深入的研究,。這些化石的年代大約為2.6億年，研究人員將它們歸成緊密相關(guān)的六個(gè)種類,。有明顯的證據(jù)表明,，它們都具有大的、似鼓膜的結(jié)構(gòu),。在保存比較完好的標(biāo)本中,，發(fā)現(xiàn)有與現(xiàn)代動(dòng)物耳相似的內(nèi)耳骨骼，包括鐙骨,。此外,，研究人員還發(fā)現(xiàn)，這些標(biāo)本的耳膜及傳遞信息到內(nèi)耳的鐙骨部分與現(xiàn)代陸地脊椎動(dòng)物的非常相似,，這有力地表明了,，這種遠(yuǎn)古的耳已經(jīng)具有聽取空氣載聲的功能。
瑞典烏普撒拉大學(xué)的進(jìn)化生物學(xué)家Martin Brazeau表示,，此次研究發(fā)現(xiàn)的結(jié)構(gòu)與耳非常相似,，具有探測(cè)空氣振動(dòng)的能力。這應(yīng)該是迄今為止發(fā)現(xiàn)的最為古老的耳,。
之前,，英國(guó)劍橋大學(xué)的古生物學(xué)家Jennifer Clack曾提出一種理論認(rèn)為，脊椎動(dòng)物耳的進(jìn)化是為了方便捕捉大量出現(xiàn)的嗡嗡叫的昆蟲,。最新研究對(duì)此提出了挑戰(zhàn),，因?yàn)樵?.6億年前，嗡嗡叫的昆蟲并不多,。新的研究數(shù)據(jù)表明,，聽覺器官很可能是脊椎動(dòng)物為適應(yīng)黑暗環(huán)境而進(jìn)化產(chǎn)生的。這與現(xiàn)代很多具有敏銳聽覺的動(dòng)物適應(yīng)黑暗生活的事實(shí)是一致的,，比如貓頭鷹,、壁虎和貓等。對(duì)此,，Clack表示,，新的理論聽起來倒是振振有辭，不過她倒想看看作者怎樣來證明它,。（科學(xué)網(wǎng) 梅進(jìn)/編譯）
原始出處：
Received: June 2, 2007; Accepted: August 20, 2007; Published: September 12, 2007
Impedance-Matching Hearing in Paleozoic Reptiles: Evidence of Advanced Sensory Perception at an Early Stage of Amniote Evolution
Johannes Müller*, Linda A. Tsuji
Humboldt-Universität zu Berlin, Museum für Naturkunde, Berlin, Germany
Background
Insights into the onset of evolutionary novelties are key to the understanding of amniote origins and diversification. The possession of an impedance-matching tympanic middle ear is characteristic of all terrestrial vertebrates with a sophisticated hearing sense and an adaptively important feature of many modern terrestrial vertebrates. Whereas tympanic ears seem to have evolved multiple times within tetrapods, especially among crown-group members such as frogs, mammals, squamates, turtles, crocodiles, and birds, the presence of true tympanic ears has never been recorded in a Paleozoic amniote, suggesting they evolved fairly recently in amniote history.
Methodology/Principal Findings
In the present study, we performed a morphological examination and a phylogenetic analysis of poorly known parareptiles from the Middle Permian of the Mezen River Basin in Russia. We recovered a well-supported clade that is characterized by a unique cheek morphology indicative of a tympanum stretching across large parts of the temporal region to an extent not seen in other amniotes, fossil or extant, and a braincase specialized in showing modifications clearly related to an increase in auditory function, unlike the braincase of any other Paleozoic tetrapod. In addition, we estimated the ratio of the tympanum area relative to the stapedial footplate for the basalmost taxon of the clade, which, at 23:1, is in close correspondence to that of modern amniotes capable of efficient impedance-matching hearing.
Conclusions/Significance
Using modern amniotes as analogues, the possession of an impedance-matching middle ear in these parareptiles suggests unique ecological adaptations potentially related to living in dim-light environments. More importantly, our results demonstrate that already at an early stage of amniote diversification, and prior to the Permo-Triassic extinction event, the complexity of terrestrial vertebrate ecosystems had reached a level that proved advanced sensory perception to be of notable adaptive significance.
Figure 1. Phylogeny of the non-pareiasaurian parareptiles from the Mezen River Basin.
With the exception of Nyctiphruretus, all relevant taxa form a clade that is sister to Pareiasauridae, and are characterized by the presence of a large temporal emargination indicative of a prominent tympanum (in pink). Not to scale.
