德国科学家近日通过对远古爬行动物化石的研究，发现耳在大约2.6亿年前就已出现。他们由此提出一种新的理论认为，耳是脊椎动物为适应黑暗环境而进化产生的。相关论文发表于9月12日的《公共科学图书馆·综合》（PLoS One）上。
哺乳动物等陆地脊椎动物的耳能听到空气载声（airborne sounds），在历史上大约经历了六次独立的进化。这些动物的耳虽然各有不同，但大体上都具有以下结构：一个似鼓膜的隔膜用来捕获声波振动；细小的骨骼，比如镫骨，用来将声音传输至内耳。很多科学家根据化石记录认为，这种类型的耳最早可追溯到大约2亿年前。
为了更进一步探索陆地脊椎动物听觉器官的进化历史，德国洪堡大学的古生物学家Johannes Müller和Linda Tsuji对几十块早期爬行动物化石标本展开了深入的研究。这些化石的年代大约为2.6亿年，研究人员将它们归成紧密相关的六个种类。有明显的证据表明，它们都具有大的、似鼓膜的结构。在保存比较完好的标本中，发现有与现代动物耳相似的内耳骨骼，包括镫骨。此外，研究人员还发现，这些标本的耳膜及传递信息到内耳的镫骨部分与现代陆地脊椎动物的非常相似，这有力地表明了，这种远古的耳已经具有听取空气载声的功能。
瑞典乌普撒拉大学的进化生物学家Martin Brazeau表示，此次研究发现的结构与耳非常相似，具有探测空气振动的能力。这应该是迄今为止发现的最为古老的耳。
之前，英国剑桥大学的古生物学家Jennifer Clack曾提出一种理论认为，脊椎动物耳的进化是为了方便捕捉大量出现的嗡嗡叫的昆虫。最新研究对此提出了挑战，因为在2.6亿年前，嗡嗡叫的昆虫并不多。新的研究数据表明，听觉器官很可能是脊椎动物为适应黑暗环境而进化产生的。这与现代很多具有敏锐听觉的动物适应黑暗生活的事实是一致的，比如猫头鹰、壁虎和猫等。对此，Clack表示，新的理论听起来倒是振振有辞，不过她倒想看看作者怎样来证明它。（科学网 梅进/编译）
原始出处：
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.
