Nummela Sirpa, Thewissen J G M, Bajpai Sunil, Hussain Taseer, Kumar Kishor
Department of Anatomy, Northeastern Ohio Universities College of Medicine, Rootstown, Ohio, USA.
Anat Rec (Hoboken). 2007 Jun;290(6):716-33. doi: 10.1002/ar.20528.
The whale ear, initially designed for hearing in air, became adapted for hearing underwater in less than ten million years of evolution. This study describes the evolution of underwater hearing in cetaceans, focusing on changes in sound transmission mechanisms. Measurements were made on 60 fossils of whole or partial skulls, isolated tympanics, middle ear ossicles, and mandibles from all six archaeocete families. Fossil data were compared with data on two families of modern mysticete whales and nine families of modern odontocete cetaceans, as well as five families of noncetacean mammals. Results show that the outer ear pinna and external auditory meatus were functionally replaced by the mandible and the mandibular fat pad, which posteriorly contacts the tympanic plate, the lateral wall of the bulla. Changes in the ear include thickening of the tympanic bulla medially, isolation of the tympanoperiotic complex by means of air sinuses, functional replacement of the tympanic membrane by a bony plate, and changes in ossicle shapes and orientation. Pakicetids, the earliest archaeocetes, had a land mammal ear for hearing in air, and used bone conduction underwater, aided by the heavy tympanic bulla. Remingtonocetids and protocetids were the first to display a genuine underwater ear where sound reached the inner ear through the mandibular fat pad, the tympanic plate, and the middle ear ossicles. Basilosaurids and dorudontids showed further aquatic adaptations of the ossicular chain and the acoustic isolation of the ear complex from the skull. The land mammal ear and the generalized modern whale ear are evolutionarily stable configurations, two ends of a process where the cetacean mandible might have been a keystone character.
鲸的耳朵最初是为在空气中听觉而设计的，在不到一千万年的进化过程中就适应了在水下听觉。这项研究描述了鲸类动物水下听觉的进化，重点关注声音传播机制的变化。对来自所有六个古鲸亚目家族的60个完整或部分头骨、分离的鼓膜、中耳小骨和下颌骨化石进行了测量。将化石数据与两个现代须鲸科家族、九个现代齿鲸科家族以及五个非鲸类哺乳动物家族的数据进行了比较。结果表明，外耳廓和外耳道在功能上被下颌骨和下颌脂肪垫所取代，下颌脂肪垫向后与鼓室板（鼓泡的侧壁）接触。耳朵的变化包括鼓泡内侧增厚、通过气窦使鼓室-耳周复合体隔离、用骨板功能性取代鼓膜以及小骨形状和方向的变化。巴基鲸是最早的古鲸亚目动物，具有陆地哺乳动物用于在空气中听觉的耳朵，在水下借助厚重的鼓泡利用骨传导。雷明顿鲸和原鲸是最早展示真正水下耳朵的，声音通过下颌脂肪垫、鼓室板和中耳小骨到达内耳。龙王鲸和矛齿鲸展示了听骨链进一步的水生适应性以及耳朵复合体与头骨的声学隔离。陆地哺乳动物的耳朵和现代鲸类动物普遍的耳朵在进化上是稳定的结构，这是一个过程的两端，在这个过程中鲸类动物的下颌骨可能是一个关键特征。
