Museum of Paleontology, Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, USA.
Proc Natl Acad Sci U S A. 2011 Aug 30;108(35):14545-8. doi: 10.1073/pnas.1108927108. Epub 2011 Aug 22.
Eocene archaeocete whales gave rise to all modern toothed and baleen whales (Odontoceti and Mysticeti) during or near the Eocene-Oligocene transition. Odontocetes have asymmetrical skulls, with asymmetry linked to high-frequency sound production and echolocation. Mysticetes are generally assumed to have symmetrical skulls and lack high-frequency hearing. Here we show that protocetid and basilosaurid archaeocete skulls are distinctly and directionally asymmetrical. Archaeocete asymmetry involves curvature and axial torsion of the cranium, but no telescoping. Cranial asymmetry evolved in Eocene archaeocetes as part of a complex of traits linked to directional hearing (such as pan-bone thinning of the lower jaws, mandibular fat pads, and isolation of the ear region), probably enabling them to hear the higher sonic frequencies of sound-producing fish on which they preyed. Ultrasonic echolocation evolved in Oligocene odontocetes, enabling them to find silent prey. Asymmetry and much of the sonic-frequency range of directional hearing were lost in Oligocene mysticetes during the shift to low-frequency hearing and bulk-straining predation.
始新世古鲸类动物在始新世-渐新世过渡期或附近进化为所有现代齿鲸类动物(齿鲸和须鲸)和须鲸类动物(须鲸)。齿鲸类动物的头骨不对称,不对称与高频声音产生和回声定位有关。须鲸类动物通常被认为具有对称的头骨,缺乏高频听力。在这里,我们表明,原鲸类和龙王鲸类古鲸类动物的头骨明显且定向不对称。古鲸类动物的不对称性涉及颅骨的曲率和轴向扭转,但没有伸缩。始新世古鲸类动物的头骨不对称性是定向听力相关特征的一部分,这些特征包括下颚的pan 骨变薄、下颌脂肪垫和耳部区域的隔离,这可能使它们能够听到捕食的产声鱼类的高频声音。在渐新世的齿鲸类动物中,超声回声定位进化了,使它们能够找到无声的猎物。在渐新世向低频听觉和大规模捕食的转变过程中,须鲸类动物失去了不对称性和大部分定向听觉的超声频率范围。