Gray Noel-Marie, Kainec Kimberly, Madar Sandra, Tomko Lucas, Wolfe Scott
Department of Biology, Hiram College, Hiram, Ohio 44234, USA.
Anat Rec (Hoboken). 2007 Jun;290(6):638-53. doi: 10.1002/ar.20533.
Previous analyses have shown that secondarily aquatic tetrapods, including whales, exhibit osteological adaptations to life in water as part of their complex buoyancy control systems. These structural specializations of bone span hyperostosis through osteoporosis. The past 15 years of paleontological effort has provided an unprecedented opportunity to examine the osteological transformation of whales as they make their transition to an obligate aquatic lifestyle over a 10-million-year period. It is hypothesized that whales manifest their osteological specialization in the same manner as extant semiaquatic and fully aquatic mammals. This study presents and analysis of the microstructural features of bone in early and late archaic cetaceans, and in a comparative sample of modern terrestrial, semiaquatic, and aquatic mammals. Bone histology was examined from the ribs of 10 fossilized individuals representing five early cetacean families, including Pakicetidae, Ambulocetidae, Protocetidae, Remintonocetidae, and Basilosauridae. Comparisons were then made with rib histology from nine genera of extant mammals including: Odocoileus (deer), Bos (cow), Equus (horse), Canis (dog), Lutra (river otter), Enhydra (sea otter), Choeropsis (pygmy hippo), Trichechus (sea cow), and Delphinus (dolphin). Results show that the transition from terrestrial, to semiaquatic, to obligate aquatic locomotion in archaeocetes involved a radical shift in bone function achieved by means of profound changes at the microstructural level. A surprising finding was that microstructural change predates gross anatomical shift in archaeocetes associated with swimming. Histological analysis shows that high bone density is an aquatic specialization that provides static buoyancy control (ballast) for animals living in shallow water, while low bone density is associated with dynamic buoyancy control for animals living in deep water. Thus, there was a shift from the typical terrestrial form, to osteopetrosis and pachyosteosclerosis, and then to osteoporosis in the first quarter of cetacean evolutionary history.
先前的分析表明，包括鲸鱼在内的次生水生四足动物，作为其复杂浮力控制系统的一部分，表现出对水生生活的骨骼适应性。这些骨骼结构特化涵盖了从骨质增生到骨质疏松的范围。过去15年的古生物学研究工作提供了前所未有的机会，来研究鲸鱼在1000万年的时间里向专性水生生活方式转变过程中的骨骼变化。据推测，鲸鱼的骨骼特化表现方式与现存的半水生和完全水生哺乳动物相同。本研究展示并分析了早期和晚期古代鲸类以及现代陆生、半水生和水生哺乳动物比较样本的骨骼微观结构特征。对代表五个早期鲸类家族（包括巴基鲸科、陆行鲸科、原鲸科、雷明顿鲸科和龙王鲸科）的10个化石个体的肋骨进行了骨组织学检查。然后与9个现存哺乳动物属的肋骨组织学进行了比较，这些现存哺乳动物包括：白尾鹿属（鹿）、牛属（牛）、马属（马）、犬属（狗）、水獭属（水獭）、海獭属（海獭）、倭河马属（倭河马）、海牛属（海牛）和海豚属（海豚）。结果表明，古代鲸类从陆地运动到半水生运动，再到专性水生运动的转变，涉及通过微观结构层面的深刻变化实现的骨骼功能的根本性转变。一个惊人的发现是，微观结构变化早于古代鲸类与游泳相关的大体解剖结构转变。组织学分析表明，高骨密度是一种水生特化，为生活在浅水区的动物提供静态浮力控制（压载），而低骨密度与生活在深水区的动物的动态浮力控制有关。因此，在鲸类进化史的第一阶段，出现了从典型陆地形态到骨质石化和厚骨硬化，再到骨质疏松的转变。
