Department of Earth and Planetary Sciences, University of California Davis, Davis, California, United States of America.
Department of Geology and Geological Museum, Peking University, Beijing, China.
PLoS One. 2013 Dec 11;8(12):e66075. doi: 10.1371/journal.pone.0066075. eCollection 2013.
Mesozoic marine reptiles and modern marine mammals are often considered ecological analogs, but the extent of their similarity is largely unknown. Particularly important is the presence/absence of deep-diving suction feeders among Mesozoic marine reptiles because this would indicate the establishment of mesopelagic cephalopod and fish communities in the Mesozoic. A recent study suggested that diverse suction feeders, resembling the extant beaked whales, evolved among ichthyosaurs in the Triassic. However, this hypothesis has not been tested quantitatively. We examined four osteological features of jawed vertebrates that are closely linked to the mechanism of suction feeding, namely hyoid corpus ossification/calcification, hyobranchial apparatus robustness, mandibular bluntness, and mandibular pressure concentration index. Measurements were taken from 18 species of Triassic and Early Jurassic ichthyosaurs, including the presumed suction feeders. Statistical comparisons with extant sharks and marine mammals of known diets suggest that ichthyosaurian hyobranchial bones are significantly more slender than in suction-feeding sharks or cetaceans but similar to those of ram-feeding sharks. Most importantly, an ossified hyoid corpus to which hyoid retractor muscles attach is unknown in all but one ichthyosaur, whereas a strong integration of the ossified corpus and cornua of the hyobranchial apparatus has been identified in the literature as an important feature of suction feeders. Also, ichthyosaurian mandibles do not narrow rapidly to allow high suction pressure concentration within the oral cavity, unlike in beaked whales or sperm whales. In conclusion, it is most likely that Triassic and Early Jurassic ichthyosaurs were 'ram-feeders', without any beaked-whale-like suction feeder among them. When combined with the inferred inability for dim-light vision in relevant Triassic ichthyosaurs, the fossil record of ichthyosaurs does not suggest the establishment of modern-style mesopelagic animal communities in the Triassic. This new interpretation matches the fossil record of coleoids, which indicates the absence of soft-bodied deepwater species in the Triassic.
中生代海洋爬行动物和现代海洋哺乳动物通常被认为是生态类似物，但它们的相似程度在很大程度上尚不清楚。特别重要的是中生代海洋爬行动物中是否存在深潜抽吸进食者，因为这将表明中生代中已建立了中层带头足类和鱼类群落。最近的一项研究表明，在三叠纪的鱼龙中进化出了各种类似于现生喙鲸的抽吸进食者。然而，这一假说尚未得到定量检验。我们检查了与抽吸进食机制密切相关的四种有颚脊椎动物的骨骼特征，即舌骨体骨化/钙化、舌弓装置坚固性、下颌骨钝度和下颌骨压力集中指数。测量值取自 18 种三叠纪和早侏罗纪的鱼龙，包括假定的抽吸进食者。与现生鲨鱼和已知饮食的海洋哺乳动物的统计比较表明，鱼龙的舌弓骨明显比抽吸进食的鲨鱼或鲸类更细长，但与食鱼鲨鱼的相似。最重要的是，除了一种鱼龙之外，所有鱼龙都没有附着舌骨牵引肌的骨化舌骨体，而文献中已确定的骨化舌骨体和舌弓角的牢固结合是抽吸进食者的一个重要特征。此外，鱼龙的下颌骨不会迅速变窄，从而无法在口腔内产生高抽吸压力集中，这与喙鲸或抹香鲸不同。总之，最有可能的是，三叠纪和早侏罗纪的鱼龙是“食鱼者”，其中没有任何喙鲸样的抽吸进食者。当与相关三叠纪鱼龙推断出的暗光视力能力丧失结合起来时，鱼龙的化石记录表明，在三叠纪并没有建立现代风格的中层带动物群落。这种新的解释与头足类动物的化石记录相吻合，表明在三叠纪没有软躯体深海物种。
