title: Absence of suction feeding ichthyosaurs and its implications for triassic mesopelagic paleoecology.
abstract: 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.
authors.affiliation: Department of Earth and Planetary Sciences, University of California Davis, Davis, California, United States of America.
authors.forename: Ryosuke
authors.initials: R
authors.lastname: Motani
authors.affiliation: Department of Geology and Geological Museum, Peking University, Beijing, China.
authors.forename: Cheng
authors.initials: C
authors.lastname: Ji
authors.affiliation: Department of Earth and Planetary Sciences, University of California Davis, Davis, California, United States of America ; Hokkaido University Museum, Hakodate, Japan.
authors.forename: Taketeru
authors.initials: T
authors.lastname: Tomita
authors.affiliation: Department of Earth and Planetary Sciences, University of California Davis, Davis, California, United States of America.
authors.forename: Neil
authors.initials: N
authors.lastname: Kelley
authors.affiliation: Paläontologisches Institut und Museum, Universität Zürich, Zürich, Switzerland.
authors.forename: Erin
authors.initials: E
authors.lastname: Maxwell
authors.affiliation: Department of Geology and Geological Museum, Peking University, Beijing, China.
authors.forename: Da-yong
authors.initials: DY
authors.lastname: Jiang
authors.affiliation: Steinmann Institute, Division of Palaeontology, University of Bonn, Bonn, Germany.
authors.forename: Paul Martin
authors.initials: PM
authors.lastname: Sander
mesh_terms: D000818:Animals; D005075:Biological Evolution; D005247:Feeding Behavior / Q000502:physiology*; D005580:Fossils; D010802:Phylogeny; D012104:Reptiles / Q000145:classification / Q000502:physiology*