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.

The evolution and extinction of the ichthyosaurs from the perspective of quantitative ecospace modelling

“…Ecotype D represents the four known large species with reduced dentition, implying a specialized feeding strategy (figures 1 and 2) [9,10]. This ecotype appears to have been lost, and convergently re-evolved by Temnodontosaurus azerguensis during the Early Jurassic radiation.…”

Feeding kinematics and performance of basal otariid pinnipeds, Steller sea lions (Eumetopias jubatus), and northern fur seals (Callorhinus ursinus): implications for the evolution of mammalian feeding

“…There is historical evidence supporting the effectiveness of a biting feeding mode in mesopelagic habitats. Morphological and paleoecological data indicate that a ram-biting feeding mode, but not a suction feeding mode, was predominant among Triassic ichthyosaurs, even those feeding in mesopelagic habits (Motani et al, 2013;Pyenson et al, 2014). It is not certain whether a biting feeding mode in northern fur seals was driven by ecological pressures or phylogenetic constraint.…”

A new phylogeny of ichthyosaurs (Reptilia: Diapsida)

“…character 149:0→1). The more derived members of the intermediate grade than Mixosauridae, which includes traditional 'shastasaurids', modify the forelimb further than earlier taxa, particularly through the loss of digits and shortening and rounding of the phalanges (see Online Supplemental Material Document S1.5; Thorne et al 2011;Sander et al 2011;Motani et al 2013).…”