古生物学：スピノサウルス科恐竜が水生だった可能性

Evidence that spinosaurids had adaptations for an aquatic lifestyle is presented in a Nature paper. The findings challenge the hypothesis that most non-avian dinosaurs were restricted to terrestrial environments.

Although it can be difficult to infer anatomical adaptations in extinct species, only a few non-avian dinosaur species are thought to be partly or predominantly aquatic. In the past decade, fossils of spinosaurs characterized by paddle-like feet and fin-like tails have emerged, but whether these dinosaurs lived predominantly on land or in the water remains unknown. Bone density is used as a proxy for aquatic adaptation, as even aquatic animals that are not clearly shaped for an aquatic lifestyle — such as the hippopotamus — have very dense bones. Compact bones are a feature that often precedes the evolution of more clearly visible bodily adaptations for life in the water, such as fins or flippers.

To investigate aquatic adaptations in dinosaurs, Matteo Fabbri and colleagues analysed and compared the densities of 380 bones from a broad range of extinct and non-extinct amniotes (mammals, lizards, crocodiles and birds, including marine reptiles, and flying reptiles), including non-avian dinosaurs. The authors found that spinosaurids — a family of predatory dinosaurs — had dense bones, suggesting they were adapted to life in the water. Their increased bone density may have facilitated buoyancy control when immersed in water, relating to underwater foraging in Spinosaurus and Baryonyx, and to more terrestrial environments in Suchomimus, the authors suggest.

The findings imply that adaptations to aquatic environments appeared in spinosaurids during the Early Cretaceous (around 145 million years ago to 100.5 million years ago), following their divergence from other large carnivorous dinosaurs during the Early Jurassic.