【進化】 先史時代の小さな目の「カモノハシ」は触覚によって獲物を見つけていたかもしれない

Two new specimens of a marine reptile from the Early Triassic (about 250 million years ago), called Eretmorhipis carrolldongi are described in a study published in Scientific Reports this week. The reptile, which shows similarities to the modern duckbilled platypus, had very small eyes relative to its body size. The specimens thus represent the oldest known record of amniotes (reptiles, birds, and mammals) with reduced visual capacity, and are likely to have used non-visual cues for prey detection.

Animals with small eyes that rely on non-visual sensory cues usually show enhancement of other sense organs. They tend to be active in visually challenging conditions, such as when hunting in the dark. Ryosuke Motani and colleagues found that Eretmorhipis had a relative eye size equal to or smaller than that of some of these species. As soft tissue was not preserved in the fossils presented in this study, the authors were unable to test which non-visual sense was enhanced in Eretmorhipis. However, based on its morphology and the environment the species would have lived in, the authors conclude that the tactile sense is most likely.

The discovery that Eretmorhipis may have hunted under low-light conditions, using its sense of touch, suggests that amniotes started using touch to discriminate between predator and prey at least as early as the Early Triassic. The findings challenge the traditional view that diversification of marine reptiles was delayed following the end-Permian mass extinction, the Earth’s most severe known extinction event, which occurred around 252 million years ago.