【化石】シルル紀の大型魚という大発見

Fossils of a fish that lived around 423 million years ago, described in Scientific Reports this week, provide new insights about the ancient creatures present at that point in history. With an estimated length of roughly 1 metre, the new form is far larger than any other vertebrate (backboned animal) known from that time. Previously, the apparent lack of large fishes in rocks over 380 million years in age was thought to indicate a restricted growth limit due to lower oxygen levels, but the new finding refutes that assumption.

Min Zhu, Brian Choo, and colleagues describe the new fish, based on fossils discovered in late Silurian sediments in Yunnan, China. The creature has an estimated jaw length of 17 cm with blunt, strong teeth, ideal for eating hard-shelled prey, the authors suggest. These features are reflected in the name assigned to this fish: Megamastax amblyodus, derived from Greek words meaning “big mouth, blunt tooth”.

The Devonian period (around 358-419 million years ago) has been considered to mark a major transition in the size and diversity of early jawed vertebrates, including the earliest appearance of large predatory fishes. Until recently the largest jawed vertebrates in the preceding Silurian period were roughly 35 cm in length. The discovery of a metre-long, predatory Silurian fish provides evidence that pre-Devonian creatures could attain comparatively large sizes.