A team in China led by researchers from the University of California, Davis have discovered the first fossil of an amphibious ichthyosaur. Ichthyosaurs were dolphin-like marine reptiles that thrived for around 150 million years during the Age of the Dinosaurs. The discovery dates to the Lower Triassic period and marks the creature’s transition from land back to the sea. As the first evidence linking the marine ichthyosaur to its terrestrial ancestors it fills a significant gap in the fossil record.
The discovery of the fossil, named Cartorhynchus lenticarpus, is described in a paper recently published in the journal Nature. The fossil is about 248 million years old and measures roughly 16 inches (40 cm) long. UC Davis professor Ryosuke Motani and his colleagues discovered the specimen in China’s central-eastern Anhui Province. Unlike the later ichthyosaurs that were fully adapted to living in the sea, the fossil has unusually large flippers with flexible wrists, which could have allowed it to move around on land like a seal. Most ichthyosaurs also had long, beak-like snouts, but the new discovery shows a nose as short as that of land reptiles. It also appears adapted for suction feeding from the sea floor.
The fossil also has thicker bones than previously discovered ichthyosaurs. This is in keeping with the theory that most marine reptiles that moved back to the sea from land first became heavier by developing thicker bones, in order to swim through rough coastal waters before entering the deeper sea. Professor Motani says, “Cartorhynchus represents a stage of the land-to-sea transition that was somehow lacking in the fossil record of the ichthyosaur lineage, while known in most other marine reptile and mammal lineages.”
Motani also notes that the study’s implications go beyond evolutionary theory because the animal lived about four million years after the worst mass extinction in Earth’s history. Scientists have long wondered how long it took life on Earth to recover from the mass die-off 252 million years ago, particularly since it was associated with global warming. Montani says, “This was analogous to what might happen if the world gets warmer and warmer. How long did it take before the globe was good enough for predators like this to reappear? In that world, many things became extinct, but it started something new. These reptiles came out during this recovery.”