LIFE × EVOLUTION

Reptiles Go Back to the Sea

~250 to 242 million years ago

Phanerozoic · Mesozoic · Triassic

Land reptiles walked back into the water and stayed. Within a few million years their legs had become paddles, their bones had gone spongy and light, and some had grown large enough to eat one another. The oldest fully open-water forms, from Spitsbergen, are about two million years younger than the end-Permian extinction, close enough to recast them as survivors of the crisis rather than latecomers filling an empty sea.

Why it matters

Air-breathing animals have gone back to the ocean over and over, and this is the first time it happened on a large scale. The whale and the sea turtle are running the same experiment, with the same constraints and mostly the same answers. What is startling here is the speed: the descendants of walking reptiles were living out of sight of land while the sea was still recovering from the worst extinction on record.

Dating & uncertainty

The Spitsbergen fossils give about 250 Ma, some two million years after the end-Permian extinction; until they were found, the oldest well-dated remains were Chinese, at 248.8 Ma. The choice between the two dates changes what these animals were: at 248.8 Ma they look like beneficiaries of the sea the crisis emptied, at 250 Ma like animals that came through it. Either way the transition from land animal to ocean animal is fast enough to be uncomfortable.

Sources

1. Kear et al. (2023), Current Biology · Earliest Triassic ichthyosaur fossils push back oceanic reptile origins
2. Jiang et al. (2023), Palaeogeography, Palaeoclimatology, Palaeoecology · Emergence and ecological transition of the Mesozoic marine reptiles: evidence from the Early Triassic Chaohu and the Middle Triassic Xingyi faunas
3. Wang et al. (2026), Science Advances · Highly aquatic marine stem archosaur with soft tissue preservation

Earth history as one day

22:40:421 h 19 min before midnight

if Earth’s 4.54 billion years were compressed into a single day