Archosauria / Lepidosauria (not Dinosauria)

Marine Reptiles

250–66 Ma

vault species

184

million years

What is a Marine Reptiles?

The "marine reptiles" of the Mesozoic are not a single evolutionary group but several independent lineages — ichthyosaurs, plesiosaurs, and mosasaurs — that each independently evolved from land-dwelling ancestors to become fully aquatic ocean predators.

Ichthyosaurs

~250–90 Ma — first fully aquatic reptiles

Plesiosaurs

~203–66 Ma — long-necked and short-necked forms

Mosasaurs

~98–66 Ma — giant Late Cretaceous lizards

Not dinosaurs

None are dinosaurs; each group independently evolved

Largest

Mosasaurus hoffmannii (~17 m)

Evolution & History

The Mesozoic seas were ruled by reptiles in the same way the land was ruled by dinosaurs, but the story is more complex: the great marine predators of the Triassic, Jurassic, and Cretaceous represent at least three separate evolutionary experiments in returning to the sea, each producing remarkably similar body plans through convergent evolution.

Ichthyosaurs were the first and most fish-like, appearing in the Triassic and achieving a dolphin-shaped body plan so efficiently that they're often mistaken for fish in silhouette. Plesiosaurs split into two very different forms: the long-necked plesiosauroids (Elasmosaurus, with 72 neck vertebrae) and the short-necked, enormous-headed pliosauroids — built more like killer whales. Both groups persisted throughout the Jurassic and Cretaceous.

Mosasaurs were the last to arrive and the most successful in the final chapter of the Mesozoic. True lizards (relatives of modern monitor lizards), they evolved from land-dwelling ancestors in the Late Cretaceous and within 20 million years had produced apex predators over 17 meters long. At the time of the asteroid impact, mosasaurs were the dominant large predators in every ocean on Earth.