Life › The age of dinosaurs › Life recovers in the Triassic
Marine reptiles (ichthyosaurs, plesiosaurs, mosasaurs)
c. 248–66 million years ago· Organism or group· Animals
Well establishedStrong fossil, genetic or geological evidence.
Several separate groups of reptiles that returned to the sea and became top ocean predators during the age of dinosaurs.
See it on the Life map →What happened
Land reptiles took to the sea several times, independently. Ichthyosaurs, later dolphin-shaped, appeared about 248 million years ago, early in the Triassic recovery, and died out about 90 million years ago. Four-flippered plesiosaurs, many long-necked, lived from about 203 to 66 million years ago. Mosasaurs, giant sea lizards related to monitor lizards and snakes, ruled late Cretaceous seas. Many gave birth to live young at sea. None were dinosaurs; the last died in the K–Pg extinction.
Why it matters
Again and again, land animals re-adapted to the sea and converged on similar streamlined shapes, a pattern later repeated by whales.
The evidence (fossils, genes, rocks)
Skeletons from England's Jurassic Coast, Germany's Holzmaden shales (some with body outlines and embryos) and Kansas chalk; early key finds by Mary Anning in the 1810s–1820s.
Still debated
How the land ancestors of ichthyosaurs made the move to sea, and how warm-blooded these reptiles were, are still debated.
Connections (4)
▸Life recovers in the Triassic saw the rise of Marine reptiles (ichthyosaurs, plesiosaurs, mosasaurs)Well established
The connection
As oceans refilled, reptiles found empty top-predator roles. Within about 4 million years of the extinction, ichthyosaurs and other marine reptiles appeared and diversified; by about 246 million years ago one ichthyosaur, Cymbospondylus youngorum, reached about 17 m. Their prey included fish and ammonoids, which also recovered quickly.
When and how
Early to Middle Triassic, about 248–240 million years ago; key finds come from Anhui and Guizhou in China, Nevada and Europe.
The evidence
Early ichthyosaurs and relatives occur in dated Early Triassic marine rocks, including Cartorhynchus from China, a small form that may have moved on land too.
Debates and open questions
Why so many reptile lines took to the sea so soon after the extinction, and which land reptiles were their ancestors, are debated.
▸K–Pg extinction: end of the non-bird dinosaurs ended the last of the Marine reptiles (ichthyosaurs, plesiosaurs, mosasaurs)Well established
The connection
Ichthyosaurs had already disappeared about 90 million years ago, but plesiosaurs and giant mosasaurs were still top ocean predators. When plankton collapsed after the impact, the food chains feeding these hunters failed, and both groups vanished. Sea turtles and crocodile relatives, also marine reptiles, survived.
When and how
Oceans worldwide, 66 million years ago; latest Cretaceous mosasaur and plesiosaur fossils come from Morocco, the Netherlands (Maastricht) and the United States.
The evidence
Abundant mosasaur and plesiosaur fossils occur in rocks right up to the boundary, and none above it.
Debates and open questions
Why sea turtles and crocodilians survived when other marine reptiles did not is unclear; slower metabolisms and varied diets are leading ideas.
▸Mary Anning discovered key fossils of Marine reptiles (ichthyosaurs, plesiosaurs, mosasaurs)Well established
The connection
Anning's finds at Lyme Regis gave science its first good look at ichthyosaurs and plesiosaurs: the first ichthyosaur skeleton to win wide scientific attention (1811–12) and the first complete plesiosaur (1823). The plesiosaur's neck was so long that Cuvier at first suspected a fake, then accepted it. Such sea reptiles showed that lost worlds lay in the rocks.
When and how
Jurassic cliffs at Lyme Regis, Dorset, about 200–195 million years old; finds from 1811 into the 1840s, described by Everard Home, William Conybeare and others.
The evidence
Many of her specimens survive at London's Natural History Museum and elsewhere, with sale records and published descriptions from the 1810s–1820s.
Debates and open questions
How many of her finds were published without credit, and which surviving museum specimens she personally found, are still being traced by historians.
▸Marine reptiles (ichthyosaurs, plesiosaurs, mosasaurs) foreshadowed the body shape of Whales return to the seaStrong evidence
The connection
Ichthyosaurs evolved a dolphin-like body, streamlined with a back fin and tail fluke, long before whales existed, though their tails were upright like a fish's while whale flukes are horizontal. Whales, from entirely different land ancestors, converged on a similar design. This is convergent evolution, with no ancestry link: fast swimming favors similar solutions.
When and how
Ichthyosaurs in seas about 248–90 million years ago; dolphin-shaped whales from about 40 million years ago onward.
How sure are we?
Ichthyosaur fossils with body outlines from Holzmaden, Germany, show fins and flukes; family trees confirm the two groups evolved them independently.
Debates and open questions
Whether ichthyosaurs had blubber and warm blood like whales is debated; a 2018 study reported blubber-like tissue in a Jurassic fossil.