Dino card No. 1311 of 2,027

Oneirosaurus

Say it: oh-NYE-roh-SORE-us

Name means: “dream lizard”1

Ate: MeatMosasaursLate CretaceousNot technically a dinosaur

Oneirosaurus was a fierce marine lizard that swam in the warm seas of Colombia about 87 million years ago. It was a mosasaur, a relative of modern snakes and lizards, and it hunted fish and other sea creatures with its sharp teeth.2 1

Fun fact: this sea reptile swam while dinosaurs walked on land. Everyone calls them dinos, but they were a different branch of reptiles.

Lived

90 to 86 million years ago3

Late Cretaceous

Fossils found in

Lebrija, Santander, Colombia1

Food

Meat2

When did it live?

Triassic

Jurassic

Cretaceous

252 million years ago20114566, the asteroid

cool stuff

Fun facts

Oneirosaurus had a unique feature: two holes in its skull for nerves X, XI, and XII to exit, a trait not seen in any other plioplatecarpine mosasaur.1

Its name means 'dream lizard,' because the scientists who described it thought finding such a complete fossil was like a dream.1

Oneirosaurus lived during the Coniacian stage, about 89.8 to 85.7 million years ago, in a shallow sea that covered what is now Colombia.2

The only known fossil of Oneirosaurus is a nearly complete skull with a lower jaw and a disarticulated atlas (the first neck vertebra).2

Oneirosaurus belonged to a group of mosasaurs called Plioplatecarpinae, which were medium-sized, fast-swimming predators.2

Its teeth had two sharp edges (carinae) arranged 180 degrees apart, perfect for slicing through flesh.2

AI-made picture of Oneirosaurus — An AI-made picture of how Oneirosaurus might have looked, made by a computer from what scientists know. It is a guess: the real animal may have looked different, and nobody knows its true colours.

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For dino experts

Scientific name: Oneirosaurus
Named by: Páramo Fonseca et al. 20253
Group: Mosasaurs
Family / clade: Mosasauridae3
Classification: Mosasauroidea3
Age of the rocks: 90 to 86 million years (Coniacian)3
Rock formations: Galembo3
Countries: Colombia3
Fossil records: 1 in the Paleobiology Database3
Discovery: Oneirosaurus was named in 2025 by María Eurídice Páramo Fonseca and colleagues, based on a skull found in the Galembo Formation of Colombia.1
Still debated: Scientists are still debating exactly how Oneirosaurus is related to other plioplatecarpine mosasaurs, especially its closeness to Yaguarasaurus columbianus and Ectenosaurus clidastoides.1

Where the facts come from

Oneirosaurus caballeroi gen. et sp. nov., a new mosasaur from Colombia | Earth Sciences Research Journal (revistas.unal.edu.co)
PBDB Taxon (paleobiodb.org)
Oneirosaurus in The Paleobiology Database (CC BY 4.0): age, places, rock formations, classification.

Every fact on this page has a number that points to its source. If we got something wrong, sorry, we are dino fans, not a science journal. Tell us at [email protected] and we will fix it.

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