Short answer: a mosasaur was not a dinosaur. Mosasaurs were marine reptiles in the Squamata, the group that includes lizards and snakes. They lived in the oceans at the same time as dinosaurs lived on land, during the Late Cretaceous, and both disappeared in the extinction event about 66 million years ago.
What was a mosasaur?
Mosasaurs were large predatory reptiles fully adapted to life at sea. Their limbs had become paddle-like flippers, their bodies were long and streamlined, and many had a powerful tail for swimming. Their jaws carried rows of teeth adapted to different diets: slender teeth for catching fish, robust conical teeth for large prey, and in Globidens, rounded teeth for crushing shells.
The name comes from the Meuse River near Maastricht in the Netherlands, where the first mosasaur fossils were found in the 18th century. That same region gave its name to the Maastrichtian, the final stage of the Cretaceous, when mosasaurs were at their most diverse.
Why mosasaurs are not dinosaurs
“Dinosaur” is a specific scientific group, not a word for any large prehistoric reptile. Dinosaurs were land animals with an upright posture, their legs held directly under the body. Mosasaurs belong to a completely different branch of the reptile family tree, closer to today’s monitor lizards and snakes than to any dinosaur.
| Mosasaurs | Dinosaurs | |
|---|---|---|
| Reptile group | Squamata (lizards and snakes) | Dinosauria |
| Habitat | Oceans | Land (birds are living dinosaurs) |
| Limbs | Flippers | Legs held under the body |
| When they lived | Late Cretaceous | Triassic to end of Cretaceous |
| Moroccan examples | Mosasaurus, Prognathodon, Thalassotitan, Globidens, Halisaurus | Spinosaurus, Carcharodontosaurus |
What about plesiosaurs?
Plesiosaurs, such as the long-necked Zarafasaura oceanis from Morocco, were another group of marine reptiles. They swam with four large paddles and were neither dinosaurs nor mosasaurs. Mosasaurs and plesiosaurs shared the same Late Cretaceous seas off Morocco.
Morocco: the world’s richest mosasaur fossil site
The phosphate deposits of Morocco preserve some of the most diverse mosasaur faunas known anywhere. Around a dozen mosasaur species lived in the waters that covered the region at the very end of the Cretaceous, from the giant apex predator Thalassotitan atrox, described in 2022, to the shell-crushing Globidens. Learn more in our guide to the Oulad Abdoun Basin.
Why the difference matters to collectors
Many listings online describe mosasaur skulls and jaws as “dinosaur fossils.” That is a scientific error. A correct label tells you the seller understands what the specimen is. When you buy a mosasaur fossil, look for the genus (for example Prognathodon), the age (Late Cretaceous, Maastrichtian) and the locality (for example the Oulad Abdoun Basin).
Frequently asked questions
Did mosasaurs and dinosaurs live at the same time?
Yes. Mosasaurs lived during the Late Cretaceous, when dinosaurs such as Tyrannosaurus lived on land.
What were mosasaurs related to?
They were squamates, related to modern lizards and snakes. Monitor lizards are often cited as among their closest living relatives.
Were mosasaurs bigger than dinosaurs?
The largest mosasaurs reached lengths comparable to a large orca, but the biggest sauropod dinosaurs were far larger. Size is not what separates the two groups.
Explore Moroccan mosasaur fossils
Sources
- Longrich et al. (2022), Thalassotitan atrox, a giant predatory mosasaurid from the upper Maastrichtian phosphates of Morocco. Cretaceous Research. doi:10.1016/j.cretres.2022.105315
- Muséum national d’Histoire naturelle, The phosphates of Morocco