Bryan McAllister’s personal reference notebook
July 30, 2026Short & citedBlog
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Field Notes • Paleontology

Mosasaurus

Mosasaurus is an extinct genus of large marine reptiles from the Late Cretaceous.

Updated July 30, 20261 min readCited sources

It was not a dinosaur, but a marine reptile related to modern snakes and lizards.

What it is

Mosasaurus is a genus of mosasaur, an extinct group of marine reptiles within Squamata. It was not a dinosaur, but more closely related to modern snakes and lizards. It had a long body, powerful tail, paddle-like limbs, and a skull adapted for catching prey.

Why it matters

Mosasaurus was a top marine predator in Late Cretaceous seas, feeding on fish, cephalopods, and other marine animals. Its fossils have been found on multiple continents, including Canada, showing its wide distribution in Atlantic and related seaways.

Key details

Mosasaurus lived about 70–66 million years ago. The largest species reached up to 15–17 metres in length. The first scientifically described fossils were found near Maastricht, Netherlands, in 1764. It went extinct at the end of the Cretaceous.

In short

  • Not a dinosaur, but a marine reptile related to snakes and lizards
  • Top predator of Cretaceous seas, eating fish and other marine animals
  • Fossils found worldwide, including Canada
  • Extinct about 66 million years ago
Canadian angle

Mosasaur fossils, including mosasaur remains, have been found in Canada, and Canadian museums and public education resources commonly feature them as part of the country’s fossil record.

Quick questions

What is a Mosasaurus?
Mosasaurus is an extinct genus of large marine reptiles from the Late Cretaceous, known for being a top ocean predator.
Is a Mosasaurus a dinosaur?
No. Mosasaurus was a marine reptile in the squamate lineage, not a dinosaur.
How big was a Mosasaurus?
Some estimates place the largest species at about 15–17 metres long, though size varies by species and source.

Sources

  1. Natural History Museum — https://www.nhm.ac.uk/discover/what-is-a-mosasaur.html
    Supports: Mosasaurus is not a dinosaur; mosasaurs are marine reptiles and squamates; fossils found in Canada and around the world.
  2. National Park Service — https://www.nps.gov/articles/000/mosasaur.htm
    Supports: Mosasaurs as marine reptiles; size up to 50 feet; predatory diet; fossils from the Western Interior Seaway.
  3. Fossil Guy — https://www.fossilguy.com/gallery/vert/reptile/mosasaur/index.htm
    Supports: Late Cretaceous age, extinction about 66 million years ago, global fossil distribution, and non-dinosaur classification.
  4. Dino Tail — https://www.dino-tail.com/en/extinct/mosasaurus/mosasaurus.html
    Supports: Genus-level classification, approximate length estimates, diet, first documented fossil near Maastricht, and non-dinosaur status.