A mosasaur is a large, carnivorous marine reptile that lived during the Late Cretaceous period, about 70-65 million years ago. The name “mosasaur” comes from the Latin Mosa, meaning “Meuse River”, and Greek sauros, meaning “lizard”. They were so named because the first fossils were found in Maastrichtian limestone deposits near the Meuse River in the Netherlands.
Mosasaurs were similar in appearance to snakes or lizards, and ranged in size from 6 to 40 feet (2 to 12 meters) long. They had long tails and four flipper-like limbs that they used for swimming. Their bodies were streamlined and covered in smooth scales, which may have been helpful for reducing drag while swimming.
Mosasaurs are thought to have evolved from an extinct group of land-dwelling reptiles called varanids, or monitor lizards. They probably arrived in the ocean by way of freshwater rivers and lakes.
Once in the water, mosasaurs thrived; they were top predators during their time on Earth. They fed on fish, other marine reptiles (including Plesiosaurs), and ammonites (a type of mollusk). Some species may even have eaten baby dinosaurs that fell into the water!
Because mosasaurs lived during a time when there was high global sea level (due to all of the melting ice at the poles), many of their fossils are now found inland, far from any current coastline. This is why most mosasaur bones that you see in museums come from North America; it was once part of a huge inland sea called the Western Interior Seaway.
Today, we know a lot about mosasaurs thanks to their well-preserved fossil record. We have even found complete skeletons with soft tissues still intact! Studying these remains has helped us understand more about these amazing creatures – how they lived, what they ate, and how they eventually went extinct.