Just before the end of the Cretaceous Period, the seas were home to gigantic lizards as terrifying and as huge as any mythological sea serpent. Called mosasaurs, these monsters were close relatives of today’s lizards and snakes. They evolved from small, land-living lizards that took to the water in search of food. As the lizards adapted to life in the sea, their legs turned into flippers and their bodies, supported by water, became enormous.
FAMILY FACT FILE
■ Lizardlike bodies with flippers
■ Powerful jaws lined with sharp teeth
■ Breathed air at the water surface
Mosasaurs lived in the Cretaceous Period, between 85 and 65 million years ago. They were killed along with dinosaurs and most other large reptiles in the mass extinction at the end of the Period.
In the Cretaceous, an ocean ran down the middle of North America, cutting the continent in two. Its muddy bed has now turned to rock, forming the Niobrara chalk of North America. Niobrara chalk contains a wealth of amazing fossils, including mosasaurs and plesiosaurs.
■ When: 70–65 million years ago (Late Cretaceous)
■ Fossil location: USA, Belgium, Japan, Netherlands, New Zealand, Morocco, Turkey
■ Habitat: Oceans
■ Length: About 50 ft (15 m)
■ Diet: Fish, squid, and shellfish
Mosasaurus was one of the largest mosasaurs. It looked a bit like a crocodile with flippers, and swam by moving its long body in slow waves. As a result, it couldn’t swim fast over long distances, but might have been capable of sudden bursts of speed. Scientists think Mosasaurus lived in the well-lit surface waters of oceans, hunting slower-moving prey. Bite marks left by its huge, conical teeth have been found on turtle shells and ammonites. Mosasaurus survived to the end of the Cretaceous, when it vanished with the dinosaurs.
■ When: 85–80 million years ago (Late Cretaceous)
■ Fossil location: Worldwide
■ Habitat: Oceans
■ Length: 14 ft (4.2 m)
Platecarpus was not the largest mosasaur but it was certainly was one of the most abundant. Its fossils have been found worldwide, most commonly in the Niobrara chalk beds of North America. Like other mosasaurs, Platecarpus used its long, muscular tail to drive itself through water in a zigzag, snakelike manner. It had fewer and smaller teeth than other mosasaurs, suggesting a diet of softer prey such as fish and squid.
The belly of one specimen was found to contain fish scales and fish bones—the remains of one of its last meals.