Mosasaur

Mosasaurs were a group of large aquatic reptiles that lived during the Late Cretaceous period.

Their name comes from the Latin word for the Meuse river and the Greek word for lizard. They belonged to the order Squamata, which is the same group that includes modern snakes and lizards. During the last 20 million years of the Cretaceous, mosasaurids became the dominant marine predators. This happened after other large hunters, such as ichthyosaurs and pliosaurs, went extinct. However, mosasaurs also went extinct about 66 million years ago during the K-Pg event.

These animals were perfectly adapted for life in warm, shallow inland seas. Mosasaurs breathed air and were powerful swimmers. Their bodies were elongated and streamlined, similar to modern monitor lizards, but built for the water. Their limbs were reduced in length, and their paddles were formed by webbing between long finger and toe bones. Most advanced mosasaurs had broad, laterally compressed tails. These tails ended in a large, crescent-shaped fluke, much like those seen on sharks. Instead of undulating their entire bodies like eels, they likely kept their bodies stiff to reduce drag. They used their powerful tails for propulsion and may have used their front flippers in a breaststroke motion.

Mosasaurs were diverse in size and specialized in how they hunted. The smallest known species was Dallasaurus turneri, which was less than 10 feet long. In contrast, Mosasaurus hoffmannii was a massive apex predator. It could reach lengths of up to 50 feet, though some scientists believe this may be an overestimate. Different species had different diets based on their teeth. Most were active predators of fish and ammonites, which were molluscs with shells. Some species, like Globidens, had blunt, spherical teeth specifically for crushing mollusk shells. Others, like the smaller Platecarpus, likely hunted smaller prey in both salt and fresh water.

Their hunting methods were highly efficient due to unique skull features. Mosasaurs possessed double-hinged jaws and flexible skulls, similar to the mechanics of a snake. This allowed them to gulp down prey almost whole. Evidence of their feeding can be seen in the fossil record. For example, triangular puncture holes in the shells of ammonites like Placenticeras match the bite marks of medium-sized mosasaurs. Skeletons of the species Tylosaurus proriger have even been found with the remains of fish, sharks, and even flightless birds inside them.

Recent scientific discoveries have changed how we view their biology. For a long time, artists drew mosasaurs with large dorsal crests on their backs. Scientists now know these were errors caused by misidentifying tracheal cartilage. We also have new information about their skin. Fossils from Jordan show that mosasaurs had small, overlapping, diamond-shaped scales like snakes. These scales were often keeled, or ridged, on the upper body and smooth on the lower body. This might have helped them hide while lurking as ambush predators. Furthermore, studies suggest mosasaurs were endothermic, meaning they were warm-blooded. They could internally regulate their temperature to stay warmer than the surrounding water.

Even their teeth followed a complex biological process. Mosasaurs had thecodont dentition, which means their tooth roots were deeply cemented into the jawbone. They did not have permanent teeth but instead constantly shed and replaced them. This happened through an eight-stage process involving a resorption pit inside the original tooth root. A new tooth crown would develop in the pit, grow to full size, and then develop its own root. Eventually, the new tooth would push the old one out and anchor itself in the vacancy.

Understanding mosasaur life cycles provides a window into ancient marine ecosystems. While many reptiles lay eggs on land, mosasaurs likely gave birth to live young in the open ocean. This adaptation allowed them to remain fully marine. Some evidence, such as a large fossilized egg found in Antarctica, suggests that some large marine amniotes lived near the end of the Cretaceous. Finally, we know their color patterns through the study of melanin in fossils. They likely used countershading, with dark backs and light underbellies, to remain camouflaged in the water.