Mosasaurus, the namesake of the group and first discovered mosasaur, was also one of the largest and most fearsome. Image by Andrey Atuchin.
was among the last of its kind, living at the end of the Age of Dinosaurs. It was also the largest carnivore of its day, reaching more than 17 meters (55 feet) long and 15 tons, with a powerful skull as big as a grown man. This real-life leviathan preyed on huge turtles, sharks and even other mosasaurs in the shallow Maastricht seas. When the Mesozoic Era came to an end 66 million years ago, Mosasaurus
and its fellow mosasaurs were the last of a relatively brief yet highly successful dynasty that ruled the seas during the last 25 million years of the Cretaceous.
At the end of the Cretaceous, there were several types of mosasaurs with rounded teeth for crushing shells, as well as lithely-built, fast-swimming pursuit predators that hunted fish in open water. Then there were whale-sized ambush predators, like Mosasaurus itself, with jaws built for both crushing and cutting up its victims.
Mosasaurs, like this Tylosaurus, had large shark-like tail fins used for fast swimming. Image by Julio Lacerda.
Despite the differences in diet and habitat, they all had a similar body plan: muscular and cylindrical bodies, long jaws and a powerful tail for propulsion. The limbs had evolved into smooth paddles used only for steering. For a long time, it was thought that mosasaurs swam like eels or crocodiles by moving their entire body from side to side, but a fossil of the medium-sized Platecarpus told a different story. It had what looked like an upside-down shark’s tail, with the fleshy upper lobe smaller than the lower. The animal also had a powerfully rigid body, with the tail being the principal mode of propulsion. Skin impressions from the same fossil indicate that it had scales designed to reduce the forces of drag in the water.
Another mosasaur fossil, a specimen of Prognathodon, also shows the presence of this fishlike body shape and a tail very similar to the Platecarpus specimen, thus making the fluke and shark-like body a feature of the mosasaur family in general. The mosasaurs were clearly much more powerful swimmers than we had realized, with incredibly sleek bodies for effortless movement through the seas.
They were so well adapted to life in the oceans that they gave birth to live young, possibly bearing several small offspring rather than just a few large ones like whales do today. Fossils of baby Clidastes from Kansas have been found in open-water settings and not in sheltered nurseries near the coast. That, and the birthing of many babies at a time indicate that mosasaurs did not care for their young for extended periods of time. Their growth was similar to most other reptiles.
Never before had a marine reptile group reached such diversity in such a short time. Not only was Mosasaurus itself huge, but it was incredibly successful. Its fossils have been found from Russia to North Dakota to Morocco. Other mosasaurs had achieved a worldwide distribution towards the end of the Cretaceous, like the shellfish-eating Globidens and Prognathodon and the small Halisaurus. Freshwater mosasaurs like Pannoniasaurus grew as large as crocodiles and probably competed with crocs for the same resources. There are even mosasaur remains from Antarctica. This worldwide distribution highlights just how adaptable these creatures were.
Some mosasaurs, like this Globidens, had teeth modified for crushing shells. Image by Brittany Bostain.
Where exactly did these creatures come from? And how were they able to mushroom into such a plethora of forms in such a short period? The fossils of the earliest mosasaurs date back to about 98 million years ago, but they were not exactly the most diverse or widespread marine reptiles. Their success proper was due in part to sheer luck. About 92 million years ago, an extinction event occurred that was caused by large-scale underwater volcanic activity. It wiped out several groups of animals in the seas, among these were the ichthyosaurs or “fish lizards”, and the pliosaurs, large-headed and more predatory cousins of the plesiosaurs. With their demise, there was a huge gap in the ocean food web.
During the middle of the Cretaceous Period, about 98 million years ago, a family of lizards related to modern monitors called the aigialosaurs began returning to the oceans. They looked like and were around the same size of most monitor lizards, just over a meter and a half in length. They bred on land, and still had the characteristic features of a regular lizard, with a long tail and separated toes, although this was soon to change.
Currently, two definitive mosasaur ancestors are known from North American rocks. One is Dallasaurus turneri, a small semi-aquatic lizard, while the other is Russellosaurus coheni. Both lived in Texas at the time of the extinction of the ichthyosaurs. They were long-bodied aquatic lizards barely over a meter in length. The better-known one of the two, Dallasaurus turneri had very reduced limbs and a slim body. It also had a slim snout characteristic of the later mosasaurs. The legs would soon evolve into flippers and the body would become streamlined as the family evolved. The blueprint for the typical mosasaur is thus contained in tiny Dallasaurus.
Upon its discovery, the little lizard was famously hailed as the ancestor of the entire mosasaur line, especially due to it being one of the more complete early mosasaur fossils. Later research revealed that Dallasaurus was only ancestral to the mosasaurines and prognathodontids. Russellosaurus on the other hand, is ancestral to the tethysaurines, plioplatecarpines, tylosaurines and halisaurines.
The tethysaurines are among the most primitive mosasaurs known, and the most similar to their lizard ancestors. The freshwater Pannoniasaurus from Hungary is one such tethysaurine. Plioplatecarpines and halisaurines were usually medium-sized animals that fed on small prey, while tylosaurines were the exact opposite. The tylosaurines were mostly very large but lightly-built animals built for ambush. Russellosaurines had highly moveable lower jaws to help them swallow their prey whole. On the other hand, the mosasaurines like Mosasaurus and Clidastes include both some of the biggest and smallest of the mosasaurs. Most of them had immobile lower jaws and had teeth for crushing and slicing both large and small prey. The related prognathodontids like Globidens and Prognathodon were mostly powerfully built animals with blunt teeth for dealing with hard-shelled prey.
The Western Interior Seaway was home to huge numbers of mosasaurs like Tylosaurus. Image by Henry Sharpe.
The best place to see the increasing success of the mosasaurs is the Smoky Hill Chalk member of the vast Niobrara Formation in Kansas, USA. It contains the sediments of the Western Interior Seaway, a large body of water that cut North America in two parts, eastern and western, from around 100 to 66 million years ago. The Smoky Hill deposits in the Niobrara showcase an ever-changing environment full of both long-necked and short-necked plesiosaurs, massive sea turtles, flightless diving birds, some decidedly frightening carnivorous fish, and giant clams. These chalk beds run during the later stages of the period, from 87 to about 82 million years ago.
The three best-known mosasaurs of the chalk, also known as the “first-wave” mosasaurs, are the mosasaurine Clidastes, the plioplatecarpine Platecarpus and the tylosaurine Tylosaurus. Remains of all these mosasaurs were described in the 19th Century by famous rival paleontologists Othniel Charles Marsh and Edward Drinker Cope. During the beginning of their reign, all three of them were medium-sized animals, with Tylosaurus being the biggest, about as big as the larger sharks of the time.
By the end of the Smoky Hill Chalk though, all of the “first-wave” mosasaurs doubled in size and Tylosaurus emerges as the apex predator of the Niobrara Sea. One large Canadian specimen, nicknamed “Bruce,” is the biggest mosasaur skeleton on display in a museum. It was an animal roughly 13 meters (42 feet) in length, much smaller and lighter than the later Mosasaurus but no less fearsome a predator.
Tylosaurus’ primary feature was a cylindrical ram projecting from its upper jaw. It had a highly flexible lower jaw compared with the robust and stiff skulls of many later mosasaurs, with fossils showing relatively large prey items having been swallowed whole. It was, however, not averse to scavenging as seen in tooth marks on a drowned dinosaur carcass from Alaska. Fossils of Tylosaurus also helped in understanding aspects of mosasaur life that had previously been the realm of guesswork: Their color.
Microscopic examination of mosasaur skin shows that these animals were dark on their backs and lighter on their bellies, like this Clidastes. This pattern is a type of camouflage known as countershading. Image by Andrey Atuchin.
In 2014, a study compared fossilized skin specimens of a Tylosaurus against those of an ichthyosaur and a leatherback turtle. Johan Lindgren and colleagues tested the specimens for signs of preserved skin pigments like melanin in structures called melanosomes. The tests proved that not only was Tylosaurus very darkly colored on top but that it and probably other mosasaurs were countershaded. This meant that like large marine animals today, they were darker on top and lighter on below. Countershading is a kind of camouflage which enables an animal to break up its outline from both the bottom and the side, very useful for an ambush predator. However, we do not know if all mosasaurs were similarly countershaded or even similarly colored.
The placement of mosasaurs among the reptiles is still poorly understood even with the fossil record in place to give us a framework on their evolutionary history. Most modern-day studies place mosasaurs as a relative to both snakes and monitor lizards. A group called “Pythonomorpha”. There is also plenty of doubt concerning the early evolution of the mosasaur line, and how the primitive and advanced species are related. One of the greatest problems concerning mosasaur research involves their last days in the Western Interior Seaway. The shallow seas regressed towards the end of the Cretaceous, thus causing a collapse of the mosasaurs’ home territories.
The mosasaurs were one of the greatest success stories of the Mesozoic Era, and it took the mass extinction at the end of the Cretaceous Period to wipe them out completely. Their rapid evolution not just tipped the scales in their favor in terms of long-term domination of their oceanic realm but also paved the way for some of the most diverse and adaptable of all the ancient marine reptiles. Had it not been for the mass extinction at the end of the Mesozoic, the mosasaurs might have even taken the place of whales today.
Special thanks are owed to Nathan Van Vranken in research and preparation of this story.