Pictures of a mosasaur often highlight its vicious teeth, which it used to eat all kinds of smaller animals. © Dotted Yeti/ Shutterstock
Towards the end of the dinosaurs’ reign on land, a group of fearsome marine reptiles called mosasaurs dominated the oceans. They’re named after Mosasaurus, the most famous member of the group.
Discover more about Mosasaurus’ size, where its fossils are found and how realistic the mosasaur in the film Jurassic World really is.
In the mid-eighteenth century, Dutch miners made an extraordinary discovery.
As they picked away at the rocks in a limestone quarry near the city of Maastricht, in the Netherlands, the glint of fossil teeth shone in the darkness. More teeth followed, then bones, revealing the head of an ancient animal unlike anything ever seen before.
Little did they know, but they’d actually discovered the first known Mosasaurus fossils. Over the following decades, the bones of this animal and its relatives would come to change how we understood life on Earth.
Though they lived at the same time as Mosasaurus, dinosaurs aren’t closely related to these prehistoric marine reptiles. Mosasaurus was also discovered decades before the dinosaurs were named. Public domain image by G R Levillaire via the Library and Archives at the Natural History Museum.
Mosasaurus isn’t a dinosaur. In fact, when the first fossil of this species was discovered in 1766, it would be almost 80 years until the term ‘dinosaur’ was coined.
However, it’s understandable to be confused. Scientists didn’t really pay attention to what’s now known as Mosasaurus until a second, better preserved skull was also discovered near Maastricht around a decade later. The discovery was widely publicised and came to the attention of many different experts, who suggested it could be a fish, a crocodile or even a whale.
In 1795 the French army invaded the Netherlands and seized the skull, taking it to Paris where it’s still on display today. Nonetheless, scientists continued to research and debate what the animal was. Adriaan Camper was the first to notice the skull’s similarities with living reptiles and suggested the bones came from a giant lizard.
This idea was backed by the famous French naturalist Georges Cuvier, which settled the argument in favour of the animal being a reptile. In 1822, the fossils were finally named Mosasaurus, meaning ‘Meuse lizard’, after the river that flows through Maastricht.
Dr Marc Jones, our curator of fossil reptiles and amphibians, says that we now know that Mosasaurus and all other mosasaurs are marine reptiles. Specifically, they’re squamates – the group of reptiles that snakes and lizards are also a part of.
“Mosasaurs have very squamate-like skulls, such as their very characteristic lizard-like ear region,” Marc explains. “They also have a bone known as the quadrate that’s very mobile, just like snakes, which allows different parts of the mosasaur skull to move independently to consume larger prey.”
“However, we’re not sure what part of the squamate family they’re part of. While there are arguments that they could be marine relatives of monitor lizards or snakes, mosasaurs are so specialised that there’s not currently enough evidence either way.”
The scientific name of a Mosasaurus varies depending on the species, but none of them are called Mosasaurus mosasaurus. The first named species was Mosasaurus hoffmannii.
The five recognised Mosasaurus species vary in size. It’s thought that the original species, classified as Mosasaurus hoffmannii by Gideon Mantell in 1829, is the largest of the five.
This large range of variation is because the skull is much more commonly found than the rest of the Mosasaurus skeleton. It means scientists normally have to estimate overall mosasaur length from the size of their heads, which can be up to two metres long.
Some scientists suggest that the biggest Mosasaurus hoffmannii could have measured 17 or 18 metres in length, though others believe 11 to 12 metres is more likely.
Whichever estimate is more accurate for Mosasaurus hoffmannii, the other four species would have been significantly shorter. Mosasaurus missouriensis and Mosasaurus beaugei, for example, are estimated to have been around 8–10 metres long.
A Mosasaurus’ weight is just as difficult to estimate because only the bones survive. The best estimates, using comparisons with living animals, suggest that an adult Mosasaurus might have weighed around 10 tonnes, but we can’t say for certain.
When it comes to megalodon vs mosasaur size, megalodon would have been the bigger animal at over 24 metres in length. In comparison, the largest Mosasaurus length estimates come in at around 18 metres. Megalodon image adapted from © Scarlet23 via Wikimedia Commons, licensed under CC BY-SA 3.0, and Mosasaurus and diver public domain images via Wikimedia Commons.
While these animals didn’t all live at the same time, here’s how they would’ve measured up to each other:
Though it plays a key role in the film Jurassic World, scientists say that there’s work to be done to make its Mosasaurus realistic. Image © Troutfarm27 via Wikimedia Commons, licensed under CC BY-SA 4.0.
Anyone who’s seen the film Jurassic World will remember its Mosasaurus, an enormous creature that lives in an equally vast tank in the theme park. Its swallowing of unlucky personal assistant Zara is certainly one of the film’s most memorable moments, but how accurate were the filmmakers? It turns out the one in the film is not that true to the real Mosasaurus.
“Though it’s great to see a cinematic Mosasaurus, Jurassic World’s is just too big,” says Marc. “It’s gigantic, and probably around the same size as the blue whale – the largest animal that’s ever lived.”
“The real Mosasaurus certainly wouldn’t have been big enough to eat a Tyrannosaurus rex-sized animal in one go.”
Another issue with Jurassic World’s Mosasaurus is how it swims. We first see it in the tank swimming like an eel, with a long thin tail extending out behind it. While early mosasaurs are likely to have swam like this, more evolved species such as Mosasaurus probably moved differently through the water.
A preserved tail discovered in a relative of Mosasaurus suggests it could have had a tail fluke with a large, downward-pointing fin that it used to push itself through the water. This is much more efficient than the eel-like swimming method and would’ve made Mosasaurus more manoeuvrable in the water.
This would’ve been helped by the snake-like scales covering the body of Mosasaurus – we know a relative called Plotosaurus had these too. These scales would’ve made mosasaurs more streamlined, allowing them to cut through the water more easily. Hollywood’s Mosasaurus, however, doesn’t seem to be very scaly.
Though there are issues with its mosasaur, Jurassic World’s marine reptile isn’t completely inaccurate.
“I was very pleased to see the pterygoid teeth on the roof of the Mosasaurus’ mouth, which is a characteristic found in its fossils,” Marc notes. “They would’ve helped it to grab prey, and similar arrangements are still found in snakes today.”
During the film’s finale, Mosasaurus jumps out of the water to grab a nearby dinosaur. Though there’s no clear evidence Mosasaurus did this in real life, some marine predators such as orca are able to briefly lunge onto land while hunting so it’s not totally out of the question.
How big is a mosasaur? Their size varies, but this 14-metre long Tylosaurus skeleton found in Kansas, USA, is among the largest known. Image © MCDinosaurhunter via Wikimedia Commons, licensed under CC BY-SA 3.0.
Mosasaurus lends its name to the much wider group of marine reptiles that it’s a part of. This family is called Mosasauridae and any member of it is called a mosasaur.
Mosasaurs are split into five main groups. As well as the mosasaurines, which contains Mosasaurus and its relatives, there are the tethysaurines, tylosaurines, halisaurines and plioplatecarpines.
While these animals are ultimately descended from squamates, their closest relatives are a group of reptiles called the aigialosaurs. Fossils of these animals are found in rocks in North America and Europe dated to between 100 million and 90 million years ago.
Aigialosaurs show signs of transitioning from life on land to life in the water, but were probably able to live in both environments. Their limbs are similar to those of lizards that live on land, but they have very long tails that they probably would have used to swim.
Over time, some aigialosaurs developed flipper-like limbs – these made them better adapted to life in the water. Eventually, these animals evolved into the first mosasaurs.
One of the key facts about mosasaurs is that their fossils are only found in a small part of the fossil record covering less than 40 million years.
Mosasaurs first evolved in the Late Cretaceous Period, which lasted from around 100 million to 66 million years ago. Some of the oldest known fossils are around 94 million years old, suggesting the first mosasaurs must have evolved slightly earlier.
At this time, the world was around 5–10°C hotter than it is now. It’s thought undersea volcanoes were releasing vast amounts of carbon dioxide into the atmosphere and causing rapid global warming.
This had a particular impact on the oceans, with oxygen levels dropping dramatically and tropical water temperatures reaching 35°C or more.
Many marine species were unable to cope with these changes, causing widespread extinctions. Even long-lived marine reptiles such as the ichthyosaurs and pliosaurs, which had existed for more than 100 million years, went extinct in the face of such severe climate change.
The extinction of these animals provided mosasaurs with an evolutionary opportunity, as did major changes to the world’s oceans.
Globidens dakotensis is a shellfish-eating mosasaur. Its remains have been found in rocks in central USA, in an area which millions of years ago used to be covered in a large body of water called the Western Interior Seaway. Map image © Sampson et al. via PLOS One, licensed under CC BY 4.0, and Globidens skull image © Jonathan Chen via Wikimedia Commons, licensed under CC BY-SA 4.0.
While the first known mosasaur fossils were found in the Netherlands, the remains of these animals have since been found in other parts of Europe and around the world. Their teeth and bones have been uncovered at a range of sites stretching from Canada all the way to islands off the coast of Antarctica.
Their fossils are found over such a large area because sea levels were as much as 220 metres higher in the Cretaceous Period than they are today. High temperatures meant there were no ice caps to lock away water at the poles, while shallower ocean floors meant the seas were pushed onto land.
The evidence of this can be seen today in Canada and the USA, which during the Cretaceous Period were cut in half by an ancient body of water known as the Western Interior Seaway. States such as Saskatchewan and Kansas are rich in mosasaur skeletons, even though today they’re hundreds of kilometres away from the nearest ocean.
At the same time, Earth’s large landmasses were breaking up into the continents we’re familiar with today, providing many new habitats for mosasaurs. The Atlantic Ocean was getting wider as the Americas moved away from Africa and Europe. Antarctica and Australia were also drifting further apart.
Mosasaurs were well adapted to spread into new areas thanks to some crucial differences from their land-based relatives. Analysis of mosasaur fossils suggests that, unlike other squamates, at least some species were warm blooded and better able to tolerate everything from cool polar waters to warm tropical oceans.
They also didn’t need to return to land to give birth, with fossils of pregnant mosasaurs showing that these marine reptiles could give birth to live young in the open ocean.
Some mosasaurs, however, preferred freshwater to saltwater. Pannoniasaurus is currently the only named freshwater mosasaur and was found in what’s now Hungary. It had a crocodile-like skull that would’ve helped it to grab prey in shallow water as well as, perhaps, on the shoreline.
Other aquatic mosasaurs also existed, but little is known about their lives. A mosasaur tooth found in freshwater sediments in the USA suggests that the reptiles might have spread from the Western Interior Seaway into rivers, but more evidence is needed.
Mosasaur fossils have been found in the UK for hundreds of years but are often fragmentary. Adapted from a public domain image by James Erxleben via Flickr.
Mosasaur fossils aren’t common in the UK. A group of Cretaceous rocks found in southern England called the Chalk preserve lots of animal remains in extraordinary detail, but mosasaur remains are rare.
Nonetheless, some mosasaur remains have been found in Britain. St James’ Pit in Norfolk is a protected site due to the number of mosasaur teeth, jaws and backbones that have been found there. Mosasaur fossils have also been found in Kent, Hampshire, Surrey and elsewhere.
Marc is currently involved in a project called ChaSE to redate the UK mosasaur specimens that he looks after in order to better understand these animals.
“Mosasaur fossils from the Chalk often don’t have specific dates attached to them,” Marc explains. “As part of the ChaSE project, we’re going back and dating British mosasaur fossils, which is really exciting.”
“By refining the dates of these specimens, we’ll have a better idea of when certain species were alive and if they overlapped. Small changes in the dates of these fossils can even shift how we think these species are related, giving us a better idea of this mosasaur community.”
Mosasaurus’ teeth are curved and have sharp points, but those of other mosasaurs varied depending on their diet.
All mosasaurs are believed to be carnivores, eating other animals to survive. Unlike other squamates, mosasaurs evolved teeth with roots that sit in sockets that helped them to deliver forceful bites without hurting themselves.
The exact shape of the teeth varied depending on the prey a particular species hunted. Most mosasaur species have curved and pointed teeth that helped them grab fish, squid and sharks.
Meanwhile, the species Globidens has curved teeth that were probably used to grip and crack open ammonites. The species Xenodens, on the other hand, has saw-like teeth that might have carved chunks of flesh from its prey.
Some well-preserved mosasaur fossils still contain their stomach contents. These specimens show that mosasaurs were even capable of eating other marine reptiles.
“The lower mosasaur jaw has a joint about halfway along its length that allows it to hinge, meaning these reptiles could eat much larger prey than you might expect,” explains Marc. “We know they were able to eat plesiosaurs and other mosasaurs.”
“Some mosasaurs, such as Tylosaurus, also have a bony snout with their teeth set back behind it. It’s been suggested that this reinforcement was necessary so that it could ram other animals to stun or even kill them, similar to the behaviour of some modern dolphins.”
Extraordinarily preserved skin from one Tylosaurus fossil also shows that mosasaurs had black skin. They might have been totally black, hiding them in dark waters as they hunted, or the black skin might have been part of a larger pattern for camouflage or signalling.
In any case, the fact they had many different diets and lifestyles meant that lots of different mosasaur species could live alongside each other without competing. This supported rich and diverse ecosystems right up until the end of the Cretaceous Period.
As one of the earliest known extinct animals, Mosasaurus inspired scientists and artists alike. Image © Dreizung via Wikimedia Commons, licensed under CC BY-SA 4.0.
By the end of the Cretaceous Period, 66 million years ago, mosasaurs were still thriving. Though some smaller species had gone extinct as temperatures dropped from their peak near the start of the Late Cretaceous Period, larger species were still living all around the world.
Rocks in Morocco from the end of the Cretaceous Period, for example, contain the remains of as many as 16 different species of mosasaur. The presence of so many large predators together at the same time suggests that the marine reptiles, and the wider oceans, weren’t in decline at this time.
Instead, it’s likely that the mosasaurs were wiped out for the same reason the dinosaurs were – a massive asteroid striking Earth just off the coast of what is now Mexico.
Dust thrown up into the air would’ve blocked out the light from the Sun, causing populations of photosynthetic algae to collapse. In turn, this would have brought down the rest of the food chain as increasingly larger animals found themselves with nothing to eat.
Mosasaurs, as the top predators, would’ve been most vulnerable to the sharp decline in their prey. Without enough food to stay alive, all mosasaurs would’ve died shortly after the asteroid strike.
Fittingly, millions of years later, it was the fossils of Mosasaurus that helped to inform Georges Cuvier’s theories around extinction. The idea that species die out was groundbreaking at the time.
Georges’s work went on to inspire later theories such as natural selection that have revolutionised our understanding of nature. Without the mosasaurs, the history of science could look very different indeed.
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