The mosasaurs are one of the most spectacular success stories of the Mesozoic. In a period of time spanning less than 35 million years, they went from very small semi aquatic lizards to the apex predators of the world’s oceans. This success comes from the mosasaurs ability to rapidly adapt to a variety of aquatic habitats while at the same time attacking and hunting quite literally anything that was in the water with them. In all seriousness the only thing a big mosasaur needed to be concerned about was an even bigger mosasaur, which makes mosasaurs vicious, fascinating, terrifying and altogether awesome.
Our study into mosasaurs goes all the way back to 1764 when the first ever mosasaur skull was discovered in the Netherlands. This skull was fragmentary and initially thought to belong to a then unknown species of fish. In the early 1770‘s a second partial skull was found. After this a fellow by the name of Johann Leonard Hoffmann, a retired army physician, discovered further fossil remains, and made an effort to have them looked at by the wider scientific community. Hoffman disagreed with the idea that these remains were of a fish and instead thought they belonged to a gigantic crocodile. In 1786 a Dutch professor named Petrus Camper came to the conclusion that the fossils belonged to a toothed whale.
The fossils were stored in the Fortress of Maastricht where they were kept until the fortress fell to French military forces and the region of Maastricht was annexed by the Napoleonic Empire. The fossils were moved to France where Barthélemy Faujas de Saint-Fond studied them and came to agree with the idea that they belonged to a large crocodile. However the original notes written by Petrus Camper had not gone to France, and when Petrus’s son Adriaan Gilles Camper reviewed his father’s notes he came to what would be remembered as a surprisingly close discovery. Adriaan’s conclusion was that the fossils remains were of a gigantic monitor lizard, and this is something that we now know to have been as close to the right answer without actually being right as you could get. Support for this idea came in 1808 when Georges Cuvier, a leading figure in the field of comparative anatomy also agreed with the monitor lizard hypothesis.
It was not until 1822 that these fossils were given the name Mosasaurus by William Conybeare, and the name which means ‘Meuse River Lizard’ which was near where the fossils were located. However at this time Mosasaurus was still perceived to being a huge monitor lizard. In 1829 Gideon Mantell (famous for naming Iguanodon, the first plant eating dinosaur to be described to science) made the name official when he designated the type species as Mosasaurus hoffmanii in memory of Johann Leonard Hoffmann (it should also be noted that at this time many extinct animals that were being named were often given genus names lacking a species name, something that had to be sorted out by later naturalists and palaeontologists).
It was not 1854 that Mosasaurus was put forward as an aquatic creature that lived in the ocean. This came about from an observation by Hermann Schlegel who noted that the limbs of Mosasaurus were not suited for land movement and would have been far more use as flippers. Later discoveries would prove Schlegel right and by the closing decades of the nineteenth century mosasaurs were not just being found in marine fossil deposits of not just Europe but North America as well. By the early twenty-first century mosasaur fossils have been recorded in Late Cretaceous marine deposits on every major continent showing that the mosasaurs were so successful that they spread out across the globe wherever the habitat was suitable for them.
The Rise of the Mosasaurs
Most of the known primitive mosasauroids are currently aged as living in the Cenomanian and Turonian eras. The Cenomanian and the Turonian are the first two time periods that make up the Late Cretaceous, and about 100.5 to 89.9 million years ago. What is interesting here is that about the time of the boundary between the Cenomanian and Turonian there was actually an anoxic event. An Anoxic event is where the levels of dissolved oxygen in the water plummet, and the animals that breathe through gills asphyxiate and die, making this an extinction level event.
At this time the main kinds of large predatory marine reptiles in the water were ichthyosaurs, plesiosaurs and pliosaurs. As reptiles these were all air breathing creatures, so some may assume that a drop in water oxygen levels would not affect them, and directly it wouldn’t. Indirectly however this drop would have killed off colossal amounts of fish and cephalopods (squids, ammonites, etc.), which would have been the key prey species of the plesiosaurs, ichthyosaurs and some pliosaurs. With far less prey to eat these would die out through starvation, and indeed for the ichthyosaurs things seem to have been particularly bad. Almost all of the ichthyosaur genera that we know of that made it to see the Cenomaninan of the Cretaceous vanished at about the time of this extinction event, and the rare few that survived died out a little bit later in the Cretaceous. The large Pliosaurs with genera such as Brachauchenius that survived the extinction event itself disappear from the fossil record in the Turonian, possibly as a result of the other marine reptiles dying out making less prey available to them. Out of these three groups only the plesiosaurs are known to have made it to the end of the Cretaceous, and then seemingly in reduced numbers.
With the oceans relatively empty of the classical large marine reptiles, the stage was set for a new group of predators to arise. The early and most primitive of mosasauroid forms such as Dallasaurus and Komensaurus amongst others were well adapted to swimming, but they were not yet suited to life in the open ocean. Their tails were long and flattened for swimming, but the legs still capable of walking on land. Fossils of these primitive mosasaurs were found in what were shallow waters and coastal locations and this is most probably the reason why these reptiles not only survived the Cenomanian/Turonian extinction, but thrived during it.
Shallow waters around the coast would be less susceptible to a drop in oxygen levels of the open ocean as a greater abundance of aquatic coastal plants and water turbulence from tides would increase oxygen levels in these parts meaning that these areas would have been richer in aquatic life than say the open ocean where many of the large marine reptiles were used to doing their hunting. This in turn meant that the coastal areas would have had still had a great abundance of prey from fish, other kinds of cephalopods such as octopuses, as well as various crustaceans like shrimps would have all been viable prey options for the primitive mosasaurs. With the ability to still walk about on land, primitive mosasaurs may have also been beach combers, scavenging the dead bodies of other aquatic organisms such as fish and maybe even fully aquatic marine reptiles like plesiosaurs and ichthyosaurs.
Most of the primitive mosasaurs were actually very small, most of the currently known genera did deviate much more than about a meter in length. Again, this would have been to their benefit, because smaller bodies don’t require so much food to keep going, so it is easier to survive an extinction event. When fish levels began to rise later in the Cretaceous, the mosasaurs seem to have started venturing out further and further into the sea, until eventually they stopped returning to the land all together. In the absence of competition from other kinds of large marine reptiles, the mosasaurs began not only growing bigger, but better adapted to fast stable swimming, and high agility in the water. By the Santonian period, only about 4 million years after the Cenomanian/Turonian extinction, the mosasaurs had gone from being small one meter long semi-aquatic lizards, to very big marine apex predators, that for the remainder of the Cretaceous would continue to grow bigger and ever more specialised.
How are mosasaurs related to
snakes and monitor lizards?
For a long time mosasaurs have been speculated to have been descended from either monitor lizards or snakes, and it should be mentioned here that both monitor lizards and snakes are also thought to have possibly had a common ancestry. Mosasaurs had jaws that could open very wide and were more suited to swallowing large chunks of prey, if not actually swallowing their prey whole. Mosasaurs also had a second set of teeth growing from their palate, and these teeth would help to grip prey and larger chunks of flesh as they swallowed.
The snake to mosasaur hypothesis is no longer that widely accepted. For a start research and new snake fossil discoveries from the end of the twentieth and early twenty-first centuries is already pointing to the conclusion that while snakes and mosasaurs share some similarities, these may actually be the result on convergent evolution, or shared traits from a common but distant ancestor. Snakes are also known to have only one functioning lung, the other being vestigial (present but reduced to the point it does not work). One specimen of the genus Platecarpus has shown us that mosasaurs had two working lungs, not one. Finally primitive mosasaurs forms such as Dallasaurus show us that the direct mosasaur ancestors had legs when the adaptations were taking place.
Now it seems that the original interpretation of Mosasaurus being a monitor lizard was actually only 40 million years off the mark. The mosasaurs are now often regarded as a sister group to the Varanidae which includes prehistoric monitor lizards such as Saniwa, Palaeosaniwa and the colossal Megalania/Varanus priscus, as well as the Varanus genus which includes all modern day species of monitor lizard.
The Various kinds of mosasaur
There are two main family groups of mosasaur that you must be familiar with. The first is the Mosasuroidea, which includes all mosasaurs, including the primitive ones like Dallasaurus and Komensaurus. The second group is the Mosasauridae, which includes all the advanced form mosasaurs that are defined by Mosasaurus, but excludes the primitive forms.
The Mosasauridae can be further divided into what are currently six sub-families. First is the Mosasaurinae, which includes genera that are physically close to Mosasaurus, which means that regardless of differences in size, are noted for having proportionately robust builds meaning that members of the Mosasaurinae adapted to have an emphasis on strength and power. Members of the Mosasaurinae could be medium sized at around 6 to 7 meters long, to much larger at 12 to 15 meters long, with a few individuals exceeding even this.
Second is the Tylosaurinae, which includes the genera that are closer to Tylosaurus. So far tylosaurines and especially Tylosaurus itself make up in terms of body length some of the largest mosasaurs. However the tylosaurines are notably more gracile (lightly built) than the mosasaurines. This means that while tylosaurine mosasaurs were still powerful, there emphasis was more on speed and agility than raw strength. This means that a greater range of aquatic creatures was within their predatory scope, and indeed stomach contents of Tylosaurus do indicate that anything from fish to other marine reptiles to even sea birds were eaten by tylosaurine mosasaurs.
Third is the Plioplatecarpinae, and this includes genera such as Plioplatecarpus, Platecarpus and Angolasaurus amongst many others. These genera are notably smaller the previous two groups, with sizes commonly ranging between 3 and 6 meters long. Plioplatecarpines show some of the greatest adaptations for fast swimming, with some genera developing adaptations that were analogous to the earlier ichthyosaurs. It was the plioplatecarpine genus Platecarpus that was the first to reveal the presence of a fluked tail, something that now seems to have been a common feature for other mosasaurs. Plioplatecarpines also seem to have focused upon swimming forward using only their tails instead of their whole bodies, now an outdated idea.
Fourth, fifth and sixth are the Halisaurinae, Yaguarasaurinae and Tethysaurinae, and again these generally range from about 3 to 6 meters long. Like the Plioplatecarpine mosasaurs these seem to have been predators of fish and possibly smaller marine reptiles. A seventh group is called the Aigialosauridae after Aigialosaurus but this is a side group to the Mosasauridae which is a home for many of the primitive mosasaurs.
When mosasaurs were first reconstructed as aquatic animals mosasaurs were usually constructed as lizards with eel-like proportions. This idea existed all the way into the twentieth century but eventually a specimen of the mosasaur Platecarpus was discovered and this included partially preserved remains of soft tissue of the tail. This was the first ever indicator that mosasaurs had bi-lobed tails with the main tail dipping down and a soft tissue fluke rising up from the top. For many though Platecarpus was the exception not the rule, and so reconstructions of larger mosasaurs usually went ahead with a more classic tail.
Then in 2008 a new specimen of the genus Prognathodon was discovered, and after five years of cleaning and study, it was revealed that a second mosasaur was confirmed as having a bi-lobed tail with a fluke of soft tissue rising up from the tail. Prognathodon had a very different ecological niche than what Platecarpus had, and grew significantly bigger, and this is the discovery that forced others to sit up and think about how accurate the old reconstructions of mosasaurs were. As of 2015, there is no hard evidence to support the old hypothesis that mosasaurs had straight tails, but mounting evidence that most if not all advanced mosasaurid mosasaurs did. For this reason newer reconstructions of mosasaurs are always done with the presence of a bi-lobed tail.
Mosasaurs were air breathers which meant that they would periodically have to swim to the surface and poke their head out of the water to fill their lungs with fresh air. This was a very necessary but also dangerous thing for mosasaurs to do, especially smaller genera which were putting themselves at risk from attack from other larger mosasaurs as well as some of the larger predatory sharks of the time such as Cretoxyrhina, Squalicorax and Cardabiodon.
One specimen of Platecarpus has been found with the outlines and impression of internal organs preserved on the slab of rock that the bones are preserved upon. Here the trachea and bronchi are present, and show that Platecarpus and probably other mosasaurs had two working lungs. Further to this when the bronchi split the trachea to take air into each lung, the bronchi run parallel to each other instead of branching out to the sides like in terrestrial lizards. This may show an adaptation to the lungs working against the effects of water pressure pushing in from the sides, as the same adaptation can be seen in cetaceans (whales and dolphins) today.
It is not improbable that mosasaurs may have a directional sense of smell, something suggested by the confirmed placement of the nostrils on Platecarpus which face out to the side. These may have allowed for water to be sampled, and if one nostril picked up a smell before the other, a mosasaur might have been smart enough to figure out the direction the smell, say something like blood from an injured animal or a pheromone trace from a female ready to breed, and swim towards it. Even animals that are not considered to be very intelligent can do this, but monitor lizards which mosasaurs are close relatives of are actually considered to be some of the most intelligent lizards. There is not yet any way of knowing how smart a mosasaur was, but the innate intelligence of monitor lizards may have been transferred to mosasaurs and at least allowed them to develop much keener senses such as sight and smell, as well as learned patterns of behaviour such as knowing when certain kinds of animal would be available to hunt. There is also fresh speculation that mosasaurs may have also had forked tongues, though confirmed detail of this lacking at the time of writing.
What scales were on the body depends somewhat on the genus and body part in question. Returning again to Platecarpus, there seems to have been three main kinds. On the actual body and underside of the tail the scales are rhomboid and arranged into diagonal rows. Each row also overlaps the row behind it, perhaps to increase hydrodynamic efficiency. The upper side of the tail were similar to the rest of the body but larger in size, perhaps as a way to increase the tails rigidity for more efficient movement. On the snout of Platecarpus however scales were hexagonal and not shaped in any regular pattern, perhaps as a result of contact damage with prey animals. In specimens of other mosasaurs scales on the upper body were keeled, perhaps again to increase rigidity, or perhaps even to prevent light scattering so that they could more easily lurk in the murky depths and wait for their target prey to return to the surface for fresh air where they would be most vulnerable.
For a long time the colour of mosasaurs was unknown, and this was true until 2014 when a paper (Lindgren et al.) described the process of examining melanin pigments in a sample of fossilised mosasaur skin. The result for the specimen in question was that mosasaurs like a great many other marine creatures were counter shaded, with very dark almost black colouration on the sides and back, while very lightly coloured on the underside. It is not yet known if all mosasaurs followed suit, indeed species specialising in hunting in shallows may have been quite different. But most pelagic (open water) animals do have this patterning, and if one mosasaur did, then it’s reasonable that at least many others also did.
Mosasaurs are confirmed as being viviparous, which means that mosasaurs did not lay eggs but gave birth to live young. This is not that farfetched as while reptiles are generally associated with laying eggs, some lizards and snakes have been observed and scientifically documented to give birth to live young. In mosasaurs direct evidence from this comes from the genus Clidastes which had a much smaller juvenile Clidastes inside of it. Remains of small juveniles have been found as stomach contents before, but these are always partially corroded by acid, with distinctive pitting in the bones being evident of contact with the stomach acid. The small Clidastes in the larger one is in pristine condition meaning no exposure to stomach acid. This raises the likelihood that the adult Clidastes was pregnant at the time of death, and confirms an idea that mosasaurs did not lay eggs on land because of the fantastically great difficulties they would have had trying to crawl out of the water.
|1 - Halisaurus, 2 - Pannoniasaurus, 3 - Plioplatecarpus, 4 - Carinodens, 5 - Globidens, 6 - Platecarpus, 7 - Plesioplatecarpus, 8 - Plesiotylosaurus, 9 - Yaguarasaurus, 10 - Clidastes, 11 - Hainosaurus, 12 - Liodon, 13 - Prognathodon, 14 - Plotosaurus, 15 - Tylosaurus, 16 - Mosasaurus, 17 - Taniwhasaurus, 18 - Moanasaurus.|
How did mosasaurs swim?
A long standing idea since the early days of mosasaur research is that mosasaurs swam but undulating their entire bodies from side to side like a snake does. There are three reasons why this idea is no longer thought sound. First is that mosasaurs are probably not related from snakes, so there is no reason to assume that they would swim like them. Second is that such a method of swimming is relatively slow, and would be a question rather than an answer regarding how mosasaurs became so successful. Third, modern analysis of mosasaur bodies and scale patterning shows that when swimming at least the body would have been held fairly rigid to reduce resistance when moving through the water.
This means that the only effective method of propulsion of swimming would be to move the tail, and the tail only from side to side. The presence of a tail fluke which is now widely accepted as a fact for most if not all mosasaurs would have greatly increased the amount of push the tail had against the water making faster speeds more easily possible, as well as bursts of quick speed when attacking prey. Scales of mosasaurs when preserved are often fairly small but also shaped to decrease drag, making it easier for a mosasaur to move through the water with the least amount of effort.
One myth about mosasaurs swimming which is still put forward even in modern times is that mosasaurs would use their flippers for actual propulsion. This is just plain wrong, as moving the flippers in conjunction with the tail would only increase drag, reduce speed and increase the amount of energy required for swimming. The true function of the flippers was far simpler: steering. The problem with tails is that they push the body forward, not pull. This means that the body is always ‘nose heavy’ and will pitch down from gravity plunging the body towards the bottom of the ocean. The front flipper at least of a mosasaur would have been held at to the sides during regular everyday cruising and angled to act as hydroplanes to counteract this pitching effect.
This is actually a sound scientific principal, all fish use their pectoral fins at the front to counter this pitching effect, even submarines built by mankind have bow planes to counter the same effect. The flippers on mosasaurs could have also been used for steering allowing mosasaurs to make what were relatively tight turns in the water making them allow the more precise and deadly predators. When making a final attack however, mosasaurs may have folded their flippers back as close to their body as possible to reduce drag and increase their attack speed as much as possible, giving their prey as small as possible a chance to escape as they could.
Where did mosasaurs live?
Mosasaurs could live anywhere they had water to swim in. As air breathers it did not matter if they were in either salt water or fresh water. The overwhelming majority of mosasaur fossils are known from marine (salt water) sediments, and even more often from areas that were once shallow seas or coastlines. However, while the sea was undoubtedly their favourite haunt, the holotype fossils of the genus Pannoniasaurus and Goronyosaurus were recovered from what are percieved to have once been fresh water areas. It would be impossible for a dead mosasaur to float upstream, rivers always flow into the sea, so this can only mean that this individual made the decision to swim upstream. Since mosasaurs gave birth to live young, it is perhaps then more likely that Pannoniasaurus and Goronyosaurus were on the hunt. There is absolutely no reason to assume that other mosasaurs did not swim into river systems, and while there may have had a predator/prey interaction with some dinosaurs.
As far as geographic distribution goes, mosasaurs are known from every major continent in the world, even Antarctica (which in the Cretaceous was further North and warmer than what it is today). The hotspots for mosasaur fossils however are currently Europe, North Africa and the central portion of North America, with a special focus on the central states of the USA. This is because in the Late Cretaceous continental Europe was more like a chain of islands surrounded by shallow seas, North Africa was surrounded by shallow seas and had extensive rivers systems, and the central area of North America was submerged by the western interior seaway. This area were near perfect for preserving the remains of mosasaurs, and are also known for producing large numbers of fossils for other marine animals.
This is not to suggest that mosasaurs were more common in these areas during the Late Cretaceous, we can only study marine sediments from that time, and if they have not been preserved there is not much we can do further. Given their ancestry however mosasaurs were probably ectothermic, meaning that they were more reliant upon the temperature of the water to keep their bodies at the right temperature. For this reason the mosasaurs may have favoured waters in the warmer latitudes, which in the Late Cretaceous were more widespread given that global temperatures seem to have been higher.
What did mosasaurs eat?
In the simplest terms mosasaurs would eat anything that they could catch. Tylosaurus is the best example of this with fossils of animals found partially preserved in what would have been the stomach including remains of fish, sharks, marine reptiles, including other mosasaurs and even sea birds such as Hesperornis. It was their voracious appetites that allowed mosasaurs to be so successful, absolutely nothing in the ocean would be safe from an attack by a mosasaur.
The largest of the mosasaurines were certainly the apex predators that would focus their hunting upon other mosasaurs and plesiosaurs, as well as possibly larger sharks and fish. The smaller and faster mosasaurs were undoubtable hunters of fish, either cruising over shallow reefs and snatching out fish that darted out from amongst the rocks, or perhaps harassing large fish shoals in the open water.
The mosasaurines with their more robust skulls and heavier builds could have still attacked marine reptiles like plesiosaurs and mosasaurs that were on the surface, but they would have also been better suited to attacking more heavily armoured prey like the large protostegid turtles, typified by such genera as Archelon and Protostega. Two mosasaurine genera in particular though, Prognathodon and Globidens had specially adapted teeth that would easily break up the shells of turtles and ammonites.
Just as an extinction event may have helped clear the seas for mosasaurs to rise to power, an even greater event, the KT extinction marks their disappearance from the fossil record. The KT Extinction marks not only the end of the Maastrichtian and the Cretaceous, but the Mesozoic as well, and the mosasaurs along with the plesiosaurs, pterosaurs, and even the dinosaurs all disappear. This is event is signified by the famous asteroid strike that took place in the north of the Yucatan Peninsula, however new and ongoing research suggests that there may have been more than one major asteroid strike responsible. In addition to this volcanic activity may have been much greater than what we know today, and these factors combined may have been what wiped out the mosasaurs as well as many other types of Mesozoic animals.
Although different mosasaurs had very different diets to one another, they were all part of a global food web, and one upset, particularly at the most basic level would have larger ramifications for all of the other creatures concerned. If one or more major asteroid strikes occurred or massive volcanic eruptions continued to happen, or even a combinations of these all at once, then the skies would have become darkened as ash and dust were ejected into the atmosphere. This would reduce the rate of photosynthesis causing a large scale reduction in the amount of plankton. This would reduce available food to marine organisms like shrimps and small fish that ate the plankton, which would reduce the numbers of bigger fish that ate them. This would then in turn reduce the numbers of their predators such as the smaller mosasaurs and plesiosaurs, which in turn would reduce the numbers of their predators, the larger mosasaurs. As unfavourable conditions to life continued, the declines would continue until the population levels reached a point that a species could not some back from, then it would only be a matter of time before that species went extinct.
In addition to that, large amounts of dust and ash in the atmosphere would block out sunlight and reduce global temperatures, even the temperatures of the oceans. If ectothermic and dependent upon the water temperature to maintain a good metabolism, then this would have had a further negative impact on the mosasaurs, making them more sluggish and less able to hunt, as well as maybe even reducing their ability to reproduce. This is because many reptiles that we know today will not even begin to try and reproduce until they have been exposed to an ‘ideal’ temperature for a period of time.
After the mosasaurs went extinct the world’s oceans were relatively empty of impressively large apex predators. The sharks survived the extinction, biologically speaking sharks were and continue to be far better adapted to life in the oceans than any marine reptile. However it would not be until the mammals started venturing into the water in a similar way that the first primitive mosasaurs did that the world’s oceans would begin to see gigantic predators again. In an evolutionary pattern that would repeat itself, the first ancestors of the whales were beach combers that would occasionally venture out into the water to reach new areas and hunt for animals in the water. By the Eocene period whales were living in the water permanently, and had developed long streamlined bodies for swimming into the water. In fact some of these primitive whales were so similar to the mosasaurs that one genus named Basilosaurus was actually described as a mosasaur before it was realised to be a whale. It is also the rise of these whales that would see some gigantic sharks such as C. megalodon appear to hunt them.
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