Summary of the stream: ‘A relaxed chat with F.-L. Pélissier: Mosasaurs, the lizards of the seas’
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February 22nd 2026, Twitch : twitch.tv/Palaeowave
F.- L. is a palaeontologist and science and culture communicator at the Var Departmental Museum
1. You are working on mosasaurs – how did you come to specialise in these animals?
“Thanks to a twist of fate! I have been brought up on palaeontology from a very young age. Shortly before starting my Master’s degree, I attended a lecture by Thierry Tortosa, a palaeontologist at the Sainte-Victoire National Nature Reserve. It was during this lecture that I first heard about the discovery of mosasaur remains not far from my home region: Provence, between Marseille and Aix. I had already heard of mosasaurs, but I did not have any particular interest in these animals; I liked them just as I liked many others at the time. While chatting with him, I learnt that these animals were being studied at the National Museum of Natural History in Paris by Nathalie Bardet, a specialist in the field. I then moved to Paris to do my Master’s degree. During my first year of the Master’s, I had the opportunity to speak with her following a lecture. I kept in touch with her until my second year of the Master’s, when she suggested a dissertation topic on the subject. After completing my Master’s, I continued to take an interest in mosasaurs, about which there is still much to discover.
In my view, they are among those animals that have been widely publicised, and one might think, ‘That’s it, everything’s already been discovered about them’—but fortunately, that is not the case, and it is brilliant to realise that there is still so much to learn about them and so many questions we can ask!”
Quite so, especially as we will see, there is not just one mosasaur but a large number of mosasaur species. It is also important to bear in mind that the definition of a species in palaeontology is slightly different from the one we are familiar with in biology. In palaeontology, we focus on the physical characteristics observable in fossil remains. We therefore move away from the biological criteria whereby individuals of the same species must be able, either actually or potentially, to reproduce with one another and produce viable and fertile offspring.
“That is right, and we start from the premise that biological similarities will eventually have an impact on morphology. This is based, in particular, on studies carried out on modern organisms."
2. When and how were they discovered?
“This story takes us back to the 18th century. We are in the Netherlands, and there is a mountain, Mount Saint Pierre, which was extensively quarried for its rock, particularly chalk. The quarry is situated very close to a town called Maastricht, and this chalk, dating from the late Cretaceous period, happens to be rich in fossils. In particular, teeth and jaw remains have been found there, the oldest of which were discovered in 1766 and are currently on display at the Teyler Museum (Haarlem, Netherlands). These remains raised many questions for the naturalists of the time. They had already identified that the sedimentary rocks were of marine origin, implying that they were dealing with marine animals, but the question remained as to the nature of these animals. Another fossil of a very complete set of jaws was unearthed between 1770 and 1774; currently on display at the National Museum of Natural History (Paris, France), it was studied by several naturalists and gave rise to a number of hypotheses that would clash in the years that followed. This fossil changed hands several times before being confiscated by Napoleon’s armies, a reminder that palaeontology, like all sciences, is linked to societies and their geopolitical history.
One of the first hypotheses put forward was that of the ‘crocodile’, proposed by Faujas de Saint-Fond in a work published in 1799. This hypothesis did not gain widespread acceptance within the scientific community, and other hypotheses began to emerge. In 1786, the anatomist Petrus Camper suggested that the remains belonged to the sperm whale family. In 1790, Martin van Marum, having in his possession somewhat more complete fossils, noted that teeth were present on both the upper and lower jaws, and proposed the hypothesis that they had instead belonged to dolphins. Petrus Camper’s son, Adriaan G. Camper, apologising for disagreeing with his father’s idea, then proposed in 1800 a theory that would revolutionise the entire understanding of these fossils. He linked them to lizards. Giant marine lizards, more closely related to iguanas or other large lizards. George Cuvier then took up the subject and confirmed A. Camper’s hypothesis in 1808, as well as a connection to the group of iguanas or monitor lizards.
For years, these fossils were not given a name. The first person to name them was the palaeontologist William Conybeare, following the discovery of other similar fossils in different sedimentary deposits. As the River Meuse flows not far from Maastricht, in 1822 he named them Mosasaurus, meaning ‘lizard of the Meuse’. A few years later, in 1829, Gideon Mantell, taking his inspiration from the name of one of the fossil’s owners, gave them a full species name: Mosasaurus hoffmanni.
In the second half of the 19th century, a vast number of mosasaur and dinosaur fossils were discovered all over the world, and we witnessed the ‘Bone Wars’ between the palaeontologists E. D. Cope and O. C. Marsh. Cope, following the discovery and study of fairly complete mosasaur skeletal fossils across the Atlantic, came up with the idea of revising their taxonomy and classifying them as closely related to snakes.”
3. Complete skeletons have therefore been discovered, and they have sometimes been depicted as being very large in popular culture – as is the case in Jurassic World, for example. How tall were they?
“The one from Jurassic World: the size of its skull corresponds to the estimated size of an adult Mosasaurus hoffmanni. The largest mosasaurs of all species were probably between 12 and 15 metres long. The smallest known to date were between 1.5 and 2 metres long.”
4. The way they are portrayed today is often quite different (Ark, Jurassic World, etc.), but their portrayal has also changed over the years.
“Indeed, and the iconography we have of mosasaurs allows us to reconstruct the evolution of our understanding over the ages with a fair degree of accuracy. Artists have helped to capture in images all the major ideas we have had about how mosasaurs looked during their lifetime. There is not a single hypothesis about the biology of mosasaurs that has not been depicted by artists. We must bear in mind that there is often a slight time lag between the discoveries that are made and the representations artists create of them.”
Knight Tylosaurus, 1899
Mosasaurus Burian, 1962
Mosasaurus ichthyosaurus Heinrich Harder, 1912
I notice that in these depictions they all have fins; has this been confirmed, or is it just speculation?
“It depends on the species. Several studies have shown that their forelimbs and hindlimbs functioned as flippers, known as ‘swimming paddles’, which are similar to those found in modern cetaceans or sea turtles. For other species, the limbs appear more similar to those of modern lizards, perhaps with webbed limbs. As the preservation of soft tissue in the fossil record remains very rare, this information is still difficult to access.”
5. Where can we see mosasaur fossils today?
“There are many fossils in the Palaeontology Gallery of the National Museum of Natural History (Paris), but also at the Natural History Museum in Marseille and the Musée des Confluences in Lyon. If we look beyond France, one of the largest known specimens is at the Natural History Museum in Brussels, but there are also examples in Maastricht, and in North America, where they can be found in a huge number of museums and universities.”
By the way, just a quick reminder: for a fossil to be found in a particular place, there must first be sedimentary layers there that correspond to the period in which it lived.
6. They are not dinosaurs, so what sets them apart? Where do they fit into the phylogenetic trees?
“In fact, their group diverged from that of the dinosaurs quite early on; they belong to the squamates, that is to say, the group of “true lizards”. Phylogenetics has been something of a revolution for this group because, historically, snakes and lizards have always been treated as separate. But with the advent of phylogenetic studies, we realised that these two groups have a great deal in common. Some lizards are even closer to snakes than to other lizards. Many species of lizard have even lost their legs. Snakes are therefore now considered a group of legless lizards, distinguished by other characteristics.
As for mosasaurs, two hypotheses are currently debated regarding their place in the phylogeny: one suggesting they are cousins of monitor lizards, and the other suggesting they are close cousins of snakes. Please note that phylogenies are, above all, hypotheses regarding evolutionary relationships! I favour the hypothesis that places them closer to monitor lizards, partly based on the observation of certain fossils, and partly following a 2022 study re-evaluating the cranial characteristics of these animals. In the classification of lizards, the skull is one of the characteristics that provides us with the most information, whilst being the least subject to evolutionary convergence.”
Phylogenetic tree showing the various hypotheses regarding the position of mosasaurs in relation to other reptiles.
A quick note: What is known as evolutionary convergence occurs when two distinct groups develop similar characteristics over the course of their evolution as a result of environmental pressure. Examples often cited include the flippers of cetaceans, sea turtles and fish, or the wings of birds and bats.
A quick note (part two): There is no point in learning a phylogenetic tree! Several trees can be correct and valid at the same time, depending on the characteristics used to construct them. Furthermore, these trees are bound to change and are regularly revised as scientific knowledge advances.
“It is now accepted that mosasaurs belong primarily to four subfamilies: the Mosasaurinae, the Halisaurinae, the Tylosaurinae and the Plioplatecarpinae. These subfamilies are grouped within the family Mosasauridae. Over time, other subfamilies have been proposed, including the Tethysaurinae, which I myself have identified in my analyses but which does not enjoy consensus within the scientific community.”
7. When did they live, roughly speaking? When did they first appear and when did they become extinct?
“The earliest mosasaurs date back to the early Late Cretaceous period, around 100 million years ago, and they became extinct around 66 million years ago, at the same time as the dinosaurs during the Cretaceous–Paleogene extinction event. Mosasaurs occupied a very specific ecological niche. Indeed, they were not the only marine reptiles to have ruled the seas. We also know of Ichthyosaurs, Pliosaurs and Plesiosaurs. If we look a little more closely, we see that the vast majority of mosasaurs appeared after the extinction of the Pliosaurs, Ichthyosaurs and a large proportion of the Plesiosaurs. During their diversification in the Late Cretaceous, mosasaurs took over the ecological niches previously occupied by these other groups. They had free rein to occupy all the spaces and ecological niches left vacant by the extinct organisms.”
Extinction timelines and the coexistence of large marine reptiles at the end of the Cretaceous period
Reminder: The Cretaceous–Paleogene extinction event, which led to the extinction of numerous species, was ‘instantaneous’ on a geological timescale; however, for the species living at that time, it was the result of a series of events that unfolded over a period of time. It is now accepted by the scientific community that two major events occurred in succession: firstly, large-scale volcanic activity in the Deccan Traps, releasing enormous quantities of gas and particles and altering the global atmosphere; and secondly, the impact of a massive meteorite (the crater of which was discovered near Chicxulub, in what is now Mexico).
Furthermore, the ocean during the Late Cretaceous had a slightly different configuration from what we know today. The continents have moved and are still moving, altering the spatial layout, currents and salinity of the marine environments. At the end of the Cretaceous, the Atlantic was narrower than it is today. India was moving towards Eurasia, but the collision had not yet taken place (so there were no Himalayas) and the Panama Strait, between North and South America, was still wide open. The Mediterranean did not exist because West Africa had not yet collided with Europe; it was an open sea: the Tethys. Finally, sea levels were much higher, around 80 metres higher than they are today, and there were no ice caps at the poles. This configuration must be taken into account when we find marine fossils on the continents.
Ocean map during the K-Pg crisis
8. Are mosasaur fossils found all over the world?
“Palaeogeography had an impact on their evolution. As the spatial layout of the ocean was not the same as it is today, animals did not distribute themselves across the ocean in the same way. Early in their history, mosasaurs were mainly found along the coast and in shallow seas. Gradually, they developed the physical characteristics enabling them to live in the open sea and began to spread into other areas. Teams of palaeontologists have conducted studies and succeeded in understanding the ecology of these animals. Mosasaurs thus occupied the entire ocean and all the marine basins of the planet; we have even found plenty of them in France! In the Late Cretaceous, Europe consisted of a large number of islands, forming an archipelago. In this context, many of these mosasaurs inhabited the basins formed between these islands before spreading out into the open ocean.”
World map showing the various fossils that have been found
Were they exclusively marine species, or have we also found some that lived in freshwater (rivers and lakes)?
“Recent fossil discoveries in North America, combined with geochemical studies, have led to the suggestion that some mosasaurs may have lived in an environment that was a hybrid of freshwater and seawater. They are thought to have inhabited coastal areas, regularly venturing into freshwater. An increasing number of studies now allow us to estimate the environment in which each of the discovered species lived. We thus find species that lived in predominantly coastal environments, others in the open sea, and some species may even have occupied different environments depending on their life stage, staying closer to the coast as juveniles and moving further out to sea as adults. A recent study of a mosasaur found in a deposit in Hungary mentions, for the first time, a mosasaur that is thought to have spent most of its life in freshwater or brackish water (a mixture of freshwater and salt water). The largest individuals of this species are thought to have measured up to 6 m in length!”
Diagram showing δ13C isotopic geochemistry and the distribution of different mosasaur species from coastal to pelagic zones over time
9. These are land animals that have returned to live in a marine environment. What morphological changes and adaptations have they undergone? Have different stages of evolution been identified?
“For a long time, in evolutionary science, it was thought that a particular structure would evolve into another structure only once in the course of history. We then realised that this was not the case at all! For example, in the case of mosasaur limbs, it was discovered that the paddle-like shape, which enables swimming, appeared several times during evolution, and did so independently. Moreover, if we look in detail at the internal structure of the limbs of mosasaurinae, Plioplatecarpinae and Tylosaurinae, we see that they are quite different.”
Indeed, whilst this has been observed on several occasions in mosasaurs, the same evolutionary convergence and morphological characteristics can also be found in other major groups; as if we look at the flippers of modern cetaceans, they have a very similar structure!
“That is right, and it is a convergent trait found in all tetrapods, which are land animals that have returned to an aquatic lifestyle. Mosasaurs have other specific adaptations that are also evolutionary convergences shared with cetaceans; this is the case with the porosity of their bones. This distinctive structure is an advantage for animals that live in the open sea and have to dive to great depths.”
Have there also been changes in the shape of their caudal fin?
“Yes! We have even found a fossilised specimen with an extremely rare feature: the soft tissue making up the caudal fin has been preserved! In mosasaurs, as with swimming paddles, we observe evolutionary convergences with other organisms regarding the shape of their caudal fin. The latter closely resembles that of sharks (but ‘inverted’). Note, however, that there are several forms of caudal fins in mosasaurs, and not all of them have a tail with two lobes. This fin served to propel these organisms efficiently in an aquatic environment. There is a classification of swimming styles among modern sharks, which can also be applied to mosasaurs. In mosasaurs with a slender tail similar to that of modern monitor lizards, swimming involves undulations of the entire body and is described as ‘eel-like’. In cases of slightly more specialised caudal fin forms, where a second lobe begins to form, propulsion is achieved through movement of the animal’s posterior section, with the tail accounting for approximately 50% of the propulsive effort; this type of swimming is then termed ‘sub-carangiform’. Finally, in the most highly differentiated forms of caudal fins, where tail movement accounts for around three-quarters of the propulsive effort, the swimming style is known as ‘carangiform’.”
Classification of the different types of swimming
10. Have there been any recent discoveries that you did not expect at all/that surprised you?
“There is one that surprised me at first and now leaves me sceptical: it is a mosasaur tooth found in the famous Hell Creek Formation in the United States (which has yielded the remains of Tyrannosaurus rex, among others). The paper was published last year (2025). Chemical analyses of this tooth are said to have shown that even large mosasaurs living in the open sea would, at some point, have returned to freshwater. I’m waiting to see more, because in my view, the other animals to which this tooth might have belonged have not been ‘ruled out’ in a very convincing manner.”
11. Were they warm-blooded or cold-blooded?
“We tend to like to put what we observe into “boxes”, into categories. When it comes to living creatures today, and vertebrates in particular, we distinguish between two main categories: cold-blooded animals and warm-blooded animals. Cold-blooded animals are known as ectotherms, meaning that the source of the heat they need to survive comes from outside their bodies. They are also poikilotherms, meaning that their temperature varies with that of their environment. Warm-blooded animals, on the other hand, are endotherms; they generate their own heat and are homeotherms, meaning that their body temperature remains stable regardless of the environment in which they find themselves. Being warm-blooded or cold-blooded is closely linked to metabolism. A slower metabolism is often associated with cold-blooded animals, whilst a faster metabolism is most commonly associated with warm-blooded animals. Metabolism is not, however, merely a question of body temperature; it also encompasses various internal physiological activities, growth, digestion, and so on. There are skeletal indicators that provide information about the metabolism of mosasaurs: the rate of bone growth. By comparing these with modern snakes or monitor lizards, we can estimate what their metabolism might have been. Mosasaurs, therefore, possess fibrous and pseudo-lamellar bone tissue, which develops in the context of relatively rapid growth—an indicator of a fairly high metabolic rate (in contrast to what we see in monitor lizards or anacondas, for example). Mosasaurs, however, had a slower metabolism than that of other large marine reptiles (e.g. Plesiosaurs and Ichthyosaurs), which appear to have had faster bone development. Thanks to isotopic geochemistry, we have gone further and been able to estimate the body temperature of mosasaurs and that of the ocean at the end of the Cretaceous. The isotopic values obtained from three species of mosasaurs were compared with those from other species (ectothermic fish and turtles, and endothermic birds), and it was found that their body temperatures spanned a wide range (between approximately 28 and 40 °C), regardless of the species’ size. Taking this a step further and re-examining these values in relation to palaeolatitudes, we observe that the body temperature values do not correspond to those of the water at different latitudes. We can therefore rule out the hypothesis that mosasaurs’ temperature depended on the environment in which they lived. Recent studies therefore conclude that mosasaurs likely had a fairly fast metabolism and were warm-blooded animals. It should be noted, however, that this represents a spectrum along which different organisms are positioned. Between these two extremes, there are various intermediate situations, and the metabolism of mosasaurs was not among the fastest either.”
12. What do we know about their sensory system?
Their skin?
“This is truly remarkable, as we have discovered a large number of fossilised skin remains, exhibiting a wide variety of structures. In some mosasaurs, such as the tylosaur, we have found scales described as “carinated”, a structure that is particularly advantageous for hydrodynamics. Another strategy often observed to improve hydrodynamics is to have extremely smooth scales. Both types of strategy have been observed in mosasaur remains. Furthermore, as with all lizards, the structure of the scales varies depending on where they are located on the animal’s body. As for colour, only one specimen has so far yielded pigments: Tylosaurus, and these are rather dark pigments. This colour could have been an advantage both for camouflaging itself in its environment and for retaining the sun’s heat.”
Their view?
“By studying certain skulls, we can estimate their field of vision and, more importantly, their binocular field of vision (that is, the angle over which the vision of both eyes overlaps). This information is important because it allows us to assess how well mosasaurs were able to judge distances and perceive three-dimensional vision. This angle is estimated at around 29° for mosasaurs, which places them in the upper-middle range for the large marine reptiles of the time.”
Their taste and sense of smell?
“For their sense of smell, they have nostrils, but also, like many lizards, a tongue connected to the Jacobson’s organ. Given their classification within the lizard phylum, mosasaurs necessarily had a very forked tongue. Within the group to which they belong, the only lizards to have lost this deeply forked tongue are iguanas and chameleons, and this was for the purpose of grasping food. For mosasaurs, one of the best modern counterparts would be Lanthanotus borneensis, a semi-aquatic lizard found in Asia, which has a fairly thick forked tongue. This tongue also resembles that of heloderms, venomous lizards found in the Americas.”
13. What was their diet? What did they eat?
“A wide variety of tooth types have been found in mosasaurs, whether for cutting (pyramidal, sharp teeth), crushing (shorter, rather rounded teeth) or piercing (thin, very pointed teeth) – it’s spectacular! Each type of tooth corresponds to a preferred diet. Generally speaking, mosasaurs are fairly typical examples of what is known as homodonty, meaning that most of them have only one type of tooth in their jaws.
The different types of teeth found in mosasaurs and their associated preys
Stomach contents from mosasaurs have also been found, providing irrefutable evidence of the nature of their meals. Analyses reveal a diet consisting of seabirds, large fish, even sharks, or other mosasaurs. Even more astonishingly, a 2023 study of the stomach contents of a Prognathodon kianda specimen revealed the presence of three other mosasaurs that had been ingested, including a species previously unknown to science and an individual of the same species! This is also evidence that numerous species of mosasaurs coexisted within the same ecological niche and geographical region.”
Were they themselves prey to other animals?
“We have found numerous remains where it is not clear whether they were scavenged or preyed upon. In some cases, we can rule out the possibility of an attack, since when we are dealing with a mosasaur fossil measuring around ten metres in length that bears bite marks from sharks three times smaller, it seems unlikely that this was a case of predation. Sharks nevertheless remain the main ‘non-mosasaur’ predators of mosasaurs. However, we have found a few remains where we are certain that an attack occurred whilst the animal was still alive, as the bones have healed and scarred over, a sign that the animal continued to live after its injury.”
Could some of these signs of attacks be the result of defending resources rather than predation? We now know that defending resources (such as territory, young or food) sometimes drives animals to attack creatures much larger and stronger than themselves, without this being for the purpose of feeding.
“That’s a very good question, and one to which I do not have the answer. The literature focuses heavily on theories of predation and scavenging. In fact, I wonder whether the question has even been raised in the literature.”
14. Do we have any information on how they reproduce?
“We have tried to find out, since we know that many lizards lay eggs (they are oviparous) but this is not true of all of them; some give birth to fully formed young (they are viviparous). A study from a few years ago seems to suggest that ‘yes, perhaps’, but without committing itself too firmly. It concerns a large soft-shelled egg found in Antarctica. A species of mosasaur, of a size corresponding to that estimated for the lizard that laid this egg, was found in the same geological formation. That said, in terms of shell composition, lizards are not the only animals that could have laid this egg. The same applies to dinosaurs (some of which have also been found in Antarctica) or large pterosaurs. This egg could therefore have belonged to a reptile other than a mosasaur.
As mentioned earlier, there are many known species of reptiles that are viviparous. This is the case, for example, with slow worms and vipers. It also means that females in these groups undergo a gestation period. Indeed, a fossil of a pregnant female has been found, whose anatomy suggests an aquatic or semi-aquatic lifestyle. A modern example of a group very closely related to mosasaurs would be Shinisaurus crocodilurus, which has a similar ecology and in which the female is viviparous and gives birth in water. It is therefore easy to imagine mosasaurs giving birth at sea, or even on the open ocean. Another modern example would be Hydrophis platurus, a sea snake that gives birth to its young on the open ocean.”
“Mosasaurs were therefore a highly diverse group of reptiles with a complex evolutionary history, making them a rich subject for research.”
15. Do you have any particular anecdotes from the field that you would like to share with us?
“I haven’t done any fieldwork on mosasaurs. The last fieldwork I did was in the Jurassic period, on dinosaurs.”
16. What are you currently working on in relation to these animals?
“I am currently working on a study of their reproduction, as well as their ecology and their potential existence in freshwater. Following my findings, I have also begun work on revising the phylogeny of mosasaurs.”
17. And finally, where can we find you (online and in person) over the next few days? Any upcoming releases?
“I'm slowly getting back into Instagram and preparing a comeback on YouTube: F-L reptile.”
Questions from the chat:
1. Could mosasaurs, like some dinosaurs, have had feathers?
“So, for now, feathered dinosaurs appear to be limited to certain very specific groups of dinosaurs. This has been discussed in relation to some pterosaurs, but it is now highly controversial, since many researchers and palaeontologists generally believe that pterosaurs did not have feathers, but rather other skin structures that may have resembled them. Mosasaurs are not among the groups suspected of having feathers, as they belong to the group of lepidosaurs, which have only scales.”
Are feathers a specific evolution of certain scales?
“Well, that is what we thought for years, but we now know that is not quite right. In terms of embryology, the mechanisms underlying the development of a scale or a feather are virtually identical. It is not that scales turned into feathers, but rather that whatever enables the formation of a scale can also enable the formation of a feather. It is at other molecular levels that changes occur to produce the feather. The difference lies not in the evolution of the general form but in the way it develops.”
2. Has the number of phalanges within the swimming web changed across different groups, species or over the course of evolution?
“Yes, the number of phalanges increases over the course of evolution; this is known as polyphalangy. It is something we also see in sea turtles and cetaceans; these are also examples of evolutionary convergence among many tetrapods that return to live in an aquatic environment. In mosasaurs, there is another phenomenon: in addition to having more phalanges, the phalanges tend to become longer. This gives them a longer swimming paddle and makes it easier for them to swim.”
3. We know that, nowadays, marine species that lay eggs tend to return to coastal waters, or even to freshwater, in order to access a suitable environment, whilst other species are generally viviparous. There are examples among various shark species, such as the lemon shark, which lays eggs. Is there a link among mosasaurs between the different modes of reproduction and the suspected behaviour of untimely returns to freshwater?
“That is a very good question. It is worth noting that in the 20th century, there was a hypothesis that “nurseries” existed at the mouths of rivers and estuaries, by analogy with certain sharks that do this, but this hypothesis has since been abandoned. There are several species that presumably did not need them. If we take, for example, the results of isotopic analyses on the Clidastes species, we are justified in concluding that these animals returned to freshwater at some point in their lives. However, if we look at the fossils of juvenile Clidastes, it appears that they were born in the open sea, and we have not found any fossils of them in river or brackish water sediments.”
And could we imagine that they might have led a lifestyle involving migration from one habitat to another depending on the stage of life and reproductive cycles, much like salmon do, for example?
“So this is a hypothesis I have regarding the Tethysaurinae, and in particular Pannoniasaurus. I recently reviewed the literature on the ecology of various modern reptiles, particularly crocodilians. Take, for example, the famous saltwater crocodile: the young hatch in freshwater, and the females build nests inland, which creates significant concentrations of individuals in these areas. In the case of Pannoniasaurus, we have found a cluster of individuals, including very young ones, and we have isotopic analysis results showing that they did not strictly speaking live in freshwater. Based on these findings, it is possible that the adults moved between the two environments.
Furthermore, if we look at the monitor lizard group, which is much closer to mosasaurs than crocodiles, we see that some possess salt glands. One might ask whether mosasaurs had them or not, and if so, at what point in evolution this adaptation might have emerged. It is reasonable to assume that for certain species of mosasaurs, forays into freshwater were part of their life cycle.”
Salt glands help animals living in saltwater to expel excess salt from their bodies.
4. Mosasaurs did not develop extreme morphological features, as is the case with cetaceans, but how far back on the skull did the nostrils migrate?
“In mosasaurs, the furthest position observed is roughly in the middle of the snout. Cetaceans are unique in having a blowhole system, which is, after all, the pinnacle of evolution. Cetaceans did not follow the same evolutionary path as mosasaurs, and they were subject to different constraints. One hypothesis explaining why the position of the nostrils did not shift as far in mosasaurs as in cetaceans, despite tens of millions of years of evolution, is a morphological barrier. The ancestors of cetaceans were land mammals, whereas the ancestors of mosasaurs were lizards; their internal anatomies and skull structures were therefore completely different.”
March 2026
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