Environnement

Marine “monsters” in the time of the dinosaurs

Ichthyosaurs, plesiosaurs, mosasaurs and other super-predators populated the seas more than 60 million years ago. What are these reptiles, often wrongly called “marine dinosaurs”? Visit of the “monsters” gallery with three of the authors of “The sea in the time of the dinosaurs”.

How many marine reptiles did they live in Mesozoic, when dinosaurs ruled the land? And today, how many are there?
Nathalie Bardet. During the Mesozoic (-252 to -66 million years ago), there were at least five hundred species belonging to ten orders of reptiles. Moreover, we discover new ones every year. Currently, among the so-called “marine” reptiles, there is only one crocodile, a lizard, a few turtles and snakes.

The word comes from reptile, “Which crawls” in Latin. The ancestors marine reptiles were terrestrial, weren’t they?
N. B. Yes all. Then, in the Mesozoic, they turned to the sea, as mammals such as cetaceans and sirenians did much later in the Tertiary era. This return occurs following the Permian-Triassic crisis. Occurring 252 million years ago, it sees 95% of animal species disappear. Thanks to this extinction, the reptiles “jump into the water”!

Alexandra Houssaye. Like all crises, the Permian-Triassic crisis leaves ecological niches vacant. There are hardly any predators or competitors. Easier to diversify and proliferate under these conditions! Following this extinction, there is a radiation phase and we very quickly see the emergence of new species.

It is assumed that placodonts foraged by burrowing on the seabed in search of invertebrates.

Do we know who their earthly ancestors were?
Peggy Vincent. Not always. For example, when the emblematic group of ichthyosaurs appears in the fossil record, these animals are already so altered that it is difficult to find their terrestrial ancestor.

A. H. For some groups, on the contrary, we can clearly see the evolution. For example, the first mosasaurs looked like monitor lizards. They still had legs instead of palettes natatoiresTo closeTerminal part of a limb, flattened, full and rounded, reminiscent of the end of an oar. and their compact bones prove that they were slow swimmers who stayed at shallow depths. Then, they became active swimmers; their elongated tails allowed them to propel themselves, their paws became palettes natatoires and their skeleton is lightened. These changes happened very quickly, in just a few million years.

Did marine reptiles occupy all the rungs in the food chain?
N. B. Of the approximately five hundred known species, only one ate aquatic plants. All the others were carnivorous, and ate, depending on the species, shellfish, crustaceans, fish and, for the larger ones, other marine reptiles. We basically had small predators, large predators, and huge predators. They therefore occupied the top of the food chain.

What are the main adaptations that have allowed reptiles to conquer the sea?
P. V. The first is to be able to move in an aquatic environment, to be agile enough to feed and come to the surface to breathe. All have adapted in their own way, even if we find common points. Most have limbs that have changed into swimming paddles. The most hydrodynamic form is that of ichthyosaurs, large cruisers that can be found in all seas. They were mostly equipped with a dorsal fin and a bilobed tail like that of current sharks or tunas.

Jaws with teeth sharp as knives, the palate lined with teeth curved like hooks, the Mosasaurus was a formidable mega-predator.

What other changes have they undergone?
N. B. There is a whole range of physiological, morphological and behavioral adaptations. In addition to what Peggy has just mentioned, there is an example that I like: the nostrils. They are no longer at the end of the muzzle as in a lambda reptile, but go up towards the forehead. It is the same evolution as in current cetaceans: the vents of dolphins and whales are in fact their nostrils.

PV Their thermophysiology has also changed. Today’s marine reptiles are confined to seas where the temperature is pleasant. However, some Mesozoic reptiles were found near the poles. Analysis of isotopesTo closeAtoms with the same number of protons but a different number of neutrons. They therefore have identical chemical properties but different physical properties. For example, carbon-14, an isotope of carbon, is radioactive and is used in dating techniques. Stable bone oxygen shows that they could have a high body temperature, up to 35 or 36 ° C for plesiosaurs and ichthyosaurs.

A. H. These results are in line with those obtained from the analysis of the nature of their bone tissues, which is also a good indicator of physiology since it varies depending on whether the animals are cold blooded or warm blooded. Slow-growing bones, which belong to species with low metabolism, and fast-growing bones, can be recognized in animals capable of producing body heat. The bones of ichthyosaurs and plesiosaurs have, in this respect, characteristics similar to those of birds and mammals and therefore testify to a very active metabolism.

It’s amazing. We always think of reptiles as cold-blooded animals …
N. B. That’s right, but take an ichthyosaur that traversed huge expanses of ocean. He couldn’t wait like a lizard to warm up in the sun every morning before being active!

And for reproduction, what were the adaptations?
N. B. Many terrestrial reptiles are oviparous: they lay eggs. But, in the vastness of the ocean, it’s complicated. Large marine reptiles cannot come back to the beach to lay eggs because they could run aground like whales. And laying just a few hard-shelled eggs in water is not viable either. Marine reptiles have then developed a type of viviparity where there is no longer an egg but where the young emerges entirely from the womb of its mother. This viviparity is acquired very quickly. Thanks to exceptional fossils, we could see that in the first ichthyosaurs of the Triassic, the little one came out head first. But in an aquatic environment, this is not ideal because, by the time the rest of the body comes out, it can drown. In later ichthyosaurs, the tail comes out first, as in modern cetaceans. I find this change in the mode of reproduction extraordinary!

Exceptional fossil of a female ichthyosaur and its embryo expelled at the time of death (Holzmaden site, Germany).

Did they raise their young?
N. B. Hard to say. These animals are so different from today’s reptiles that no comparisons can be made with them. Hypotheses concerning the behavioral domain are difficult to demonstrate when working with fossils.

PV Often the difficulty is that we have very few fossils. For plesiosaurs, we only have one skeleton of a mom with the embryo in the womb, so it’s hard to generalize. But one can imagine that, if she only had one, then she had to deal with it more intensely. This idea is based on hypothetical models of reproductive strategies: according to the R strategy, we “bet” on the number, and according to the K strategy, on the contrary, we have few pups that we take care of a lot.

A. H. A recent study on the bone structure of a plesiosaur embryo suggests that they gave birth to a single large baby.

66 million years ago, a meteorite fell, marking the end of the dinosaurs and the Cretaceous-Tertiary crisis. What does this cause among marine reptiles?
P. V. There are indeed great upheavals at the end of the Cretaceous. Before this meteorite fall, a very strong volcanic activity also began in India, two phenomena that shook the planet. Not all reptiles reacted the same way. Some have disappeared, such as plesiosaurs and mosasaurs, and others have passed the crisis, such as turtles and crocodiles. What needs to be studied is the state of the populations before the crisis, to see if the groups gradually died out or if it was like a sledgehammer. What we see in the fossil record is that populations of mosasaurs and plesiosaurs were spread across the globe and were very diverse just before the crisis. We therefore think of a sudden extinction.

Do we know why some survive and others do not?
N. B. The crises are selective. The tendency is that smaller animals that are not directly linked to primary production, such as scavengers, resist better. But there are counter-examples, such as sea turtles, in which we do not see a drop in biodiversity. In the current state of our knowledge it is still difficult to say why some survive and others do not in mass extinctions.

With a long neck, the Elasmosaurus stimulates the imagination: some dreamers believe they have seen it in the murky waters of Loch Ness …

Finally, among this diversity of extinct species, is there one that is more important to you?
P. V. I like theElasmosaurus. It is a species of plesiosaur with a gigantic neck. We see a multiplication of the vertebrae, he has up to seventy in the neck, with a very small head at the end.

N. B. For me that would be the Mosasaurus. It’s like a giant marine lizard. The smallest of the same family are three meters, the tallest, like him, are fifteen meters. It was Georges Cuvier, the father of paleontology, who was the first to describe them.

A. H. If it is necessary to choose, I would say the placodonts, animals endowed with a carapace, and which however have no relation to the turtles. Everything suggests that they were slow, heavy animals, which crawled on the bottom and ate shells. Yet when we look at their bones, we see rapid growth suggesting a high metabolism. There is something wrong here, a mystery. If I had to resuscitate one, I would choose a placodont because I don’t understand them.

To read
The sea in the time of the dinosaurs, N. Bardet, A. Houssaye, S. Jouve and P. Vincent, Belin, Hors collection Sciences, September 2021, 208 pages, 26 euros.


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