It is commonly believed that there were marine dinosaurs, since many reptiles that lived with them reached large sizes and had similar shapes. But the truth is that they are part of different lineages. So today we are going to take a tour of the main groups of reptiles that have adapted to the aquatic environment.
You don’t have to go very far after the appearance of reptiles to find the first adaptations. Already in the Permian period, 300 million years ago, the group of Mesosaurs appeared, primitive anapsid reptiles that show adaptations to the aquatic environment in their anatomy: they had an elongated body, apparently webbed feet and, most significantly, their bones. they were very dense. This feature is called pachyostosis , and we see it in many lineages that adapted to the aquatic environment, such as primitive cetaceans and early mosasaurs. Having dense bones – and therefore weighing more – helps you sink in the water and maintain your balance during diving, in a similar way to weights used in scuba diving.
As a curious fact, the fossils of mesosaurs, being found both in the south of the American continent and in South Africa, were used by Alfred Wegener as evidence of his theory of Continental Drift , which over time would give rise to the current Plate Tectonics.
But hey, during the Permian the dinosaurs had not yet appeared. It took the great mass extinction at the end of this period – which is considered the greatest extinction that life on Earth has faced – for the first dinosaurs to appear, already in the Triassic .
In this period, the first of the Mesozoic era , many groups of reptiles set out to reconquer the aquatic environment. A lineage of diapsid reptiles, the Sauropterygians , specialized in this environment, eventually giving rise to the placodonts, notosaurs, and plesiosaurs . The first two were very abundant during the Triassic, with plesiosaurs reaching until the end of the Cretaceous . To this group belong the typical marine reptiles with long necks that are very often confused with dinosaurs, but there were also forms with short necks and large skulls, the Pliosaurs such as Liopleurodon.
Turtles or testudines also appeared during the Triassic, which today still have limbs adapted to the marine environment. In fact, the oldest known tortoise, Odontochelys, is already interpreted as being at least an amphibious tortoise.
Another group of reptiles with adaptations to the aquatic environment that appeared in this period are also still alive today: the Crocodylomorphs, to which the current crocodiles belong. Although today’s crocodiles are already adapted to amphibious life, their relatives in the past were much more so. This is the case of the Thalatosuchians, a name that literally means “marine crocodiles”, which changed their hands into fins. To this group belong, among others, Metriorhynchus, Dakosaurus or Machimosaurus .
The maximum expression of the adaptations to this environment in reptiles is found in the Ichthyosaurs , which came to have a fusiform body, which can be reminiscent of that of a tuna or a shark, as well as an elongated snout and legs converted into fins. In addition, they had a dorsal fin like dolphins, and a caudal fin, as we have learned thanks to deposits of exceptional conservation. They were so adapted to the aquatic environment that they were ovoviviparous : the eggs hatched inside the mother, where they finished developing until giving birth. In this way, it was no longer necessary for them to return to the terrestrial environment to lay eggs, as sea turtles do.
The last group of reptiles that have adapted to the aquatic environment are the Squamous, to which lizards, chameleons, iguanas and snakes belong. And many of them have adapted to a greater or lesser extent to the aquatic environment: we have marine iguanas or aquatic monitor lizards. And some of his relatives who came to adapt much more and became great predators in the marine environment. We are talking about the Mosasaurs , relatives of monitor lizards and snakes, gigantic aquatic lizards with fins that lived during the Cretaceous period .
Most of these marine reptiles disappeared in the end of the Cretaceous extinction, in which non-avian dinosaurs and many other forms of life disappeared. But to this day we still have aquatic reptiles. The last representatives of a gigantic adaptive radiation that began about 240 million years ago.
References:
Sepkoski, J.J. 2002. A compendium of fossil marine animal genera. Bulletins of American Paleontology, Vol. 363. pp. 1–560.
mash, MW; Matzke, AT 2000. The ichthyosauria. Stuttgart Contributions to Natural History. Series B. Geology and Palaeontology, 298: 1-159.
Benton, M.J. 2014. Vertebrate Palaeontology (4th edition). Wiley-Blackwell.
Young, JZ 1977. The life of vertebrates. Editorial Omega, Barcelona.
