Millions of years ago, during the Devonian period, lobe-finned fish made an evolutionary leap and ventured onto land as amphibians. Their descendants spread across all continents and even conquered the skies. However, some of these creatures returned to their original habitat—the water. These animals are known as secondary aquatic species, as they had to readapt to life in water. Independently of one another, they developed similar adaptations because their new environment, the ocean, was universal. The first to return to the marine world were reptiles, and their golden age occurred during the Mesozoic era. In this article, you will be introduced to the main groups of marine reptiles that inhabited our planet during the time of the dinosaurs.

Ichthyosaurs (Ichthyosauria) were an order of marine reptiles whose distant ancestors diverged early from the main evolutionary line of Lepidosauromorpha and transitioned to a fully aquatic lifestyle. The first ichthyosaurs appeared in the Early Triassic, around 250 million years ago, and quickly spread across the planet. The Triassic period saw the greatest diversity of ichthyosaur ecological forms, ranging from fish hunters to benthic feeders. In the Jurassic period, pelagic species capable of fast swimming and deep diving evolved, intensifying competition with sharks and plesiosaurs. By the Cretaceous period, the diversity of ichthyosaurs declined, leaving only large predators, which eventually disappeared after the Cenomanian-Turonian extinction event 94 million years ago.

Ichthyosaurs had elongated, fish-like bodies with long skulls. Their limbs were transformed into flippers, with the front limbs being longer than the hind ones. They had long tails with a bifurcated fin at the end, with the spine extending into the lower part of the fin. The dorsal fin was composed of stiff connective tissue. The simplified structure of the spine and ribs was compensated by a robust muscular corset. Their skin was smooth, with very fine or absent scales. Ichthyosaurs had either uniformly dark coloring with a bluish tint or a countershading pattern (dark on top, light on the bottom). They typically ranged from 2 to 4 meters in length, though larger species like Shastasaurus sikanniensis could grow up to 21 meters or more. Ichthyosaurs had a high metabolism, allowing them to maintain a constant body temperature of around 35-39°C, with a layer of subcutaneous fat aiding in thermoregulation.

Ichthyosaurs were the fastest marine reptiles, propelled by strong, rapid tail movements. They might have even been capable of leaping out of the water like dolphins. Their flippers were used for maneuvering and maintaining balance. The bones of their limbs were highly modified, appearing as disc-like elements connected by connective tissue. Ichthyosaurs had an increased number of digits (up to 10) and phalanges (up to 30). Their bones bear traces of decompression, likely resulting from regular ascents from great depths. It is believed that some ichthyosaurs could dive to depths of 1.5 km or more in pursuit of prey.

Vision played a crucial role in the sensory system of ichthyosaurs. They had large eyes, protected by sclerotic rings to withstand water pressure. Some ophthalmosaur species had eyes that were record-breaking in size among vertebrates—up to 20 cm in diameter. Their hearing was weak, but they had well-developed senses of smell and electroreception. The skulls of ichthyosaurs show evidence of nerves and blood vessels connected to electroreceptors.

Ichthyosaurs were typical pelagic predators, inhabitants of the open seas. Their remains have been found worldwide, with well-preserved fossils discovered in the Volga region of Russia. All ichthyosaurs were viviparous, giving birth to numerous offspring, sometimes more than ten. Depending on their size, ichthyosaurs occupied different ecological niches. Most hunted relatively small fish and cephalopods, with long, thin, sharp teeth. During the Triassic period, there were species that fed on benthic animals, mainly mollusks. In the Jurassic period, ichthyosaurs with broad, straight teeth capable of attacking large fish and other marine reptiles appeared. Natural predators included sharks, pliosaurs, and crocodylomorphs.

Plesiosaurs (Plesiosauria) were an order of marine reptiles belonging to the group Sauropterygia, within the infraclass Lepidosauromorpha. The earliest plesiosaur fossils date back to the Late Triassic, about 203 million years ago. The group reached its peak during the Jurassic period, with a wide variety of forms. In the Cretaceous period, giant forms like Elasmosaurus emerged, but the overall diversity of the order began to decline. The few species that survived the Cenomanian-Turonian extinction 94 million years ago faced strong competition from mosasaurs. Plesiosaurs finally went extinct at the end of the Cretaceous period.

Plesiosaurs were characterized by a broad, barrel-shaped body with flippers and a short tail. Their pelvic bones and the shoulder girdle formed wide bony plates on the underside of the body, providing attachment points for limb muscles. A small rhomboid fin was located at the end of the tail. Their skin was smooth, without visible scales, featuring folds and wrinkles. The most distinctive feature of plesiosaurs was their long necks and relatively small heads. The length of the neck was achieved by increasing the number of vertebrae rather than their size (as in giraffes, for example). Elasmosaurus set a record among vertebrates with 76 cervical vertebrae. The neck structure was quite rigid and did not allow for swan-like bending, contrary to older reconstructions. Plesiosaurs ranged in size from 1.5 meters to over 12 meters. The largest, such as Elasmosaurus, could grow to 15-20 meters. Plesiosaurs had a high metabolic rate, as indicated by the analysis of their bone growth rates.

The large, long flippers of plesiosaurs were sometimes comparable in size to their bodies. The flat limb bones and increased number of phalanges formed large, lift-generating surfaces on the flippers. According to modern models, the movement of plesiosaur flippers resembled the wing strokes of birds. These reptiles essentially “flew” underwater, using their front flippers for propulsion and their rear flippers and tail for steering.

Plesiosaurs had large eyes protected by sclerotic rings, and the structure of their nasal bones suggests a well-developed sense of smell. Plesiosaurs were inhabitants of both coastal waters and the open ocean, found even in polar regions. Earlier theories suggested that they laid eggs on land, but modern findings indicate that they were viviparous, giving birth to one or two large offspring at a time. It is possible that plesiosaurs cared for their young in the early stages. Their diet consisted mainly of fish and cephalopods. The long neck may have helped them sneak up on schools of fish, as the small head would not be perceived as a threat. Some species may have fed on benthic organisms, hovering in one place and “weeding” the seabed around them, similar to how sauropods grazed on land. Plesiosaurs also had gastroliths—stones that served as ballast or helped grind mollusk shells.

Pliosaurs (Pliosauroidea) were a group of plesiosaurs characterized by short necks and large, elongated heads. Pliosaurs are not a single biological taxon but a composite group of representatives from different families sharing a common morphotype.

Like all plesiosaurs, pliosaurs had dense, robust bodies, flipper-like limbs, and short tails. Their distinctive features included a shortened neck and a large head. For example, Kronosaurus had a skull that was 2.8 meters long, making up a quarter of its total body length (12 meters). Pliosaurs, like plesiosaurs, had a high metabolic rate.

The underwater flying technique characteristic of plesiosaurs was also used by pliosaurs. However, their short necks made their bodies more compact, increasing their speed. During an attack, the powerful synchronized movement of all four flippers allowed for a rapid lunge at the target. Bone structure analysis suggests that pliosaurs could dive to significant depths in pursuit of prey.

Pliosaurs’ sensory systems were similar to those of other plesiosaurs. They relied on vision for hunting, with a keen sense of smell aiding in locating prey.

Pliosaurs were as widespread as plesiosaurs. Unlike their long-necked relatives, they were apex predators at the top of the food chain. Their large, conical teeth and strong jaws were designed to tear muscle and crush bone. Their prey included large bony fish, sharks, marine turtles, plesiosaurs, and ichthyosaurs.

Mosasaurs (Mosasauridae) were a family of marine reptiles belonging to the suborder Lacertilia (lizards). Along with monitor lizards and helodermatids, they belong to the infraorder Varanoidea. Mosasaurs appeared after the Cenomanian-Turonian extinction event 94 million years ago. The extinction of ichthyosaurs and most plesiosaurs left ecological niches open. By the end of the Cretaceous, mosasaurs had become the dominant marine predators in fierce competition with sharks. The global catastrophic changes in the biosphere at the end of the Cretaceous period disrupted established trophic chains in marine ecosystems, leading to the extinction of mosasaurs and other large predators.

Mosasaurs had long, streamlined bodies resembling those of monitor lizards, but with relatively larger heads. Their flipper-like limbs were short, and the long tail of later mosasaur species had a bifurcated fin. Soft tissue traces found in a specimen of Platecarpus tympaniticus suggest that the arrangement of kidneys and bronchi was similar to that of cetaceans. Their skin was covered in small scales similar to those of snakes. Early mosasaurs were small animals, up to 1.5 meters long. Classic species, such as Mosasaurus hoffmannii, could reach 17 meters in length. The warm-blooded nature of mosasaurs is supported by isotope analysis of their teeth. Calculations indicate they could maintain a body temperature of 33-36°C. Their metabolic rate was comparable to that of leatherback turtles but lower than that of ichthyosaurs.

Early mosasaurs used the traditional reptilian swimming method, with their bodies undulating from side to side. As they evolved, they developed a more efficient swimming style, relying on powerful tail strokes, a trait characteristic of fast marine animals. The tail became flattened, with a fin at the end. It is believed that mosasaurs could achieve greater speed than pliosaurs but were slower than ichthyosaurs. Like other large reptiles, they could dive deep but preferred to stay in shallow coastal waters.

Like other marine reptiles, mosasaurs relied heavily on vision. Their eyes were large, but they were positioned on the sides of the head, limiting binocular vision. Their sense of smell and hearing were weak.

Mosasaurs occupied the ecological niche of large and medium-sized marine predators, preferring shallow coastal seas worldwide. Like other marine reptiles, they were viviparous. Even small mosasaurs specialized in large and powerful prey. Their mobile jaw joints allowed them to perform sawing motions with their teeth and carve large chunks out of struggling victims. Their diet included sharks, large fish, marine turtles, and other reptiles, with ammonites and belemnites occasionally on the menu. Intraspecific aggression was high among mosasaurs, with younger individuals often bearing the tooth marks of larger members of their species.

Marine crocodiles (Thalattosuchia) were a suborder of crocodylomorphs that, along with modern crocodiles, belonged to the clade Neosuchia, or “new crocodiles.” Thalattosuchia includes two families: Teleosauridae and their descendants, the Metriorhynchidae. Thalattosuchians were the only archosaurs, aside from marine birds, to fully adapt to a marine lifestyle. Like ichthyosaurs, they disappeared in the early Cretaceous period.

Teleosaurs were less adapted to aquatic life and retained a crocodile-like appearance. A characteristic feature was the armor of bony plates covering their dorsal and ventral sides. Their limbs remained limb-like, with webbed toes. They had long, powerful tails and ranged in size from 3-4 meters to 7 meters. Metriorhynchids, on the other hand, fully transitioned to life in the water, losing their armor, developing flippers, and acquiring a tail fin. Most metriorhynchids ranged from 1.5 to 2.5 meters, except for the six-meter Plesiosuchus. The warm-blooded nature of teleosaurs is debatable, while metriorhynchids likely had a high metabolism.

Teleosaurs swam like crocodiles, tucking in their limbs and undulating their bodies, with the tail providing the primary propulsion. They may have been able to venture onto land for short periods. Metriorhynchids completely severed ties with terrestrial life. Like mosasaurs, their primary means of movement was their tail with a fin, while their flippers were used for steering.

The sensory system of thalattosuchians may have resembled that of crocodiles. If so, they were the only marine reptiles with good hearing, in addition to vision.

The home of marine crocodiles was the shallow seas of Western Europe, where most of their species were found. However, thalattosuchian fossils have also been discovered outside Europe, in Argentina and Madagascar. Their long, narrow jaws and sharp teeth indicate a diet of small fish. Teleosaurs had not completely lost their connection to the land and may have come ashore to lay eggs. Metriorhynchids, on the other hand, were viviparous.

Marine turtles are a group of turtle families that are not directly related but have all adapted to aquatic life. Most modern marine turtles belong to the family Cheloniidae. The family Dermochelyidae, or leatherback turtles, is represented today by a single species. Related to leatherback turtles is the now-extinct family Protostegidae, among others. The first marine turtles appeared in the Late Jurassic and reached great diversity by the Cretaceous period. The Cretaceous-Paleogene extinction event 66 million years ago did not significantly affect marine turtles. Their feeding habits and long reproductive cycles may have helped them survive this difficult period.

Marine turtles retain a crucial anatomical element of their terrestrial relatives—the shell. Cheloniidae have a streamlined, heart-shaped or oval shell, while Protostegidae and leatherback turtles have a more reduced shell composed of bony plates forming ridges along the body. Instead of horny scutes, the shell is covered with thick skin. Their flippers and large heads cannot retract into the shell. Most marine turtles are about 1 meter in length. The largest living species, the leatherback turtle, can reach up to 2.5 meters in length. Even larger was the extinct Archelon, which measured 4.6 meters and weighed around 2 tons. Cheloniidae have a low metabolism typical of cold-blooded animals, while leatherback turtles (and possibly protostegids) have an elevated metabolism. Constant movement generates muscle heat, allowing leatherback turtles to maintain a body temperature 8-10°C higher than the surrounding water.

Marine turtles use their long front flippers for powerful strokes, while their rear flippers primarily serve as rudders. Their swimming technique resembles the flight of birds, with low speed compensated by excellent maneuverability. Marine turtles can remain underwater for extended periods, although they do not dive to great depths. During active periods, dives can last up to an hour, while during sleep, they can last 4-7 hours.

Turtles have well-developed color vision, which they rely on when foraging. Their hearing is weak but better than that of terrestrial turtles. However, they are sensitive to the Earth’s magnetic field, which helps them navigate the ocean.

Modern marine turtles inhabit shallow coastal waters, often staying among seaweed beds. However, they can also be found far from shore. During the breeding season, they migrate to the beach where they hatched 20-30 years earlier to lay hundreds of eggs. Most turtles are omnivorous, feeding on algae, mollusks, crustaceans, echinoderms, and cnidarians, and they can even eat sponges. The diet of leatherback turtles consists largely of jellyfish, though they may also consume other animals. The diet of extinct species, including Archelon, was likely similar. Turtles themselves are prey for sharks and, in the past, marine reptiles like mosasaurs or pliosaurs. Many turtles die during hatching. Nonetheless, they have outlasted other marine reptiles. Of all the reptiles that ventured into the sea during the Mesozoic era, turtles have proven to be the most successful in terms of survival.

Our story of ancient marine reptiles ends here, but much has been left unsaid. The diverse groups of Permian and Triassic reptiles that explored aquatic environments deserve their own tale. Attempts by reptiles to return to the sea did not end with the Mesozoic era. Today, marine snakes are the successful conquerors of tropical waters. The Galapagos iguanas lead a semi-aquatic lifestyle. The saltwater crocodile easily swims hundreds of kilometers from shore into the ocean and even engages in battles with sharks. Who knows, perhaps millions of years from now, new marine dragons will become the rightful rulers of the ocean.