"Nothosaurus of the Triassic seas, an animal which attained an average length of 7-10 feet, but was sometimes longer. In some respects it displays close relationship to the Plesiosaurs, but it cannot definitely be said to have actually been their ancestor."
Prehistoric Sea Monsters. Written by Dr. Josef Augusta. Illustrated by Zdeněk Burian. 1966.
Scientists have found a new species of marine reptiles called nothosaurs from around 240 million years ago.
Around 240 million years ago, large reptiles called nothosaurs ruled the seas. These now-extinct sea monsters grew roughly five meters long or longer and flicked their long tails to speed through the water, chasing down fish.
Now researchers have found fossils of a type of mini nothosaur with features that suggest the creature lived a very different lifestyle from its bigger cousins. Brevicaudosaurus jiyangshanensis may have lurked on the shallow seafloor, floating motionless until unsuspecting prey got too close, say researchers, who describe the new species October 28 in the Journal of Vertebrate Paleontology.
The various families of marine reptiles that ruled the seas during the dinosaur ages - icthyoaurs, plesiosaurs, mosasaurs and more - are as iconic of the Mesozoic as non-dinosaurs can get. They’re equally as famous as the pterosaurs, and equally as misunderstood. In this, the first of two posts acting as a quick and easy guide to marine reptiles, I hope the shed some light on how these diverse groups of creatures evolved.
Warning: This post is big as hell.
The Sauropterygians
Sauropterygia includes perhaps the best-known marine reptiles - the plesiosaurs - as well as their more obscure, primitive relatives. The phylogenetic location of the sauropterygians on the reptile family tree can be seen on the incredibly scientific graph below.
Current paleontological evidence suggests that sauropterygians and testudines (turtles, tortoises, and terrapins) are derived from a common ancestor, uniting them under the clade Pantestudines. Pantestudines is in turn united with the archosaurs (the group that includes birds, dinosaurs, pterosaurs, and crocodilians) under the clade Archelosauria. (If we went down one step further, we’d see Archelosauria united with Lepidosauria - lizards, snakes, tuataras, and their extinct relatives - to form the clade Sauria, which encompasses the vast majority all reptiles.)
The Pantestudines emerged in the Late Triassic, around 240 million years ago. All primitive members of Pantestudines were amphibious or aquatic, suggesting that the common ancestor of sauropterygians and testudines was already adapted to life in the water to some degree.
Primitive sauropterygians (we’ll get back to testudines later) diverged into several orders. One of these was Placodontia, whose members (called placodonts) developed thick, flattened teeth to crush the shells of mollusks and crustaceans. Early placodonts resembled giant lizards, but later members of the group developed thick, bony plates for protection, causing them to resemble sea turtles. Despite their close phylogenetic relation to Testudines, this is believed to have been an example of convergent evolution; sauropterygians and testudines had already diverged by this point.
The placodonts went completely extinct by the end of the Triassic period, leaving no surviving descendants.
Another family of primitive sauropterygians were the nothosaurs. These amphibious reptiles are believed to have lived like modern seals, living on land but diving into the water to catch fish and squid. Their hydrodynamic body shapes and webbed feet would have made them powerful swimmers.
The nothosaurs went extinct by the end of the Triassic as well, but they are believed to be the direct ancestors of the plesiosaurs - or at least close relatives of those ancestors.
Plesiosaurs went a step further than their amphibious ancestors, becoming completely adapted to life in the water. They developed unique arrangements of the shoulder girdle (otherwise known as the clavicle and scapula) that allowed them to withstand high amounts of force, granting them powerful and high-speed swimming abilities, even in high-pressure oceanic environments. However, this came at the cost of an inability to return to the land. Plesiosaurs, like modern dolphins and whales, were completely sea-bound, surfacing only to breathe.
One of the mysteries of how this could be possible for a reptile was solved recently, when a one-in-a-million-fossil was discovered that depicted a plesiosaur giving live birth to a large, self-sufficient offspring.
Plesiosaurs were divided into two groups - “long-necked” and “short-necked”. While these were once thought to be two different orders of marine reptile, their morphologies later proved similar enough to unite them under Sauropterygia. (Older dinosaur books often refer to short-necked plesiosaurs as “pliosaurs”, a term that now refers only to a specific subset of short-necked plesiosaurs.)
The earliest true plesiosaurs are believed to have had necks of intermediate length. Their ancestors took the neck length to one of two extremes: stretching it out greatly, allowing them to sneak their small heads into schools of fish and chow down; or shrinking it down to near-nonexistence, granting the entire body a compact, submarine-like shape, suited for for active pursuit of larger prey.
The plesiosaurs radiated in diversity through the Jurassic and Cretaceous periods, before becoming extinct as a result of the same event that wiped out all non-avian dinosaurs - despite what cryptozoologists might have you believe. With their extinction, the superorder Sauropterygia was wiped out completely.
The Ichthyosaurs
“Ichthyosaur” means “fish lizard”, and looking at the body shapes of these Opthalmosaurus, you can probably see why. These aquatic reptiles were remarkably fishlike, even possessing dorsal fins and tail flukes, giving them a striking resemblance to dolphins - another example of convergent evolution.
The placement of ichthyosaurs in the reptile family tree is completely unknown. No one’s sure if they’re members of Sauropterygia, a third group belonging to Pantestudines, or a completely unrelated variety of ocean-going reptile. The main contributor to this mystery is a lack of transitional fossils; the earliest known ichthyosaur (Utatsusaurus, pictured above) was already adapted for a completely aquatic lifestyle. Utatsusaurus also predates the earliest known sauropterygian by a few million years, throwing another snag into the equation.
Early ichthyosaurs had elongated, streamlined bodies, and swam by undulating in an eel-like manner. By the Late Triassic, these ichthyosaurs had been replaced by more fish-like animals with dorsal fins and tail flukes. The ichthyosaurs reached their peak of diversity in the Early Jurassic, occupying a wide variety of ecological niches. Some were apex predators; others hunted small prey; some were specialized to crunch through clam shells, like the placoderms of the Triassic; and some simply sucked up large amounts of water and ate whatever they caught along with it. Like the plesiosaurs, they gave live birth to self-sufficient young.
Unfortunately, the ichthyosaurs underwent a decrease in diversity as the Jurassic period progressed, restricting themselves to streamlined, dolphin-like body shapes and hunting small active prey. They continued to dwindle as the Cretaceous period began, and were extinct by 90 million years ago, long before the Cretaceous mass extinction. It is speculated that they were unable to compete with plesiosaurs, as well as the next creature on our list...
The Mosasaurs
Mosasaurs were relative latecomers to the marine reptile scene, appearing about 92 million years ago - shortly before the extinction of the icthyosaurs, which may or may not have been coincidence. They are not sauropterygians or relatives of the icthyosaurs. Instead, they are descended from the aigialosaurs (pictured below) - semi-aquatic varanid lizards, related to monitor lizards, goannas, and Komodo dragons.
Mosasaurs are frequently depicted as sinuous, eel-like animals, like the one at the top of this section. However, recent fossils indicate that mosasaurs had large tail flukes, giving them a more shark-like appearance and method of swimming. They lived like sharks as well, with every known genus being actively predatory.
Additional light was shed on the lifestyles of mosasaurs when a mosasaur skin sample was found to contain fossilized melanin, revealing that it had a dark back and a light underbelly. This adaptation is seen in many modern marine predators, such as great white sharks and killer whales, camouflaging them simultaneously from both above and below.
Mosasaurs were active on earth for less than 30 million years before the Cretaceous extinction wiped them out. However, in that time, they were immensely successful - the largest and most powerful marine predators of the Late Cretaceous.
The Thalattosuchians
Not content to allow a bunch of non-archosaurs to dominate the Mesozoic oceans, some crocodilians adapted to marine environments as well. Although commonly known as “marine crocodiles”, the members of Thalattosuchia were not actually crocodiles; they were distant relatives of modern crocodilians, and are grouped together with them and a variety of other extinct crocodilians under the clade Neosuchia.
The thalattosuchians were divided into two families: the metriorhynchids (like Dakosaurus, pictured above), which lost their scutes and became quite fishlike in appearance; and the teleosaurids (like Pelagosaurus, pictured below), which looked more traditionally crocodilian.
The thalattosuchids were never as widespread or successful as other groups of marine reptiles. They originated during the Jurassic period and lasted until its end, at which point they died out. Today, all crocodilians are restricted to freshwater environments, except for the saltwater crocodile of Southeast Asia and Australia.
Sea Turtles
As I said earlier in this post, the Pantestudines emerged about 240 million years ago. The earliest known Pantestudine is Pappochelys, a lizard-like animal with expanded ribs that likely gave rise to the shells of modern turtles. Pappochelys was a semi-aquatic creature whose swimming ability was improved by its thickened bones, which made the body less buoyant.
Odontochelys, a much more turtle-like proto-turtle, lived about 20 million years after Pappochelys. It possessed even more broadened ribs and a plastron-like protective underbelly. As this adorable reconstruction shows, it was a fully aquatic animal.
Early sauropterygians, such as placodonts and nothosaurs, possessed broadened ribs similar to Pappochelys, but lacked Odontochelys’s plastron, indicating that the testudines diverged from the sauropterygians between 240 and 220 million years ago - a hypothesis currently supported by the fossil record.
Turtles diverged into land-dwelling and fully aquatic forms during the Late Triassic and Early Jurassic, about 210 to 190 million years ago, and began adapting to both environments quickly. Sea turtles, in particular, developed into essentially modern forms by the Early Cretaceous period. Fossils of Toxochelys, a sea turtle from this time, are common in Western Interior Seaway fossil deposits.
Without a doubt, the most famous prehistoric sea turtle is Archelon. With a maximum length of 13 feet, Archelon is the biggest sea turtle to have ever existed. it lacked a solid shell, instead possessing a leathery carapace supported by a framework of bone - similar to the modern leatherback sea turtle, believed to be its direct descendant or close relative.
Archelon was quite a hardy animal. The type specimen is missing a hind flipper, believed to be the result of a mosasaur attack, but did not die from this injury. Another specimen, estimated to have been over 100 years old at the time of its death, is believed to have died while hibernating on the ocean floor - one of the few known animals to have fossilized peacefully in its sleep.
Unlike the other marine reptiles of the Mesozoic, sea turtles are still around today, with seven species still swimming Earth’s seas. However, hunting and pollution are taking their toll, and most sea turtle species are endangered. Consider donating to a sea turtle conservation organization, such as the Sea Turtle Foundation.
I considered doing a second post in which I ran through the history of marine reptile discovery, similar to the one I did for pterosaurs. However, that post would be enormous, covering centuries of paleontological history, and I think a post as long as this one is already pushing it.
What I think I’ll do instead is make a sporadic series of smaller posts, highlighting individual discoveries or people involved with the study of marine reptiles. I probably won’t do that for a while, though. As amazing as these animals are, I’m a little burned out on them at the moment.
