Reaching around 11m long (36′), with half of that being entirely neck, it lived during the late Cretaceous period about 83-80 million years ago in what is now the American Midwest – a region that at the time was submerged under a large inland sea.
With pointy interlocking teeth in its proportionally tiny head, Styxosaurus would have fed on slippery aquatic animals like fish and cephalopods, possibly using its long neck to get up close to its targets while the bulk of its body remained out of sight in dark murky waters. Large numbers of gastroliths found in the stomach regions of some specimens would have been used to grind up the hard parts of prey items after they were swallowed whole.
Discovered in the Mazon Creek fossil beds in Illinois, USA, this species dates to the late Carboniferous period around 310 million years ago. A single near-complete specimen about 5cm long (~2″) preserves impressions of the body outline and numerous tiny scales, giving us a pretty good idea of what it looked like in life.
Joermungandr had a long streamlined tubular body with small limbs and a short tapering tail, and a stubby snout with fused bones heavily reinforcing its skull. Along with microscopic ridges on its body scales that resemble the dirt-repelling scales of some modern reptiles, this combination of features suggest it was a headfirst burrower that wriggled its way through soil with snakelike motions.
Remarkably similar-looking gliding reptiles have appeared multipledifferenttimes over the group’s evolutionary history, including the modern Draco – and despite being unrelated to each other almost all of them have achieved this in the exact same way, supporting their wing membranes on extremely elongated rib bones.
These early members of the neodiapsid lineage were the very first vertebrates known to have experimented with gliding, all the way back in the late Permian period 260-252 million years ago. And while they superficially resembled all the later rib-gliders, their wings were actually something never seen before or since in a gliding reptile.
Basically, these animals were the closest that Earth life ever came to legitimately evolving a dragon.
Coelurosauravus elivensis here was a weigeltisaurid living in what is now Madagascar, which at the time was part of southern Pangaea. About 40cm long (1’4″), its body was adapted for a life climbing and gliding around in the treetops, with pneumatized air spaces lightening its bones and long slender limbs similar to those of modern tree-climbing lizards.
Its large wings were formed from around 30 pairs of long hollow rod-shaped bones extending out from the sides of its belly. These flexible structures could furl and unfurl with a motion like a foldable fan, and are thought to have been highly modified from osteoderms in the skin, creating an entirely new part of its skeleton.
Towards the front of the wing the rods were arranged in several closely-packed “bundles”, and one specimen of Coelurosauravus preserves an impression of what seems to be the outline of the wing membrane’s leading edge – showing a stiffened pointed shape resembling the alula of a bird wing, which may have served a similar aerodynamic stabilization function.
But aside from the wings, the most striking feature of weigeltisaurids were their heads. Their skulls featured large crest-like frills resembling those of chameleons and ceratopsid dinosaurs, and their edges were adorned with prominent bumps and spikes. These were probably used for visual display and might have been a sexually dimorphic feature, with males having larger spikier crests than females. The crests may also have anchored large powerful jaw muscles, giving weigeltisaurids a wider gape and faster bite speed, helping them to snap up their fast-moving insect prey.
Bipedal running has convergently evolved multiple times in squamate reptiles, known in over 50 modern species – and fossil evidence shows this is nothing new, with lizards repeatedly developing the ability to sprint on their hind legs for well over 100 million years.
Its limb proportions indicate it would have been a bipedal runner, making it one of the earliest known examples of this type of locomotion in lizards. Its skull also had some features convergent with varanids, suggesting it may have had a similar sort of active-pursuit-hunting ecology.
And back in the 1890s, a specimen of this species was discovered with soft tissue impressions showing a diamond-shaped tail fin.
But despite us knowing about plesiosaur tail flukes for such a long time, they’re surprisingly under-represented in reconstructions, never seeming to have become associated with the popular image of these animals in the same way that early pterosaur’s tail vanes did. It doesn’t help that no other direct impressions of plesiosaur tail fins have ever been found, or that the Seeleyosaurus specimen’s soft tissue got painted over at some point in the mid-1900s, making it incredibly difficult to study without causing further damage.
(Perhaps modern non-invasive scanning techniques could be able to see under the paintjob, but as far as I’m aware nobody’s tried that yet.)
These tail fins are usually assumed to have been vertically oriented like those of other aquatic reptiles, moving side-to-side and acting like a rudder. However, there’s also a hypothesis that their fins might have actually been horizontal more like those of modern cetaceans and sirenians, based on several anatomical quirks – such as their tail regions being very wide and flat at the base, and the vertebrae at the tip being unusually pygostyle-like, very different from the way the tail bones of vertically-finned reptiles look.
Living in Poland during the Late Triassic (~230 million years ago), it was a quadrupedal animal roughly the size of a large modern dog, about 50cm tall at the shoulder (1’8″) and 2m long (6’6″). The front of its lower jaw was toothless and covered with a keratinous beak, and there may have been a corresponding much smaller beak at the very tip of its upper jaw, too.
The modern tuatara is the only living representative of an entire major lineage of reptiles known as sphenodontians – an evolutionary “cousin” group to all lizards and snakes, last sharing a common ancestor with them over 240 million years ago.
And during the Triassic and Jurassic these lizard-like animals were a widespread and diverse bunch, found worldwide and occupying many of the ecological roles that were later taken over by true lizards. They ranged from tiny insectivores to omnivores, relatively large herbivores, and specialized shell-crushers – and some even adapted to a fully aquatic fish-eating lifestyle.
Pleurosaurus ginsburgi here lived during the Late Jurassic, about 150-145 million years ago, in the warm shallow seas and lagoons that covered most of Europe at that time. Fossils of this particular species are known from southern France, with the closely related Pleurosaurus goldfussi found in both the same region and the German Solnhofen Limestone.
These swimming sphenodontians could grow to around 1.5 in length (~5′), with elongated bodies, pointed triangular snouts with retracted nostrils, short flipper-like forelimbs, and especially long eel-like tails. Soft tissue impressions also show scaly skin covering their bodies and a “frill” running along the top of the tail.
And one of the groups that rose to prominence during this time were the rhynchosaurs. Part of the archosauromorph branch of reptiles, they were closely related to the ancestors of crocodilians, pterosaurs, and dinosaurs, and evolved from small superficially lizard-like forms living in southern Africa during the very start of the Triassic, around 250 million years ago. But within just a few million years they became larger and bulkier, specialized for herbivory and scratch digging, and they soon spread all over Pangaea and became incredibly abundant in some fossil deposits.
Stenaulorhynchus stockleyi was one of larger member of this lineage, around 1.2m long (4’), known from Tanzania about 247-242 million years ago. It had a typical triangular rhynchosaurian skull, with wide deep cheeks supporting powerful jaw muscles and multiple rows of grinding teeth, along with a narrow hooked “beak” formed from the premaxillary bones of its snout.
Along with another recently-discovered species, Skybalonyx skapter, and the weird burly arms of Drepanosaurus, this suggests that instead of tree-climbing some drepanosaurs were instead much more specialized for digging. They may have been Triassic equivalents to modern anteaters or pangolins, using their enlarged claws to excavate burrows and rip their way into insect nests.