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.
Fossils of at least a dozen different species of these predatory marine reptiles have been found in the area, and they seem to have all been occupying different ecological roles to avoid being in direct competition with each other. Many had conical piercing teeth adapted for gripping onto slippery soft-bodied prey, but others had rounded blunt teeth for crushing hard shells, and some even had sharp shark-like teeth for tearing flesh.
This 7m long (23′) mosasaur was part of the plioplatecarpine lineage, but it had uniquely long and narrow jaws with pointy interlocking teeth and highly retracted nostrils. Its snout shape resembled that of a crocodilians like modern gharials more than any of its short-skulled close relatives, and it was probably specialized for a similar diet of small fast-moving fish.
Mainly known from mid-Triassic deposits on the Swiss-Italian border, dating around 247–235 million years ago, fossils of the species Tanystropheus longobardicus have been found in two different “morphs” – small forms less than 2m long (6’6″), and larger ones up to 6m long (19’8″).
For a long time the smaller fossils were thought to be juveniles, but while they certainly had juvenile-looking facial proportions they also had very different teeth compared to the larger forms. They had pointed teeth at the front of their mouths and multi-cusped cheek teeth further back, and the “adults” had jaws containing only the pointed teeth, suggesting very different diets and lifestyles between the two size classes.
Extreme changes in dentition and diet during maturation aren’t unheard of in fossil species, but something particularly odd was going on here. Larger forms over 2m long always had just the pointed teeth, and there were no signs of intermediate tooth arrangements at all.
Turns out the smaller Tanystropheus longobardicus were all skeletally mature adults, already fully grown at that size. The larger ones were a completely separate species occupying a different ecological niche to their smaller relatives, and have been named Tanystropheus hydroides in reference to the mythical hydra.
While the exact lifestyle of Tanystropheus is an ongoing paleontological argument, Tanystropheus hydroides at least appears to have been much more on the aquatic side of things, with nostrils positioned on the top of its snout and its pointed teeth forming a “fish trap” in its jaws.
Stomach contents suggest it mainly ate fast-moving aquatic prey like fish and cephalopods, but its body wasn’t really adapted for strong swimming and so it couldn’t have been catching them via active pursuit. Instead it was probably an ambush predator hunting in a similar manner to some plesiosaurs, using its incredibly long neck and relatively small head to carefully approach prey species without the rest of its body startling them, and then catching them with fast snapping sideways lunges.
It was previously thought to be a slow swimmer with a low and poorly-developed tail fin, and whether it even had a dorsal fin or not was unclear. But now new specimens with soft tissue impressions have given us a big surprise.
Not only did it actually have a fairly well-developed semilunate tail fin, but it also had a dorsal fin positioned much further forward on its body than expected, giving it a shape similar to some small sharks and representing the current earliest known dorsal fin of any amniote.
Bundles of stiffening collagen fibers inside its fins were very similar to those known from later Jurassic ichthyosaur species, indicating that this adaptation evolved much earlier in the lineage than previously thought. Along with stomach contents showing it mainly ate both cephalopods and small fish – fairly fast-moving prey – this suggests it was a capable open-water swimmer. It wouldn’t have been quite as speedy as its much more specialized Jurassic relatives, but it may have still been about as efficient as the small modern sharks it resembled.