Champsosaurus

Champsosaurus might look a lot like an unarmored crocodilian, but it was actually only very distantly related to them – this animal was part of a completely extinct reptile lineage known as choristoderes, and its very gharial-like appearance was the result of convergent evolution.

Found in freshwater habitats across North America and Europe, several different species of Champsosaurus are known from around the middle of the Late Cretaceous through to the end of the Paleocene, surviving through the devasting K-Pg mass extinction 66 million years ago.

Champsosaurus laramiensis here lived in western North America and ranged right across the time of the extinction event, dating to between about 70 and 62 million years ago. Around 1.5m long (~5′), it had a flattened skull that was very wide at the back, supporting powerful jaw muscles, with a long narrow toothy snout that could sweep rapidly through the water to snap at fish in a similar manner to modern gharials. Its nostrils were right at the tip of its snout, and it may have used it like a snorkel, only sticking the very end out of the water to breathe.

Skin impressions show it was covered in numerous tiny scales, most less than 0.5mm in size (0.01″), which wouldn’t have been particularly visible from a distance.

There also seems to have been some sexual dimorphism in this species, with females having much more well-developed limb bones – allowing them to occasionally haul themselves out onto the shore to lay eggs, while males were probably fully aquatic and unable to support themselves on land.

Retro vs Modern #07: Mosasaurus hoffmannii

The first scientifically documented mosasaur fossils were skulls discovered in the Netherlands during the 1760s and 1770s, but these remains were initially interpreted as belonging to a fish, crocodile, or whale. In the late 1790s their resemblance to monitor lizards was noted, and the fossils were soon recognized as belonging to giant marine reptiles unlike any known living species – a revolutionary concept at the time, and influential in the early development of ideas about extinction.

In the 1820s Mosasaurus hoffmannii was the first species officially described. For several decades it was thought to be a giant amphibious lizard with either webbed feet or flipper-like legs, with one of the earliest popular reconstructions being the 1850s Crystal Palace statue.

By the 1870s more complete fossil discoveries in North America had revealed the paddle-like flippers and fully aquatic nature of mosasaurs. Skin impressions showed overlapping keeled diamond-shaped scales resembling those of rattlesnakes, but proportionally much smaller compared to their body size.


1890s

Then, in the late 1890s, one mosasaur specimen was interpreted as having a mane-like “fringe” of soft tissue along its back.

Only a few years later this was realized to be a mistake, actually being preserved tracheal cartilage, but it was too late. The idea had already caught on in artistic depictions and quickly became a paleoart meme, with mosasaurs frequently portrayed with elaborate frills for the majority of the next century.


2020s

Early arguments about whether mosasaurs’ closest relatives were monitor lizards or snakes had settled down by the 1920s, with the consensus at the time being monitor lizards, and the first half of the 20th century saw little mosasaur research beyond the naming of a few new species. Much like the ichthyosaurs and plesiosaurs it was only really in the wake of the Dinosaur Renaissance that interest in these marine reptiles and their paleobiology really began to pick up again.

Rather than sea-serpent-like creatures we now recognize that mosasaurs actually looked more like lizards converging on whales or ichthyosaurs, with smooth streamlined bodies and vertical tail flukes. The size and shape of their scales varied across different parts of their bodies, parts of their bodies had dark coloration (likely with a countershaded pattern), and they probably had forked tongues.

They had a higher metabolic rate than most modern lizards, and may even have been warm-blooded. They probably also gave birth to live young, although a recently-discovered fossil soft-shelled egg found in Antarctica has been suggested to have come from a large mosasaur.

The debate about their evolutionary relationships has been reignited, too, with some recent studies once again supporting a very close relationship to snakes – although there’s currently no clear consensus.

Our modern view of Mosasaurus hoffmannii is a large chunky mosasaur that grew to at least 11m long (~36′). It lived during the end of the Cretaceous period, about 70-66 million years ago, and inhabited a wide range of climates across much of the ancient Atlantic Ocean and various connected shallow seaways, with fossils known from Europe, Africa, and North and South America.

Its long jaws had a powerful bite force and it seems to have been a more visual hunter than some other mosasaurs, with relatively large eyes and a less well-developed sense of smell. It was one of the largest marine animals of its time and was probably a generalist apex predator, feeding on a wide variety of prey such as fish, ammonites, and other marine reptiles.

Retro vs Modern #05: Ichthyosaurus communis

Fossilized ichthyosaur bones have been found for centuries, but were initially misidentified as being the remains of fish, dolphins, and crocodiles. More complete skeletons began to be discovered in the early 19th century – particularly by pioneering paleontologist Mary Anning – and Ichthyosaurus communis was one of the first species of these ancient “fish lizards” to be scientifically recognized.


1830s-1870s

Early reconstructions of ichthyosaurs in the 1830s depicted flippered crocodile-like animals with long straight eel-like tails and strangely shrinkwrapped features, showing the sclerotic rings of their eyes and the internal bones of their flippers as highly visible externally. They were also frequently portrayed as being amphibious, hauling themselves out of the water to bask.

By the late 1830s impressions of smooth scaleless skin had been found, and specimens with tail-tips that were always “broken” in the exact same place were interpreted as evidence of the presence of some sort of paddle-like tail fin. The 1850s Crystal Palace Ichthyosaurus statues show this slightly updated version, along with a low dorsal ridge on their backs reminiscent of a beluga whale.


2020s

From the 1880s onwards the discovery of exceptional ichthyosaur specimens preserving whole body outlines revealed a fully aquatic streamlined shape, a triangular dorsal fin, and a crescent-shaped vertical tail fluke. Numerous examples of fossilized pregnant females also showed that ichthyosaurs gave live birth rather than laying eggs.

This highly dolphin-like version of ichthyosaurs quickly caught on and became the standard depiction into the early 20th century, frequently showing them as highly active animals – swimming in groups, chasing fish and ammonites, and leaping dramatically out of the water like their modern cetacean counterparts. While we don’t actually know if they were social or acrobatic like dolphins, it was still a surprising and refreshing contrast to the increasingly lumpy and sluggish depictions of non-avian dinosaurs that were happening around the same time.

Actual further paleontological study on ichthyosaurs was scarce for decades, however, with a general attitude that the group was already scientifically “complete” and there wasn’t much new or interesting left to learn about them anymore. It wasn’t until the late 20th century that they began to have their own “ichthyosaur renaissance” alongside the dinosaurs, with a sharp rise in research in the last few decades bringing us a lot of new information about their diversity and biology.

Ichthyosaurus communis was just one of several species in the Ichthyosaurus genus, living during the Early Jurassic, about 196-183 million years ago, in the shallow tropical seas of what is now Europe. About 3.3m long (~11′), it was adapted for high-speed long-distance swimming like a modern tuna, and it probably had a large keeled peduncle on the sides of its tail.

Bone structure and isotope analysis show that ichthyosaurs were all warm-blooded. One exceptional specimen also preserves an insulating layer of cetacean-like blubber, along with some evidence of its coloration: overall darker on the top and lighter on the underside in a countershaded pattern.

(I’ve given this reconstruction some speculative disruptive camouflage, too.)

Some of the preserved pigmentation has enough microscopic detail to show what appear to be branched melanophore cells associated with the ability to change color – suggesting that ichthyosaurs may have been able to actively darken and lighten their coloration like some modern lizards.

Spinosuchus

Allokotosaurs were a group of mostly-herbivorous archosauromorph reptiles, distantly related to the ancestors of crocodiles, pterosaurs, and dinosaurs. They lived across Eurasia, Africa, and North America during the mid-to-late Triassic period, and their lineage included some weird and diverse forms – such as the bull-horned Shringasaurus, the long-beaked Teraterpeton, and possibly also the gliding kuehneosaurids.

Spinosuchus caseanus here was yet another one of these Triassic allokotosaurian weirdos, part of the trilophosaurid family and closely related to Trilophosaurus and Teraterpeton.

Living about 221-212 million years ago in what is now northwest Texas, USA, Spinosuchus was around 2.2m long (~7’2″) and had distinctive elongated neural spines along the vertebrae of its back and the base of its tail, forming a “high back” or short “sail”. Since it’s only known from a partial spinal column the rest of its anatomy isn’t known for certain, but it probably had body proportions similar to its close relative Trilophosaurus, with sprawling limbs and a short-snouted beaked head adapted for herbivory.

Like many other fossil “sailbacked” animals the exact function of Spinosuchus’ elongated vertebrae is unclear, but the structure may have been used for visual display. I’ve depicted it here with a speculative frill of colorful elongated scales, along with a flashy dewlap.

Styxosaurus

Styxosaurus snowii here was one of the largest known elasmosaurids, named after the mythological river separating the worlds of the living and the dead.

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.

Joermungandr

Named after a legendary Scandinavian serpent, Joermungandr bolti here was a recumbisrostran “microsaur” – part of a group of animals that were traditionally considered to be lepospondyl amphibians, but more recently have been proposed to in fact be a lineage of early reptiles.

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.

Coelurosauravus

Remarkably similar-looking gliding reptiles have appeared multiple different times 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.

…Except for the weigeltisaurids.

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.

From fig 2 in Schaumberg, G. et al (2007). New information on the anatomy of the Late Permian gliding reptile Coelurosauravus. Paläontologische Zeitschrift 81, 160–173. https://doi.org/10.1007/BF02988390

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.

Huehuecuetzpalli

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.

Huehuecuetzpalli mixtecus here lived in east-central Mexico during the mid-Cretaceous, about 105 million years ago. About 25cm long (10″), it was part of an early branch of the iguanomorph lineage, related to the ancestors of modern lizards like iguanas, chameleons, and agamids.

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.

Seeleyosaurus

Seeleyosaurus guilelmiimperatoris here was a smallish plesiosaur (about 3.5m long / 11’6″) found in Germany during the early Jurassic, about 182 million years ago.

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.

Silesaurus

Silesaurus opolensis here was a type of dinosauriform – a reptile very closely related to the ancestors of true dinosaurs, but not quite actually a dinosaur itself.

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.

It was originally thought to be a herbivore, but coprolites full of insect remains suggest it was probably more of an omnivore, possibly foraging by pecking in a convergently similar manner to its distant bird cousins.

In fact, one of those pieces of Silesaurus poop was recently found to preserve a new species of tiny beetle in incredible detail.