Mambachiton fiandohana lived during the Early Triassic, about 237 million years ago, in what is now Madagascar – which at the time wasn’t yet an island, still being connected to both east Africa and India as part of southern Pangaea.

It represents the earliest known branch of the avemetatarsalians, or “bird-line archosaurs”, a major group of the archosaur reptiles that also includes pterosaurs and dinosaurs/birds

It’s only known from a few fragments but it was probably around 2m long (~6’6″), and would have been a carnivorous lizard-like animal with a long neck and semi-erect quadrupedal limb posture.

Unexpectedly for a bird-line archosaur it also had a staggered double row of bony osteoderms along its back, suggesting that the very earliest avemetatarsalians had some crocodilian-like armor. This seem to have very quickly been lost, though – there’s no sign of osteoderms in the next branches to split off after Mambachiton, the aphanosaurs and pterosauromorphs – and although they occur again later in one dinosauriform and various non-avian dinosaurs, this appears to be multiple cases of independent re-evolution rather than retaining the original ancestral trait.


Back in the 1980s, a fossil of a partial reptile skull was discovered in British Columbia, Canada, dating to the Early Triassic about 250 million years ago. Its triangular skull shape, large eye sockets, and what seemed to be distinctive spiky frills on the back of its head initially caused it to be identified as a relative of the gliding weigeltisaurids.

But the aptly-named Wapitisaurus problematicus would have had to be a very unusual member of this group. With an estimated length of up to 2m (6’6″) it was much larger than any other known weigeltisaurid, it was the only one known from the Triassic side of the “Great Dying” mass extinction event, it was found in a completely different part of the world, and its teeth seemed more like those of marine reptiles like thalattosaurs.

In recent years new discoveries and re-analysis of weigeltisaurid fossil material have resulted in much better modern understanding of their skull structure – and with that came the realization that Wapitisaurus really didn’t seem to match with them after all.

So a new study has finally identified what this problematic reptile really was… and it turns out the teeth didn’t lie! It was a marine thalattosaur all along!

Wapitisaurus had rather large eyes compared to most other North American thalattosaurs, and although the front parts of its jaws are missing it probably had a long slightly hooked snout similar to its close relative Thalattosaurus. It’s also now one of the oldest known members of the thalattosaur lineage, showing that some of their specialized skull features like retracted nostrils had actually appeared very quickly during their evolutionary history.

…Oh, and those “spiky frills” on the back of Wapitisaurus’ skull? They were actually all teeth from both the upper jaw and the palate, on broken shards of bone that had been displaced to just the right spot to muddle up its identity for over three decades.

Spectember 2023 #06: Some Big Reptiles

An anonymous request asked for a “large ankylosaur-like herbivorous notosuchian“:

A shaded sketch of a speculative ankylosaur-like animal related to modern crocodilians. It has a chunky body covered in interlocking armor plates, with a row of spikes down each side of its body and a longer pair of upward-pointing spikes on the bulbous tip of its tail. It has four squat legs, also armored, with hoof-like claws, and a short wide snout with large forward-facing nostrils.

Mitafosuchus pachysomatus is descended from Simosuchus-like notosuchians in Madagascar that survived through the K-T extinction.

Highly convergent with the now-extinct ankylosaurs, it’s a 5m long (~16’4″) squat tank-like herbivore with hoof-like claws, and a wide short snout used for grazing on low vegetation. Heavy interlocking osteoderm amor covers most of its body, protecting it against the big carnivorous crocodyliformes that also still survive in this version of Cenozoic Madagascar.

Another anon wanted to see a “giant warm blooded lizard”:

A shaded sketch of a speculative giant lizard descended from tegu. It has a small head with a slender snout, a crest on its head and a small pair of horns behind its eyes. Its neck is long and thick with a hanging fleshy dewlap, a chunky body with a sloping back, four legs in a semi-upright stance, and a long thick tail.

Atopohippus zestamenus is a descendant of invasive Argentine giant tegu lizards that became established on an island archipelago. At 2m tall (~6’6″) and around 6m long (~20′) it’s an example of island gigantism, and occupies a high-browsing-herbivore ecological niche similar to giant tortoises and prosauropods.

Its ancestors’ seasonal endothermy has become full endothermy in this species, partly due to young individuals having a very rapid growth rate and metabolism – their main defense against the predators on their island home (primarily carnivorous tegu-descendants and large birds of prey) is to simply get to a big body size as fast as they possibly can.


Hupehsuchians were small marine reptiles closely related to ichthyosaurs, known only from the Early Triassic of southwestern China about 249-247 million years ago. They had toothless snouts, streamlined bodies, paddle-like limbs, and long flattened tails, along with a unique pattern of armor along their backs made up of overlapping layers of bony osteoderms.

Hupehsuchus nanchangensis was a mid-sized member of the group, about 1m long (3’3″). Newly-discovered fossils of its skull show that its long flattened snout had a distinctive gap between the bones (similar to the platypus-like snout seen in its relative Eretmorhipis) with an overall shape surprisingly convergent with that of modern baleen whales – suggesting that this hupehsuchian may have been a similar sort of filter-feeder.

A diagram comparing Hupehsuchus' skull to that of a modern baleen whale.
Hupehsuchus skull compared to a modern minke whale
From fig 2 & fig 3 of Fang et al (2023). First filter feeding in the Early Triassic: cranial morphological convergence between Hupehsuchus and baleen whales. BMC Ecol Evo 23, 36.

Grooves in the bones along the outer edges of its upper jaws may be evidence of filtering structures similar to baleen, although with no soft-tissue preservation we don’t know exactly what this would have looked like. Its slender flexible lower jaws probably also supported a large expandable throat pouch, allowing it to filter plankton out of larger volumes of water.


Araeoscelis gracilis was a superficially lizard-like animal that lived during the mid-Permian, around 275 million years ago, in what is now Texas, USA. About 60cm long (~2′), it had a slender body, proportionally long legs, and a solidly-built skull with strong teeth, suggesting that it was a fast runner that specialized in cracking open the carapaces of thick-shelled prey.

It was one of the last known members of a lineage known as araeoscelidians, which are usually considered to be very early members of the diapsid reptiles – but a recent study has proposed they might have even more ancient roots than that, possibly being a branch of stemamniotes instead.


Proterochampsids were a group of Triassic archosauriformes, closely related to the true archosaurs (crocodilians, pterosaurs, and dinosaurs/birds).

Known only from South America between about 242 and 205 million years ago, these reptiles’ heads were wide at the back but very narrow along the snout, often with prominent bony bumps and ridges on their skulls, and they had less osteoderm armor on their bodies than other archosauriformes.

They’ve traditionally been interpreted as very crocodile-like and semi-aquatic, but their long slender limbs and presence in rather arid paleoenvironments suggest they may have been more terrestrial fast-running predators.

Tropidosuchus romeri here lived about 235 million years ago in what is now Argentina. It was one of the smaller proterochampsids, only about 50cm long (1’8″), with just a single row of osteoderms along its back, and had larger and lower-set eyes compared to its relatives.

CT scans of its braincase indicate it had a particularly good sense of smell, and it may have relied mainly on scent to locate prey.


Slavoia darevskii was a lizard that lived in what is now Mongolia and Kazakhstan during the Late Cretaceous, about 85-70 million years ago.

Around 12cm long ~(4.75″), it had a compact skull, small eyes, a short neck, shovel-like hands, an elongated body and slightly reduced hind limbs – all features that indicate it was a burrowing animal, digging tunnels and feeding on underground invertebrates.

Its exact relationships are uncertain, but recent studies have suggested it was an early amphisbaenian, representing a point in the group’s evolution before the full loss of their legs and the development of their extremely long worm-like shape.


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.


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.


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.


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.


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.