Cimolestans were one of the major mammal lineages that survived through the K-Pg mass extinction 66 million years ago. Closely related to early placentals, they had a burst of diversification during the first half of the Cenozoic and rapidly evolved into a wide range of specialized forms – some uniquely weird, and others convergently resembling more familiar modern animals like squirrels, bears, ground sloths, and hippos.

And one group known as the pantolestids were incredibly otter-like.

(Because synapsids love them some lutrinization.)

Palaeosinopa didelphoides here lived during the mid-Eocene, about 52 million years ago, in what is now the Mountain West region of the USA. It was similar in size to a small otter, about 1m long (3’3″), and had a streamlined body with a well-muscled neck, short powerful forelimbs, slightly longer hindlimbs, and a very long tail.

Inhabiting a subtropical lake ecosystem, it probably swam using both hindlimb paddling and otter-like tail undulations. Its strong jaws and teeth suggest it was specialized for crunching hard shellfish prey, but so far preserved gut contents have only shown fish bones and scales. Fairly large claws indicate it was also able to dig out burrows similarly to modern otters and beavers.

Although pantolestids were never particularly common animals they were quite widespread, expanding their range from their evolutionary origins in North America across to Europe and eventually into Asia. A cooling and drying climate at the end of the Eocene seems to have driven most of the group into extinction alongside all their other cimolestan relatives – but a few of the Asian species clung on slightly longer as the very last of their kind, with the last known fossils dating to about 28 million years ago in the early Oligocene.


Tsaidamotherium hedini was a ruminant ungulate living around 11 million years ago during the late Miocene, in the northeastern part of the Tibetan Plateau in what is now Northwestern China. Although it’s known only from partial skull remains it was probably similar in body size to a large sheep, about 80cm tall at the shoulder (2’7″).

Since its discovery in the 1930s it’s traditionally been classified as part of the muskox lineage, but in 2022 it was proposed to actually be a giraffoid very closely related to the newly-discovered Discokeryx.

Tsaidamotherium had some extremely unusual headgear, with highly asymmetrical “horns” (actually ossicones if was a giraffoid). The left one was small and positioned above the eye, while the right one was shifted back and towards the middle of the forehead, and was expanded out into a wide bony disk that would have supported a large helmet-like domed keratin covering.

Its skull also had a very large nasal cavity resembling that of the modern saiga antelope, suggesting it may have convergently evolved a similar sort of complex air-filtering snout to deal with dry cold air in its mountainous habitat.


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.


Modern birds’ upper beaks are made up mostly from skull bones called the premaxilla, but the snouts of their earlier non-avian dinosaur ancestors were instead formed by large maxilla bones.

And Falcatakely forsterae here had a very unusual combination of these features.

Living in Madagascar during the Late Cretaceous, about 70-66 million years ago, it was around 40cm long (1’4″) and was part of a diverse lineage of Mesozoic birds known as enantiornitheans. These birds had claws on their wings and usually had toothy snouts instead of beaks, and many species also had ribbon-like display feathers on their tails instead of lift-generating fans.

Falcatakely had a long tall snout very similar in shape to a modern toucan, unlike any other known Mesozoic bird, with the surface texture of the bones indicating it was also covered by a keratinous beak. But despite this very “modern” face shape the bone arrangement was still much more similar to other enantiornitheans – there was a huge toothless maxilla making up the majority of the beak, with a small tooth-bearing premaxilla at the tip.

This suggests that there was more than one potential way for early birds to evolve modern-style beaks, and there may have been much more diversity in these animals’ facial structures than previously thought.


During the Early Carboniferous, around 330 million years ago, the region that is now the East Kirkton Quarry in Scotland was located close to the equator, with a lush tropical climate and volcanic hot springs dotting the landscape. It preserves fossils of some of the earliest known fully terrestrial tetrapods, and a recent discovery shows how some of these animals were already experimenting with the shapes of their feet to better get around on land.

Termonerpeton makrydactylus is only known from a partial skeleton, and shows a mix of anatomical features that make identifying its exact evolutionary relationships rather difficult – but it was probably a very early reptilomorph, closer related to amniotes than to lissamphibians. It may also have been very closely related to the equally enigmatic Eldeceeon and Silvanerpeton from the same region, but was almsot twice their size with a estimated total length of around 70cm (2’4″).

It would have resembled a rather heavily-built lizard-like or salamander-like animal, with fairly stumpy legs and probably lacking claws on its digits. While it would have had spindle-shaped scales on its underside, and possibly small rounded scales along its sides and back, these were bony structures embedded in its skin and probably weren’t very visible externally in life.

But Termonerpeton‘s most surprising feature was its proportionally large feet with especially elongated fourth toes, which would have helped to extend its stride length for energy-efficient terrestrial locomotion and to stabilize its movement on unstable surfaces – a much more “advanced” amniote-like arrangement than expected in such an early reptilomorph, and convergently similar to to the foot shapes of some modern lizards. Its fourth toe was also unusually chunky, suggesting it may even have been bearing most of its weight on just that one digit when walking.

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.


Some of the earliest large terrestrial herbivores on Earth were the edaphosaurids – a very early-branching group of synapsids, the evolutionary lineage whose only modern surviving members are mammals. Like their more famous cousin Dimetrodon these animals sported huge elaborate sails on their backs formed from highly elongated vertebral spines, but despite the similarity in appearance they actually seem to have evolved these structures completely independently.

Known from a single partial skeleton discovered in southern New Mexico, USA, the edaphosaurid Gordodon kraineri dates to around the very end of the Carboniferous or the very earliest Permian, about 299 million years ago.

It was fairly small for an edaphosaurid at about 1.5m long (~5′), and seems to have had transitional anatomy between earlier and later members of the group. Its sail spines were thicker than those of earlier species but still less heavyset than those of later forms, and while each spine had numerous side projections these structures were small, thorn-like, and randomly distributed, unlike the more organized thick crossbars seen in Edaphosaurus.

Its head was proportionally small compared to its body, but still relatively large for an edaphosaurid, and it had an unusually long neck for an early synapsid. But its most distinctive features were its jaws and teeth – it had a narrow snout with a pair of large incisor-like teeth at the front of both its upper and lower jaws, followed by a large toothless gap (a diastema) and then a short row of small peg-like teeth. Like Edaphosaurus it also would have had batteries of grinding tooth plates inside its upper and lower jaws, but probably not as extensively.

Overall its tooth arrangment looked more like a modern herbivorous mammal than an early synapsid, much more highly specialized than anything else known to be alive at the time – the next synapsid known to convergently evolve similar teeth lived around 90 million years later!

It probably had a very different diet to its relatives, with its specialized teeth and fairly slender body suggesting it may have been a selective feeder, cropping the softer more nutritious parts of plants like the fleshy seeds and cones of gymnosperm plants.

Its discovery also hints that herbivorous edaphosaurids in general were much more diverse than we previously thought, and there may be even more surprising forms out there still to be discovered.

Spectember 2021 – Marsupial Predators

It’s September, the Cambrian series has been delayed until later this year, so instead let’s get speculative – it’s time for the return of #Spectember! I can’t manage daily content this time around, but I still have plenty of submitted concepts left over from last time.

So let’s get started with some marsupials suggested by someone crediting themselves only as Bruno Drundridge:

Continue reading “Spectember 2021 – Marsupial Predators”


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.

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.

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.