squamata – Nix Illustration

Megapterygius

Most mosasaurs all had very similar body plans: they were streamlined scaly monitor-lizard-like marine reptiles with four rounded paddle-shaped flippers, and many of them also had large shark-like tail fins.

But Megapterygius wakayamaensis here seems to have been doing something a bit different.

Living towards the end of the Cretaceous, about 72 million years ago, in the waters covering what is now western Japan, this mosasaur was around the size of a modern orca, roughly 6m long (~20′).

Unlike other known mosasaurs its flippers were huge, bigger than its own head and distinctively wing-shaped, with the back pair being larger than the front. This is an arrangement oddly reminiscent of the unrelated plesiosaurs, and may suggest a convergent sort of highly maneuverable “underwater flight” swimming ability – but unlike plesiosaurs Megapterygius also still had a powerful fluked tail, so how exactly all of its fins worked together is still unknown.

It’s also the first mosasaur known to preserve potential evidence of a dorsal fin. Some of its back vertebrae show a change in orientation at the point where a fin base would be expected to be, closely resembling the vertebrae shape of cetaceans like the modern harbor porpoise.

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.

Slavoia

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.

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 20^th 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.

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.

Gavialimimus

During the late Cretaceous period, about 72-66 million years ago, the Oulad Abdoun Basin region of Morocco was submerged under the Atlantic ocean – and the water above it was absolutely teeming with mosasaurs.

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.

And one of the most surprising recent discoveries from this diverse ecosystem was Gavialimimus almaghribensis.

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.

Eons Roundup 4

Some more recent commission work for PBS Eons!

An illustration of an extinct entelodont, an omnivorous hoofed mammal. It has a hippo-like head and a body proportioned rather like a hyena, with high shoulders and long slender legs. It has cloven-hoofed feet and a short tail, and its body is covered in a coat of short bristly fur. — Eoentelodon

The entelodonts Eoentelodon and Brachyhyops, from “The Hellacious Lives of the Hell Pigs”
https://www.youtube.com/watch?v=trJpxwMGoCw

An illustration of an extinct ichthyosaur, an ancient marine reptile. It has a short narrow snout full of rounded teeth, large eyes, a long tapering body, four flippers, and a long tail ending in a vertical fin. — Tholodus

An illustration of an extinct mosasaur, an ancient marine reptile. It has long triangular jaws full of many small sharp teeth, a long lizard-like body, four paddle-like flippers, and a long tail ending in a vertical fin. — Tholodus

The early ichthyosaur Tholodus and the mosasaur Pluridens, from “When Ichthyosaurs Led a Revolution in the Seas”
https://www.youtube.com/watch?v=V342aXQs9XY

An illustration of an extinct early bat in flight. It has claws on all five fingers on each wing, along with small ears and a long tail. — Onychonycteris

An illustration of an extinct early bat in flight. It has claws on both its thumb and the first wing finger one each wing, along with small ears and a long tail. — Onychonycteris

The early bats Onychonycteris and Icaronycteris, from “When Bats Took Flight”
https://www.youtube.com/watch?v=zWeYCULC0UQ

Island Weirdness #26 – The Mauritian Giant Skink

Along with its unique birds, Mauritius was also home to many endemic reptile species. In the absence of terrestrial mammals giant tortoises were the largest herbivores on the island, and various geckos, skinks, and snakes helped to fill out the rest of the vertebrate ecosystem.

Leiolopisma mauritiana was a very large skink, one of the biggest ever known to have existed with a total length of around 80cm (2′7″). Its ancestors originated in Australasia, over 5600km away (~3500 miles) at least 3-4 million years ago – and they must have endured a particularly long ocean rafting journey without any island hopping stops, since none of the other islands along that route seem to have ever had populations of similar skinks.

It probably lived in rocky areas, possibly also being capable of digging burrows, and would have eaten an omnivorous diet of seeds, fruits, invertebrates, and smaller lizards and birds.

By the early 1600s it was already extinct, very soon after the arrival of humans, probably due to predation from invasive mammals like rats. However, its half-sized close relative Leiolopisma telfairii does still survive on rat-free Round Island a short distance to the north of Mauritius, and recent conservation efforts have been rebuilding its population and setting up new colonies on other nearby small islands.

Almost-Living Fossils Month #23 – Enamel-Armored Lizards

The glyptosaurines were a group of lizards that first appeared in the Late Cretaceous, about 85 million years ago. They were an early branch of the anguid lineage, originating in North America, and had heavily armored bodies covered in bony osteoderms – superficially similar to those of modern beaded lizards, but structurally much more complex with the outermost layer formed from a unique enamel-like substance called osteodermine.

They were some of the few lizards to survived through the end-Cretaceous extinction 66 million years ago (which killed off over 80% of the lizard species known at the time) and went on to become quite successful in the warm climates of the early Cenozoic.

They spread across to Europe and Asia and developed much larger body sizes, going from small 10cm-long (4″) forms in the Early Paleocene (~65 mya) to over 60cm long (2′) by the mid-Eocene (~40 mya). In North America and Europe they became common enough that they were probably important parts of the local ecosystems, and their widespread distribution suggests they were able to adapt to a variety of different habitats and environmental niches.

Their teeth resembled those of modern omnivorous lizards like blue-tongued skinks, suggesting they had a similar generalist diet – although their strong jaws have also been proposed to be specializations for crushing hard-shelled invertebrates such as snails.

Helodermoides tuberculatus here was one of the largest glyptosaurines, about 65cm long (2′2″). It lived during the Late Eocene and Early Oligocene (~34-33 mya) in the northwestern and midwestern United States, with fossils known from Montana, Wyoming, and Nebraska.

One fossil shows evidence of having lost part of its tail, probably dropping it in a self-defense behavior to escape a predator. However, unlike the regenerating tails of many other lizards, the osteoderms of Helodermoides instead seem to have formed a thick rounded bony cap over the wound, preventing any significant regrowth and leaving its tail permanently stumpy.

During the Late Eocene and Early Oligocene the glyptosaurines began to disappear, probably struggling to cope with cooling and drying climates, and their last definite fossils date to about 30 million years ago. Possible fragmentary remains from as late as the Early Miocene of Central Europe (~16 mya) may indicate that a few isolated late-surviving members of the group persisted on for a while longer, but if they did hang on that long they were probably finished off by further sharp temperature drops in the mid-Miocene.

Almost-Living Fossils Month #14 – Ancient Snakes

While the evolutionary origin of snakes is still rather poorly understood, one very early branch of their lineage – known as the madtsoiids – were a particularly long-lived group.

Originating back in the mid-Cretaceous (~100 mya), these “primitive” snakes were found mostly in the southern continents of Gondwana (known from South America, Africa, India, and Australia), but a few also spread into Europe. They were either some of the earliest true snakes or perhaps ophidians very closely related to them, and may have retained small hindlimbs that were slightly more well-developed than the vestigal ones of some modern snakes.

They ranged in length from under 1m (3′3″) to at least 7m (23′), with biggest of them rivaling some of the very largest living snakes in size.

They would have been similar to pythons, non-venomous and relying on constriction to kill their prey, although they had less flexible skulls than their modern relatives and couldn’t easily swallow animals much larger than their own heads. At least some of the Cretaceous species would have preyed on smaller dinosaurs, with one fossil even preserving a mid-sized madtsoiid in a sauropod nest alongside a hatchling.

Although the madtsoiids survived the end-Cretaceous extinction quite well and kept going throughout most of their range for the first half of the Cenozoic, most of them eventually disappeared in the Eocene-Oligocene extinction about 33 million years ago.

Aside from a single possible Late Olgiocene/Early Miocene record from South America (~29-21 mya), after that point the madtsoiids were found only in Australia, where they persisted almost into modern times.

Wonambi naracoortensis was one of the last of the Australian madtsoiids, living from the mid-Miocene (~11 mya) to at least the Late Pleistocene (~40,000 years ago). It was also one of the larger members of the group, 5-6m long (16′5″-19′8″), and seems to have been an ambush predator that lurked around waterholes to catch drinking animals.

The last madtsoiids went extinct at the same time as many of the other Australian megafauna, and it’s not clear exactly what caused them to die out. Humans had arrived in Australia about 20,000 years earlier, and hunting – either directly targeting the large snakes, or simply gradually reducing their available prey – combined with a changing climate may have been too much for them to handle.