pantestudines – Nix Illustration

Jiangxichelys neimongolensis

Jiangxichelys neimongolensis was a terrestrial turtle that was part of an extinct group known as nanhsiungchelyids, whose closest living relatives today are the aquatic softshell turtles.

(This species was previously known as “Zangerlia” neimongolensis, but has since been moved into the genus Jiangxichelys instead.)

It lived towards the end of the Cretaceous, about 75-71 million years ago, in what is now the Gobi Desert – which at the time was more of a semi-arid climate with both rivers and sand dunes.

Its 60cm long (~2′) carapace had a long wide shape that made it appear rather flattened from the front, but not to quite as much an extreme as its larger American cousin Basilemys.

Several fairly well-preserved specimens have been found that appear to have been buried alive, probably either engulfed by sudden sandstorms or trapped in collapsing burrows. This has preserved some anatomical details previously unknown in nanhsiungchelyids, such as the pattern of scales on top of the head and the presence of large bony osteoderms on the underside of the front toes, which may have aided with traction on loose sandy ground.

Serpentisuchops

While the most iconic types of plesiosaur were long-necked with small heads and short blunt snouts, some of these marine reptiles actually developed the opposite sort of arrangement, with groups like the polycotylids and the pliosaurs independently evolving short necks, larger heads, and long snouts.

…Except some of them didn’t keep it quite that simple.

Serpentisuchops pfisterae here lived during the late Cretaceous, about 70 million years ago, in the ancient Western Interior Seaway covering what is now Wyoming, USA. This 7m long (~23′) plesiosaur was a member of the polycotylid lineage, but along with a long slender snout it also had an unusually long neck.

Some earlier polycotylids like Thililua had fairly long necks, too, but all of Serpentisuchops’ closest relatives were short-necked species, so it seems to have actually re-evolved this condition rather than inheriting it from its ancestors. Since no other marine reptiles in its habitat had this particular body plan, it was probably occupying a very specific ecological niche – the presence of attachment points for powerful neck muscles suggest it was able to swing its head sideways to snap its jaws at prey at high speed, with its longer neck giving it more reach than other polycotylids.

Umoonasaurus

Umoonasaurus demoscyllus was a small short-necked plesiosaur, about 2m long (6’6″), that lived in the polar shallow seas covering much of what is now Australia 115 million years ago during the Early Cretaceous.

Its known fossil remains include a specimen nicknamed “Eric”, one of the most complete opalized vertebrate skeletons ever found.

While most of its body was fairly generalized for a plesiosaur, its skull was unusually ornamented. A raised ridge along the middle of its snout shows evidence of supporting a larger keratinous crest, and smaller ridges over each of its eyes may have also had similar structures. These crests were fairly delicate so were probably mainly used for visual display, and might have been brightly colored.

Retro vs Modern #06: Plesiosaurus dolichodeirus

Plesiosaurs were first recognized as a distinct group of fossil animals in the early 1820s, only a few years after ichthyosaurs. Initially they were perceived as being closer in form to reptiles in the “chain of being” than the more fish-like ichthyosaurs were, and so the group’s scientific name ended up reflecting that early interpretation – “plesiosaur” roughly translates to “near to reptiles”.

The first named species of plesiosaur was Plesiosaurus dolichodeirus, based on a near-complete skeleton discovered by Mary Anning that revealed the strange long-necked proportions of these animals for the first time.

1830s-1850s

Early reconstructions of plesiosaurs in the 1830s compared them to “a snake threaded through a turtle”, giving them highly sinuous necks and a turtle-like body. Much like ichthyosaurs they were assumed to be amphibious, using their flippers to crawl up onto the shore like a sea turtle.

The 1850s Crystal Palace plesiosaur statues show a variant of this design with smooth skin textures and fairly flexible reptilian bodies, with powerful shoulders and flipper postures that give them an overall almost seal-like appearance.

1860s-1990s

From the 1860s onwards a more upright S-shaped neck pose became the most common depiction of plesiosaurs. The writhing snake-like necks persisted in some reconstructions of the extremely long-necked elasmosaurids, but the overall design for these animals that caught hold for the next century was an egg-shaped body with oar-like flippers and a swan-like neck – a body plan that would end up so influential in pop culture that it was incorporated into modern lake monster folklore, with the Loch Ness Monster being the most famous example.

During this period plesiosaurs were often portrayed as floating or swimming at the water’s surface, rowing along with their flippers and using their long necks to snatch up prey. They were generally assumed to still haul out turtle-style to lay their eggs on the shore, although it wasn’t clear how the very largest species would have been able to support their own weight.

2020s

Since the 1990s a boom of new plesiosaur species and biomechanical studies have brought a lot of changes to our understanding of these marine reptiles.

Their necks are now considered to have been less flexible, capable only of more gentle curving, and were probably much thicker and more streamlined with the body than previously depicted. Rather than oar-like rowing all four of their flippers were probably used in more of an “underwater flying” vertical motion similar to modern sea turtles – which is pretty fitting, considering that their closest living relatives are now thought to actually be turtles.

They gave live birth and were probably warm-blooded, with a thick layer of insulating blubbery fat and a teardrop-shaped body outline. Their skin texture was smooth, but one exceptionally well-preserved specimen shows a covering of tiny thin millimeter-sized scales that wouldn’t have been visible in life except in extreme closeup.

We now know Plesiosaurus itself was a fairly small species, around 3.5m long (~11’6″), with a broad body and a short thick tail that probably had a rudder-like fin – usually assumed to be vertically-oriented, but possibly horizontal instead. It lived during the Early Jurassic, about 201-183 million years ago, in the shallow tropical sea that covered what is now southern England, and had a rather small head compared to other plesiosaurs, with its eyes facing upwards and to the sides.

It had sharp needle-like teeth that would have been used to catch soft-bodied aquatic prey like fish and cephalopods. It’s not known whether it had extensive fleshy lips, croc-like snaggletoothed jaws, or something in-between, so the facial soft tissue on this particular reconstruction is rather speculative.

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.

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.

Eons Roundup 11

It’s time for another batch of PBS Eons commission work!

An illustration of an extinct marine reptile. It has a lizard-like body and a long paddle-like tail, and a small head with an oddly-shaped wide mouth lined with many tiny teeth – the overall shape of its bizarre jaws resembles the head of a vacuum cleaner! — Atopodentatus unicus

An illustration of an extinct marine reptile. It resembles a turtle with a wide squared-off beak, small spikes on the back of its head, and a wide carapace shaped vaguely like a horseshoe crab. — Atopodentatus unicus

The marine reptiles Atopodentatus and Henodus, from “The Triassic Reptile With ‘Two Faces'”
https://www.youtube.com/watch?v=-8W26SiCylI

An illustration of an extinct giant marine turtle. it has a ridged shell like a leatherback, and a hooked eagle-like beak. — Archelon ischyros

An illustration of an extinct sea turtle. — Archelon ischyros

The marine turtles Archelon and Euclastes, from “The Return of Giant Skin-Shell Sea Turtles”
https://www.youtube.com/watch?v=Tmb8XCwb3FI

Eons Roundup 7

It’s another PBS Eons commission roundup day!

An illustration of an extinct mammal related to marsupials, climbing on a tree branch. It's an opposum-like animal with protruding fangs and a long furred tail. — Pucadelphys andinus

An illustration of an extinct mammal related to marsupials. It's a mouse-like animal with large ears and a long naked tail. — Pucadelphys andinus

The metatherian mammals Pucadelphysand Khasia, and lineart of the sparassodont Paraborhyaena, from “How South America Made the Marsupials”
https://www.youtube.com/watch?v=l5doyrUWFbE

An illustration of an extinct relative of modern crocodiles. It has a long gharial-like snout, a slightly hunchbacked body, four paddle-like legs, and a long vertically flattened tail. — Acherontisuchus guajiraensis

An illustration of an extinct swimming turtle. It has a very wide and flat carapace, almost circular in shape. — Acherontisuchus guajiraensis

The dyrosaurid crocodyliform Acherontisuchus and the bothremydid turtle Puentemys, from “How a Hot Planet Created the World’s Biggest Snake”
https://www.youtube.com/watch?v=T-hDNbM-WLk

An illustration of an extinct early penguin, in both sitting and swimming poses. It has a long pointed beak and loon-like wings. — Waimanu manneringi

An illustration of an extinct giant penguin. It has a long pointed beak. — Waimanu manneringi

The early penguin Waimanu and the giant penguin Anthropornis, from “When Penguins Went From The Sky To The Sea”
https://www.youtube.com/watch?v=HMArjGQwLvY

Ophthalmothule

The cryptoclidids were fairly standard-looking plesiosaurs, with long necks and small heads – but those tiny skull bones were also rather fragile and so there’s very little good fossil material of their heads, making it difficult to figure out both their feeding ecology and their exact evolutionary relationships.

But a recently-discovered specimen from the Svalbard archipelago actually preserved a mostly-complete skeleton, including an unusually intact skull.

Given the name Ophthalmothule cryostea (meaning “frozen bones of the Northern eye”), this cryptoclidid lived about 145 million years ago, right at the boundary between the Jurassic and the Cretaceous.

It measured around 5m long (16’5″) and had proportionally huge eyes that faced upwards on its head – an adaptation for seeing in low-light underwater conditions, maximizing the amount of light reaching it from above.

Those big dark-adapted eyes suggest it may have been nocturnal, or spent a lot of time diving into very deep waters in search of food. Its skull had weak jaw muscles and delicate teeth, and its gut region contained a lot of fine gravelly sediment, so it probably mainly grubbed around for small soft-bodied prey on the sea floor.

At that point in time Svalbard would have been a little further south than it is today, at a subarctic latitude, but the area would have still experienced particularly long nights during the winter. So it’s possible Ophthalmothule also developed such big sensitive eyes to help it survive through those darker seasons.

Weird Heads Month #15: Hammerhead Reptiles

Atopodentatus unicus lived in Southwest China during the early-to-mid Triassic, around 247-240 million years ago. About 3m long (9’10”), it was a marine reptile – probably part of an early branch of the sauropterygians – with an elongated streamlined body and paddle-like limbs.

When it was originally described in 2014 it seemed to have a head unlike anything seen before. The skull of the only known fossil specimen was incomplete and badly crushed, but it was reconstructed as having a downward-hooking upper jaw with a vertical split in the middle forming a zipper-like row of teeth.

An illustration of the old interpretation of Atopodentatus. It has a bizarre vertical split in the front of its snout full of needle-like teeth resembling a zipper. — The original version of Atopodentatus

But then just two years later some more complete skulls were discovered and revealed something completely different: the projections on Atopodentatus‘ snout actually stuck out to each side in a wide flat “hammerhead” shape on both its upper and lower jaws.

Not quite as bizarre as before, but still a Triassic weirdo!

It also seems to have been a rare example of a herbivorous Mesozoic marine reptile, probably rooting around on the seafloor with its shovel-like mouth, using its chisel-shaped front teeth to scoop up mouthfuls of algae and other marine plants and then straining out the water through its closely spaced needle-like back teeth.