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!

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


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

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.

Leivanectes

Elasmosaurids are often depicted with noodly snake-like or swan-like necks, but they were probably actually quite stiff and inflexible in life. And while we know from fossilized gut contents that they ate relatively small prey like fish, crustaceans, and cephalopods, exactly how they used their distinctive long necks is still uncertain.

There’s some variation in the sizes and shapes of their teeth, so it’s likely each species was specialized for slightly different feeding styles – we’ve even found a filter-feeding one! – and the recently-named Leivanectes bernardoi here adds in a little more diversity, too.

Living about 115-112 million years ago during the mid-Cretaceous of Colombia, Leivanectes would have been fairly large at around 9m long (29′6″), slightly bigger than the other elasmosaurid species known from the same ancient marine deposits. It had a reduced number of teeth in its jaws, but these teeth were also proportionally larger, suggesting that it may have been tackling bigger tougher prey than its relatives.

Unfortunately it’s currently only known from a single partial skull, so we don’t have any other clues about its ecology.

Panzhousaurus

Panzhousaurus rotundirostris, a sauropterygian marine reptile from the mid-Triassic of southwestern China (~245 mya), living just a few million years after the devastating Permian-Triassic mass extinction. This small marine reptile was only about 40cm long (1′4″) and is known from a single near-complete skeleton.

Although it was a distant evolutionary cousin to plesiosaurs (and even more distantly to modern turtles), it was actually most closely related to an early sauropterygian lineage known as the pachypleurosaurs – a group of small lizard-like aquatic reptiles with tiny heads, long necks, and paddle-like limbs.

It had an unusually short and rounded-off snout compared to its relatives, and since it would have lived alongside many other diverse marine reptiles it was probably specialized for a slightly different ecological niche.

Paludidraco

Paludidraco multidentatus from the Late Triassic of Spain (~237-227 mya).

This 3m long (9′10″) animal was a member of the nothosaurs, a group of semi-aquatic seal-like marine reptiles that were closely related to plesiosaurs (and both were also evolutionary cousins to modern turtles).

It had long slender jaws full of numerous tiny teeth, creating an interlocking comb that was probably used for filter feeding – scooping up mouthfuls of fine-grained sediment from the seafloor and filtering out small invertebrates or soft plant matter.

The bones of its skeleton were also highly thickened and dense, a condition known as pachyostosis that provided ballast to weigh it down in the water. This would have made it a slow and unmaneuverable swimmer, but a very energy-efficient one, using its natural neutral buoyancy to hover or walk along the seabed.

It was essentially a reptilian manatee, filling a similar sort of ecological niche.