It was previously thought to be a slow swimmer with a low and poorly-developed tail fin, and whether it even had a dorsal fin or not was unclear. But now new specimens with soft tissue impressions have given us a big surprise.
Not only did it actually have a fairly well-developed semilunate tail fin, but it also had a dorsal fin positioned much further forward on its body than expected, giving it a shape similar to some small sharks and representing the current earliest known dorsal fin of any amniote.
Bundles of stiffening collagen fibers inside its fins were very similar to those known from later Jurassic ichthyosaur species, indicating that this adaptation evolved much earlier in the lineage than previously thought. Along with stomach contents showing it mainly ate both cephalopods and small fish – fairly fast-moving prey – this suggests it was a capable open-water swimmer. It wouldn’t have been quite as speedy as its much more specialized Jurassic relatives, but it may have still been about as efficient as the small modern sharks it resembled.
Thalattosaurs were another group of weird Triassic animals, found in coastal marine environments all around the world. Their evolutionary relationships are unclear beyond “they were some sort of diapsid reptile”, and they were well adapted for aquatic life, with streamlined lizard-like bodies, short limbs with webbed feet, and long paddle-like tails.
Hescheleria rubeli here was one of the strangest, living in Europe during the mid Triassic, about 247-235 million years ago. It was one of the smaller known species of thalattosaurs, around 1m long (3’3″), and had a particularly bizarre-looking head.
Its snout was so sharply curved downward that it formed a right-angled hook relative to the rest of its jaws, sort of resembling the initial interpretation of Atopodentatus but without the vertical split.
There were also small sharp teeth at the front of its mouth, along with a pair of large conical bony projections on its lower jaw.
This weird arrangement must have been highly specialized for something, but its actual function is still unknown. One suggestion is that the large jaw-spikes were used to crunch into hard-shelled prey, although there doesn’t seem to have been any reinforced surface in the upper jaw for them to crush against.
But I personally wonder if maybe these jaws were the equivalent of the hooked kypes seen in the males of some modern salmonid fish – structures associated with dominance fighting.
The Permian-Triassic extinction 252 million years ago was the most severe mass extinction in Earth’s history, so incredibly devastating that it’s been nicknamed the “Great Dying” – but there were still some animals that somehow just… didn’t seem to really notice it at all.
Hovasaurus lived in Madagascar both just before and for some time after the Great Dying, dating to around 252-247 million years ago. Growing up to about 90cm long (~3′), it was one of the largest tangasaurids and was also highly specialized for aquatic life in freshwater rivers, with an eel-like tail twice the length of the rest of its body and heavy thickened ribs.
Hundreds of fossils have been found representing life stages from hatchling to adult, and juvenile Hovasaurus actually seem to have been almost fully aquatic – they had proportionally shorter limbs and may have behaved similarly to modern sea turtles, crawling into the water shortly after hatching and only returning to land as adults once they had longer better-developed legs.
It’s not entirely clear why these odd little aquatic reptiles were apparently unaffected by the Great Dying. Perhaps, much like the many freshwater species that survived though the later end-Cretaceous mass extinction, Hovasaurus was simply very good at dealing with sudden changes in its environment and food availability due to the variability of river habitats, and was able to weather though the worst of the extinction without much trouble.
Thalattosaurs were a weird and rather mysterious group of Triassic marine reptiles. It’s not clear where they actually fit on the reptile evolutionary tree (we know they’re diapsids, but nobody can really agree on anything more definite than that), and they had some very strange skulls that seem to have been highly specialized for something, although their actual function is still unknown.
It’s not clear whether this odd snoot was an adaptation for hunting similar to the long bills of swordfish – there’s quite a bit of variation in length and shape between different individual specimens – or if it was serving some other purpose like the sexually dimorphic noses of some modern lizards.
Known from several near-complete fossils that include rare soft tissue impressions, it’s the first varanopid to show preserved skin details – revealing a pattern of very lizard-like rectangular scales. If it is a synapsid this is a pretty big deal, since early synapsids were previously thought to have had scale-less leathery skin.
It also had unusual mosaic-like patches of tiny osteoderms above its eyes, a feature previously known only in some temnospondyl amphibians. Whether this was the result of convergent evolution or the trait actually being ancestral to most tetrapods is unclear.
Its slender body, long digits, and highly curved claws indicate it was an agile climber. It probably mainly lived up in the treetops, feeding on insects, making it one of the earliest known tetrapods specialized for an arboreal lifestyle.
(*Maybe. There’s apparently an upcoming study that suggests varanopids might actually be sauropsids instead.)
Its short toothless snout suggests it was a suction feeder, using water pressure differences to pull small soft-bodied prey straight into its mouth like a syringe. Along with a heavily built body similar to those of hupehsuchians, and a very long tail that made up over half of its 1.6m length (5′3″), it was probably a fairly slow swimmer living in shallow coastal waters.
It was a close relative of Cartorhynchus, and may have been similarly capable of hauling itself onto land like a modern pinniped.