Most ichthyosaurs were similar in size to the modern dolphins they convergently resembled, but during the Late Triassic some of them got much much bigger.
Shonisaurus popularis lived about 222-212 million years ago, in Nevada, USA — a region that’s currently made up of dry deserts, but which was submerged under a tropical inland sea at the time.
At around 15m long (49′) it was roughly the same size as a modern humpback whale, with a long narrow snout, a fairly deep fusiform body, and four equally-sized flippers. Unlike many other ichthyosaurs it doesn’t seem to have had a dorsal fin, and its tail fluke shape was rather “primitive” indicating it was probably a slow cruising swimmer.
Juveniles had a few small teeth at the tips of their jaws, but larger adults were entirely toothless, suggesting that they may have specialized in different ecological niches at different stages of their lives. Fully-grown Shonisaurus probably mostly fed on prey such as soft-bodied cephalopods and small fish, which must have been incredibly abundant in the ancient Nevadan sea to support a population of such huge marine reptiles.
But Shonisaurus popularis wasn’t even the biggest of the Late Triassic giant ichthyosaurs. Further north in British Columbia, the closely related species Shonisaurus sikanniensis reached lengths of up to 21m (69′), and fragmentary remains from England hint at something even larger still, estimated at around 25m (82′) – close in size to the modern blue whale, and potentially being the largest non-dinosaurian reptile to ever live.
Modern mammals are the only surviving members of a much larger evolutionary group known as the synapsids – which back in the Permian period were the dominant land vertebrates.
But much like all other life on Earth at the time, the synapsids were absolutely devastated in the “Great Dying” mass extinction at the end of the Permian, 252 million years ago. Only three lineages survived into the Triassic: the dicynodonts (who briefly took over the world), the therocephalians (who went extinct not long afterwards), and the cynodonts (who eventually gave rise to early mammals).
Diademodon tetragonus here lived right in the wake of the extinction during the Early and Middle Triassic, about 251-242 million years ago. Around 2m long (6′6″), it was one of the largest known cynodonts, and it must have been a fairly successful species since it ranged across a large chunk of Pangaea, known from modern southern Africa, South America and Antarctica.
It had pig-like cheekbones and enormous jaw muscles, along with sharp incisors and canine teeth at the front of its jaws and grinding molars at the back. This arrangement suggests that much like modern pigs it may have been an opportunistic omnivore, occasionally snacking on smaller animals and carrion – although an isotope analysis of its teeth indicates the vast majority of its diet was probably still vegetation in shady damp environments.
Sclerothorax hypselonotus was a temnospondyl amphibian that lived in Germany during the Early Triassic, around 251-247 million years ago.
Measuring about 1.2m long (3′11″), it had some unusual features for a temnospondyl – a very rectangular skull with a wide blunt snout, and elongated spines on its vertebrae that gave its body a sort of “hump-backed” shape.
It was part of a lineage of temnospondyls called capitosaurs, which mostly occupied the same sort of aquatic predator niche as modern crocodiles – but unlike its close relatives Sclerothorax’s well-developed spine and limbs suggest it spent much more time walking around on land.
(And while there was another temnospondyl known to have similar extended vertebrae – the sail-backed Platyhystrix – the two weren’t actually closely related to each other.)
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.
Islands are natural sites for evolutionary experiments. Their isolation and limited resources put a lot of selective pressure on their native species, often resulting in spectacular and unique adaptations. Big animals become small, small animals become big, and ecological niches can end up being filled in unexpected ways.
From the dodo becoming the first well-known example of human-caused extinction, to Darwin’s Galápagos finches being influential in the development of the theory of natural selection, to famous-but-endangered living examples like the kiwi and marine iguana, island species are fascinating and often fragile examples of how diverse life can get even in restricted conditions.
In fact, this theme ended up containing so many species I wanted to feature that I can’t possibly fit them all into just a single month. So, for the first time, a theme is going to need two months – with part 1 happening right now, and part 2 coming later this summer.
For much of the Mesozoic Europe was an archipelago of islands in a shallow tropical sea. During the Late Triassic, about 205-201 million years ago, some of the paleo-islands in this region existed around southern Wales and South West England, near the city of Bristol.
Thecodontosaurus was actually one of the first non-avian dinosaurs ever named by modern science, discovered in the mid 1830s – several years before the term “dinosaur” was even created to classify the “great ancient lizards”.
It was an early member of the herbivorous sauropodomorphs, the group that would eventually include the largest ever land animals. But unlike its enormous later cousins it was short-necked and bipedal, and was particularly small compared to other contemporary “prosauropods”, measuring only about 2m long (6′6″). This would make it one of the oldest known examples of insular dwarfism.
Eretmorhipis carrolldongi, a hupehsuchian marine reptile from the Early Triassic of China (~247 mya).
This species was originally named back in 2015, but at the time the only known specimens were missing their heads. It was assumed that its skull would have looked similar to those of other hupehsuchians… but now new fossils have been found, and it seems to have actually been much much weirder!
Eretmorhipis’ head was surprisingly tiny in proportion to its body – sort of like a marine version of Cotylorhynchus – and its shape convergently resembled the modern platypus, with a wide “duck bill” and very small eyes. It may have hunted for food along the seafloor in a similar manner to the platypus, using either a highly sensitive sense of touch or possibly even electroreception to locate small invertebrates like worms and shrimp.
It also had much larger bony osteoderms than its other known hupehsuchian relatives, forming a distinctive protruding spiky ridge down its back. At about 85cm in length (2′9″) it was one of the largest marine animals around at the time, so this structure probably wasn’t needed for defense – but as with other hupehsuchians its actual function is still unknown.
What Triassic animal has a name that sounds like a Transformers character?
Living in Texas, USA, during the Late Triassic, about 229-226 million years ago, Triopticus was a type of archosauriform reptile (a “cousin” to crocodiles, pterosaurs, and dinosaurs). Classifying it any more specifically than that is rather difficult since it’s only known from a single partial skull.
It had five large bony bosses on its head that convergently resembled the domes of pachycephalosaurs, suggesting it may have engaged in similar headbutting or flank-butting behavior. At the back of its skull there was also a distinctive deep pit that looked like a “third eye socket”, inspiring it its name – although this feature probably wasn’t actually a parietal eye, instead just being the result of the way several of the bosses came together at that point.
The rest of its appearance is unknown, and this reconstruction is rather speculative as a result. But based on other archosauriformes it was likely to have been a small semi-sprawling quadruped, possibly around 80cm in length (2′7″).
Dicynodonts were a group of herbivorous animals with toothless beaks and protruding tusks, part of the synapsid lineage and much closer related to mammals than to reptiles. They were some of the most successful and widespread land vertebrates from the Late Permian to the Middle Triassic, with one genus even briefly taking over the world in the aftermath of the End-Permian mass extinction event.
And it turns out some of them got very big.
Fossils of a surprisingly large dicynodont were first reported in 2008, but it wasn’t until just recently (in late 2018) that this giant creature was finally given an official name – Lisowicia bojani.
Close in size to a modern elephant, at around 2.6m tall (8′6″) and 4.5m long (14′9″), it was by far the largest known example of its kind to have ever lived. And while most other dicynodonts had upright hindlimbs and sprawling forelimbs, Lisowicia seems to have developed a fully upright posture much more similar to that of quadrupedal dinosaurs and modern mammals.
It was also one of the very last of its kind, living during the Late Triassic of Poland, about 208 million years ago (although there was a possible later survivor in Australia). This was around the same time that early sauropod dinosaurs were likewise first starting to experiment with gigantism, suggesting that both groups were convergently evolving to exploit newly-available ecological niches.
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.
Caelestiventus hanseni, a pterosaur from the Late Triassic of Utah, USA. Living about 208-210 million years ago, it was very closely related to Dimorphodon – but unlike its younger coastal-dwelling relative it instead lived in a desert environment made up of a massive sand dune sea with occasional interdunal lakes.
It’s the earliest known example of a desert pterosaur, over 60 million years older than other examples, suggesting that even fairly early in their evolutionary history these flying animals had already adapted to a much wider range of habitats than previously thought.
Although only known from a partial skull and a single wing bone, it was probably one of the largest Triassic pterosaurs with a wingspan of over 1.5m (4′11″). It had a “keel” on its lower jaw that may have supported a soft-tissue crest or a pelican-like throat pouch, and there were several different types of teeth in its mouth – large pointed fangs at the front, “leaf-shaped” blades further back in its upper jaw, and numerous much smaller teeth along its lower jaw.
The skull roof also preserved the impression of Caelestiventus’ brain shape, showing that it had very well-developed vision but a poor sense of smell.