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.

Weird Heads Month #07: The Wonderful Creeping Thing

The Triassic was an incredibly weird time, full of evolutionary experiments in the wake of the worst mass extinction in Earth’s history.

Teraterpeton hrynewichorum here was part of group known as allokotosaurs, a lineage of mostly-herbivorous archosauromorphs that also included the long-necked bull-horned Shringasaurus.

Living in Nova Scotia during the Late Triassic, around 235-221 million years ago, Teraterpeton (meaning “wonderful creeping thing”) was first named in the early 2000s based on a skull and partial skeleton, with some additional skeletal material being described recently in 2019.

Its head had a confusing mix of anatomical features, with a long beak-like toothless snout at the front of its jaws, small sharp interlocking cheek teeth further back, a huge nasal opening, and a closed-up fenestra at the back of its skull making it look more like the skulls of marine reptiles.

It also had a lizard-like body, perhaps up to 1.8m long (~6′), with rather long slender limbs and large blade-like claws, and more anatomical weirdness in the pelvic region convergently resembling those of distantly related groups like rhynchosaurs and tanystropheids. It had a sprawling posture, but its hind limb musculature suggests it might have been capable of getting up into a more erect stance when walking, somewhat similar to modern crocodilians’ “high walk” gait.

It was clearly quite an ecologically specialized animal, but quite what it was specialized for is still uncertain. It was presumably a herbivore like its close relatives, but it must have been eating a very different diet with its long beak, and its deep claws could have been used for scratch digging to get at roots and tubers.

Another possibility it that it could have been an insectivore with a diet similar to modern aardvarks or armadillos, probing with its beak and digging with its claws for insects, grubs, and other invertebrates. Since termite-like social insect nests do seem to have existed around the same time, it might even have been one the earliest known animals to specialize in myrmecophagy.

Weird Heads Month #05: Crested Snorkelers

Phytosaurs were a lineage of incredibly crocodile-like archosauriformes – essentially “crocodiles before crocodiles” – convergently evolving an incredibly similar appearance at a time when the ancestors of modern crocs were still small and terrestrial.

But while they had toothy snouts and bodies heavily armored with bony ostederms, unlike crocodilians their nostrils were far back on their heads up near their eyes, often in a sort of bony “snorkel” so they could breathe while almost fully submerged underwater.

Mystriosuchus westphali lived in Germany during the Late Triassic, about 215-212 million years ago. Around 4m long (~13′), it was even more aquatic than other phytosaurs, with paddle-like limbs and long slender gharial-like jaws adapted for catching slippery prey.

And along with the typical phytosaur snorkel, it also had raised crests along its upper jaw – which may have supported even larger keratinous display structures.

Weird Heads Month #03: Big Head Mode

In the last entry we had heads that looked much too small… so now how about heads that were too big?

Erythrosuchus africanus was part of an early branch of the archosauriformes, related to the ancestors of crocodiles, pterosaurs, and dinosaurs. Living in South Africa during the mid Triassic, around 247-242 million years ago, it was the largest predator of its time, reaching about 5m long (16’5″).

It was one of the earliest archosaurifomes to develop a more upright-limbed posture, and convergently evolved a very theropod-like head with a deep narrow snout full of large serrated teeth.

A head that was absolutely massive proportional to the rest of its body, measuring about 1m long (3’3″).

As a result of such a big noggin, Erythrosuchus must have also had some bulky musculature in its neck and forequarters to support it. And while its fairly short neck wouldn’t have been very flexible buried in all that tissue, it probably didn’t need to be – some of its main prey would have been large slow-moving dicynodonts, and its hunting strategy may have consisted of simply “aim at food and lunge”.

Shonisaurus

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.

Diademodon

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

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

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.

Island Weirdness #01

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.


Thecodontosaurus antiquus

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

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.