June 2025 – Nix Illustration

Poposaurus

Despite its incredible resemblance to theropod dinosaurs, Poposaurus gracilis was actually a pseudosuchian more closely related to crocodilians than to dinosaurs.

Living in what is now North America during the Late Triassic, about 237-216 million years ago, Poposaurus grew to around 4.5m long (~15′) with roughly half of that length taken up by just its long tail. With its sharp-toothed jaws, small arms, bipedal locomotion, and counterbalancing tail, it convergently evolved the same sort of body plan and ecology as carnivorous theropods – which were still in their early days at the time, and wouldn’t really become the dominant terrestrial predators until after the end-Triassic extinction.

Unlike most other pseudosuchians Poposaurus lacked bony osteoderm armor, seems to have been capable of a digitigrade posture, and its claws were flattened and somewhat hoof-like, all adaptations that suggest it was built for running after fast-moving prey.

Exaeretodon

Cynodonts were one of the few lineages of synapsids (“protomammals”) to survive through the Great Dying mass extinction into the Triassic. And while a major branch of cynodonts known as probainognathians would eventually go on to produce the ancestors of modern mammals, for much of the Triassic a separate branch called cynognathians were initially much more diverse and numerous.

Exaeretodon argentinus was a large traversodontid cynognathian, growing up to about 1.8m long (~6′), known from the Late Triassic (~234-227 million years ago) of what is now northwestern Argentina. It was a low-slung animal with short stocky limbs, sprawling at the front and semi-upright at the back, and had a large head with a fairly short narrow snout and wide flaring cheekbones accommodating massive jaw muscles.

Although it it had large fang-like canine teeth, further back in its jaws wide molar-like grinding teeth show it was a specialized herbivore – at least as an adult. Different skull proportions in juveniles suggest that young Exaeretodon may have actually started out life as omnivorous or carnivorous, with jaws better suited for crushing hard-shelled invertebrate prey.

One Exaeretodon specimen shows evidence of severe rib injuries that would have hindered its mobility and made it very difficult to forage for food or avoid predators. But in this case those injuries were healed, suggesting this species may have lived in social groups that helped to protect each other.

Dibango

In the late 18^th century a strange fish fossil from the Monte Bolca deposits in northern Italy was described and named as Pegasus volans. This name had actually already been assigned to the living longtail seamoth (today known as Pegasus volitans), but despite this the fossil continued to be referred to as “Pegasus volans” for well over 200 years.

Now, finally, it’s been redescribed and given a proper genus name of its own: Dibango volans.

Living during the early Eocene, around 50-48 million years ago, in what was then a warm shallow reef in the western Tethys Ocean, Dibango was probably around 7-10cm long (~3–4″). It had a long flag-like first ray of its dorsal fin, a very reduced and compact abdominal region, an extremely elongated pelvic bone that appears to have supported an exterilium (external gut), long pelvic fins, and a long slender tail.

This bizarre combination of features is often seen in fish larvae, but Dibango’s level of skeletal development shows it was fully grown – suggesting it was actually an unusually neotenic fish, retaining its larval anatomy all the way into adulthood.

This also makes it very difficult to figure out what kind of fish it actually was, with the current best guess being “some sort of percomorph“.

Duonychus

Duonychus tsogtbaatari was a therizinosaurid dinosaur living in what is now the Gobi Desert in southern Mongolia during the Late Cretaceous, around 96-90 million years ago.

Like other therizinosaurids it would have been a chunky-bodied herbivore with a small beaked head atop a long neck, long rake-like claws on its hands, stout legs, and a rather short tail. But it was rather small compared to most of its close relatives, estimated at about 3m long (~9’10”), with its known fossil remains including several vertebrae, partial ribs and pelvis, and a set of nearly-complete arms and hands.

Its hands had only two well-developed fingers, with a small splint-like vestigial third finger, an anatomical condition convergently seen in some other theropod groups but previously unknown in therizinosaurids. One of its long curved claws also preserved a rare example of a thick keratinous sheath, showing that in life the claw was over 40% longer than its bony core.

Duonychus’ elbow and finger joints had a fairly limited range of motion – more similar to the forearms of Tyrannosaurus than other therizinosaurids – but its claws were able to flex almost 90° at the tips of its fingers, which may have given it the ability to reach out and grab onto foliage with a very strong and precise grip.

Panochthus

Panochthus tuberculatus was a large glyptodont – a group of giant heavily-armored armadillos – that lived in central and southern South America during the late Pleistocene, about 800,000-12,000 years ago.

Around 3.5m long (~11.5′) and 1.5m tall (~5′), it was similar in size to a modern rhino (or a small car), and its large domed “shell” made up of numerous small bony osteoderms made it resemble a mammalian tortoise. Its skull was short and deep, with ever-growing grinding teeth and downwards-flaring cheekbones that anchored powerful jaw muscles. A preserved hyoid apparatus indicates that Panochthus also had a more flexible tongue than some other glyptodonts.

The base of its tail was segmented into rings that allowed it to flex, while the end of the tail was fused into a solid bony tube that was probably studded with large keratinous knobs or spikes.

While these sort of tail weapons in glyptodonts have been proposed as being anti-predator defenses, biomechanical studies suggest they required precise aiming to be most effective and weren’t well-suited to fending off fast-moving attackers. Instead they may have been more specialized for fighting each other in ritualized forms of combat – an idea supported by injuries in fossil carapaces that appear to have been caused by blows from opponents’ tail clubs.