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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.

Megacephalosaurus

Megacephalosaurus eulerti was a one of the last of the pliosaurs – a group of short-necked big-headed plesiosaurs – living during the Late Cretaceous (~93 million years ago) in what is now the Midwestern United States, a region that at that time was covered by the Western Interior Seaway.

Although known only from fossil skulls and a few neck bones, based on the proportions of related pliosaurs it probably reached around 9m long (~30′) with its 1.75m (~5’9″) head alone making up 20-25% of that measurement.

Its elongated jaws were lined with pointed conical teeth, and pits in the bones of its snout may have housed a complex sensory system, possibly giving it the ability to detect the movements or even bioelectric fields of nearby prey.

Mosura

Mosura fentoni was a small radiodont living during the mid-Cambrian, about 508 million years ago, in near-equatorial shallow marine waters covering what is now western Canada.

Sixty specimens have been discovered in the Burgess Shale fossil deposits, ranging from 1.5cm long juveniles (~0.6″) to 6cm long adults (~2.4″), giving us a detailed look at Mosura’s anatomy and life history. It had three eyes – two on the sides of its head on short stalks and one in the middle of its face – and a pair of grasping frontal appendages each with six long sickle-shaped spines.

Unusually for a radiodont its body was divided into distinct regions: a four-segment neck, a six-segment mesotrunk with large swimming flaps, and an abdomen-like posterotrunk with up to at least sixteen segments (fewer in juveniles), all bearing gills along their undersides.

Its vaguely moth-like shape led to it being nicknamed “sea-moth” by field collectors, and inspired its genus name – “Mosura” is the Japanese name of the fictional giant kaiju moth-monster Mothra.

With a very high proportion of respiratory surface area for its size, Mosura was probably an active and agile fast-swimming predator, possibly living in low-oxygen waters around the outer continental shelf. Its wide oval central eye may have helped it stay orientated during rapid maneuvers, keeping track of the horizon line similar to the median eyes of modern dragonflies.

Shishania

Shishania aculeata lived during the mid-Cambrian, around 512 million years ago, in shallow tropical waters covering what is now southwestern China.

Up to about 6cm in length (~2.4″), its spine-covered body was initially thought to be an early mollusc, but the discovery of more specimens has resulted in a new interpretation: instead of a slug-like creature, the fossils of Shishania might instead represent a flattened and ruptured chancelloriid.

Chancelloriids were an enigmatic group of Cambrian animals that had hollow bag-like bodies armored with numerous sharp star-shaped spines. They were historically classified as sponges due to their similar body plan and immobile filter-feeding lifestyle, and they’ve also been proposed to be relatives of halkieriid molluscs due to similarities in the microscopic structure of their spines – but currently it seems most likely that chancelloriids were actually their own separate lineage of early animals, closer related to eumetazoans than to sponges.

Shishania had much simpler spines than other chancelloriids, so it may represent an early stage of the evolution of these animals’ armor, showing that these structures were developed from scratch rather than adapted from a pre-existing ancestral feature.

Thrinacodus gracia

Thrinacodus gracia* was a stem–elasmobranch – a cartilaginous fish related to modern sharks and rays – living in what is now Montana, USA during the mid-Carboniferous around 324 million years ago.

* previously known as Thrinacoselache gracia

Although the cartilaginous skeletons of chondrichthyans rarely preserve, the exceptional preservation conditions of the Bear Gulch Limestone fossil deposits mean we do actually have full-body soft tissue impressions of this species. It was about 1m long (3’3″) with an unusually slender eel-like body, a pointed snout, no dorsal fins, and an elongated tapering tail.

Preserved gut contents show that Thrinacodus gracia preyed on shrimp-like crustaceans and smaller cartilaginous fish such as Falcatus and Harpagofututor. It would have inhabited a shallow tropical bay environment, and may have had a similar sort of lifestyle to the modern eels it resembled, hiding in crevices or burrowing into sediment and ambushing passing prey.

Dinosorex

Dinosorex, the “terror shrew”, was a genus of eulipotyphlan mammal found across much of Europe for most of the Miocene, ranging from about 23 to 9 million years ago. Part of a family of stem-shrews known as heterosoricids, it was larger than most of its living relatives – probably around 15-20cm long (6-8″) – and inhabited subtropical swampy forest environments.

Dinosorex kaelini here was one of the later species in this lineage, living in what is now Switzerland around 12-10.5 million years ago.

It had massive incisor teeth at the front of its jaws and crushing teeth further back, specialized for grabbing, immobilizing, and cracking open prey like hard-shelled invertebrates. Similar to some modern shrews the tips of these teeth were also reinforced with iron in their enamel, which would have given them a striking dark red coloration.

But while Dinosorex was quite abundant and successful during its time, it seems to have had such a specific ecological preference that it couldn’t adapt when the climate shifted towards the end of the Miocene. Drier conditions and more open savannas quickly took over, and the terror shrews disappeared along with the lush humid forests they were so dependent on.

Hwiccewyrm

Hwiccewyrm trispiculum lived during the late Triassic, around 208-202 million years ago, in what is now England. It was one of the last known members of the procolophonid family, a lineage of small stocky lizard-like animals that had been widespread and abundant earlier in the Triassic.

(Traditionally procolophonids are classified as parareptiles, but some recent studies suggest this group is paraphyletic or polyphyletic, with some “parareptiles” potentially nesting within the diapsids instead.)

Measuring around 30cm long (~1′), Hwiccewyrm had wide flaring cheek bones ornamented with large spines, and like some other procolophonids it may also have had bony scute armor on its body. Its large blunt teeth suggest it was feeding on particularly tough foods such as fibrous vegetation or hard-shelled invertebrates.