Originally known only from a few fragments, this 2.5-3m long (8′2″-9′10″) animal was first considered to be an indeterminate early archosaur, then a non-archosaurian doswelliid. But new fossil material and a recent analysis have instead placed it as a member of the erpetosuchids, an early group of pseudosuchians (the branch of the archosaurs that includes modern crocodilians).
Erpetosuchids were some of the earliest well-armored archosaurs, with several rows of bony osteoderms along their neck, back, and tail, and scattered oval osteoderms covering their limbs. Their fairly gracile build and slender limbs suggest they were active terrestrial carnivores – but it’s hard to say exactly what they were preying on due to their somewhat odd skulls.
They had only a few teeth at the very front of their upper jaws, with the rest being toothless, but meanwhile the lower jaw was fully-toothed. Their skulls had narrow snouts at the front but became much wider further back, suggesting the presence of powerful jaw muscles, and they had slightly upward-facing eye sockets.
Smaller erpetosuchids are speculated to have been specialized for insect-eating, catching their small prey with their front teeth and then crushing it with the semi-toothless part of their jaws further back. But something the size of Tarjadia probably couldn’t have survived on a purely insectivorous diet, and it must have been doing something else with its weird jaws.
Known from around the North Pacific rim from about 33-15 million years ago, plotopterids were flightless diving birds which used their small but powerful wings to propel themselves through the water. They were convergently similar to penguins in body shape and lifestyle, but not actually closely related to them – instead being relatives of gannets, cormorants, and anhingas.
Smaller plotopterids were about the size of modern cormorants, around 70cm long (2′4″), but the larger known genera like Copepteryx rivalled the southern giant penguins at around 1.8m (6′).
And a second species of Copepteryx known only from a single leg bone (Copepteryx titan) may have been ever bigger. Estimated at over 2m in length (6′6″), it was possibly one of the largest diving birds to have ever lived.
About 6m long (19′8″), it had long brow horns and large curved spikes on its frill – an arrangement very similar in appearance to the centrosaurAlbertaceratops, and initially its fossils were misidentified as belonging to that particular ceratopsid. But in 2010 it was recognized as a different genus, and based on some partial frill remains it was classified as a very early chasmosaur (a different branch of the ceratopsids which includes Triceratops), related to other early forms like Mercuriceratops.
Its genus name was based on the snake-haired Medusa from Greek mythology, while its species name comes from the Norse trickster god Loki – both in reference to the years of confusion about the identity of Medusaceratops’ fossils, and the distinctive curved horns on the helmet of Marvel’s Loki.
And, true to its name, the confusion wasn’t over yet.
Recently more fossil material and a new study have shown it was still being misclassified. Now it seems like Medusaceratops was actually part of the centrosaur lineage all along, and was indeed a very close relative of Albertaceratops.
It also turns out that what were thought to be numerous Albertaceratops fossils found in the same location were all just even more Medusaceratops. Instead of a mixture of two different ceratopsids there’s a single big bonebed representing some sort of mass-mortality event of only this one animal.
Similar mass bonebeds have been found for other centrosaurs in the same area and around the same age. Perhaps there were frequent flash floods at the time, or they were attempting to migrate across fast-flowing rivers like some modern animals, but we still don’t actually know for certain why they died en masse so frequently.
Procynosuchus delaharpeae, a synapsid from the Late Permian (~259-252 mya). Measuring about 60cm long (2′), it was one of the earliest members of the cynodonts, the lineage that would eventually lead to mammals.
Its fossils are mostly known from southern Africa, but similar remains have also been found in Europe and Russia, suggesting it was actually quite widespread across the supercontinent of Pangaea that existed at the time.
It had a long vertically-flattened tail, strong leg muscles, and paddle-like feet – all adaptations that suggest it was a semi-aquatic otter-like animal capable of agile swimming. It also had forward-facing eyes, giving it good binocular vision and depth perception while pursuing fish underwater.
Nicrosaurus kapffi from the Late Triassic of Germany, about 221-205 million years ago. Although rather crocodile-like in appearance, this 4-6m long (13′-19′8″) animal was actually part of an extinct group called phytosaurs – long-snouted heavily-armored reptiles with their nostrils high up on their heads near their eyes.
Phytosaurs’ exact evolutionary relationships are still disputed, with opinions currently going back and forth between them being archosauriformes or an early branch of the croc lineage within the true archosaurs. But either way they weren’t directly ancestral to modern crocodilians, and instead developed a very similar body plan via convergent evolution.
While some phytosaurs had very slender gharial-like snouts and probably fed mostly on fish, others like Nicrosaurus had much more robust jaws and seem to have secondarily adapted to a terrestrial predator lifestyle. They had longer limbs and a more upright posture than their semi-aquatic relatives, and enlarged fangs at the hooked tips of their jaws that may have been used to deliver a powerful stabbing blow to their prey.
Capinatator praetermissus, an arrow worm from the Mid-Cambrian of Canada (~508 mya). Discovered in the famous Burgess Shale fossil deposits, it was one of the earliest known arrow worms and also much larger than most modern forms, measuring around 10cm in length (4″).
Its mouth was surrounded by 50 hooked spines, which could be extended out to grasp onto its prey – probably feeding on whatever smaller animals it could catch – but when not in use these spines would have been kept folded up inside a fleshy “hood” around its head.
It may have been a transitional form between early large-predator arrow worms and the smaller plankton-feeders that the group later became.
Australovenator wintonensis, a megaraptoran dinosaur from the Late Cretaceous of Queensland, Australia (~100-94 mya). It was a medium-sized member of the group, about 6m long (19′8″), and despite only being known from a partial skeleton it’s still one the best-known megaraptorans – and also the most complete predatory dinosaur from Australia.
Megaraptorans were a group of fairly large theropod dinosaurs, currently known from Australia, South America, and Japan (and maybe Egypt). Their relationships to other theropod groups are rather uncertain, with different studies placing them as neovenatorids, tyrannosaurids, or most recently as an early branch of the coelurosaurs.
They had very lightly-built bodies, with bird-like bones full of weight-reducing air spaces, proportionally small heads with long slender snouts, and leg bones adapted for running. But their most distinctive feature was their hands, featuring massively enlarged claws on the first and second fingers, with the third finger being much smaller and somewhat vestigial-looking. While some other theropods like allosaurids and spinosaurids also had big hand claws, megaraptorans’ almost tyrannosaurid-like mostly-two-fingered arrangement is rather odd.
Their arms and fingers were much more flexible than those of most other non-avian dinosaurs, allowing them to reach out, grab onto prey with those claws, and then pull it in close to their bodies, restraining it in a sort of death-hug while their relatively weak jaws finished it off.
A distinctive injury to the second toe of Australovenator also suggests these dinosaurs may have been able to deliver powerful kicks like modern cassowaries.
Although very lizard-like in appearance, this animal was actually part of a completely extinct group known as parareptiles – a diverse group of early sauropsids who were once thought to be the ancestors of turtles, but are now considered to instead be the evolutionary cousins to the true reptiles.
With a total length of about 25cm long (8-10″), the structure and proportions of its limbs suggest it could run fast on its hind legs, making it one of the earliest known examples of bipedal locomotion. Since its teeth were adapted for a herbivorous diet, it wasn’t using its speed to chase down prey but was instead probably sprinting away from predators.
But unlike the sprawling running of some modern lizards, Eudibamus may have been capable of holding its legs in a more upright position directly under its body, convergently evolving a more energy-efficient posture similar to that of later bipedal animals like dinosaurs.
Ergilornis rapidus, a 1.2-1.5m tall bird (4′-5′) from the Early Oligocene of Mongolia (~33-28 mya). Closely related to modern cranes, trumpeters, and limpkins, it was part of an extinct group called eogruids – flightless birds which existed across Eurasia for a large portion of the Cenozoic from roughly 40-3 million years ago.
Although the earliest known eogruids were smaller and less specialized, and may even have still been somewhat capable of flying, later forms like Ergilornis had highly reduced wings, long legs adapted for running, and convergentlyostrich-like feet with only two toes each.
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.)