pseudosuchia – Page 2 – Nix Illustration

Riojasuchus

Pseudosuchians – the evolutionary lineage whose only surviving modern representatives are crocodilians – first originated in the early Triassic and were once an incredibly diverse group. These croc-relatives experimented with fully erect limbs and bipedalism quite a few separate times, and on several occasions ended up evolving remarkably similar body plans to their distant cousins the theropod dinosaurs.

One of the earliest branches of the pseudosuchians to do this were the ornithosuchids, the best known of which is Riojasuchus tenuisceps here.

Living in Argentina during the Late Triassic, about 217-215 million years ago, Riojasuchus had a distinctive “hooked” upper jaw and two rows of osteoderm armor plates along its back.

It was only around 1.5m long (4’9″), much smaller than some of the other pseudosuchians and early theropod dinosaurs it lived alongside. Its front limbs were shorter than its hind limbs and it was probably a facultative biped – moving slowly on all fours, but getting up on just its hind legs for bursts of high speed running – which would have helped it avoid being eaten by those larger predators.

Like other ornithosuchids it had very strange ankles, with the bones in the joint articulating with each other the opposite way around compared to any other type of archosaur. The claws on its hind feet were also unusually tall and narrow, especially on the inner toes.

Its jaws were capable of delivering strong but somewhat slow bites, and the relative structural weakness of its narrow notched jaw would have made it difficult for it to deal with large struggling prey. It likely mostly hunted smaller vertebrates, and may also have been an opportunistic scavenger taking bites out of larger predators’ kills whenever it got the chance.

Island Weirdness #39 — Mekosuchus inexpectatus

Along with their weird giant birds, the islands of New Caledonia were also home to a small crocodilian unlike any alive today.

It was one of the last known members of a lineage of crocodiles known as the mekosuchines, which originated in Australia during the early Eocene about 50 million years ago and later island-hopped out into the South Pacific — mostly around the Coral Sea, but with some making it as far as New Zealand. By the start of the Holocene, about 12,000 years ago, they’d already declined and disappeared from the vast majority of their range, with only a few isolated island species remaining.

Named Mekosuchus inexpectatus, the New Caledonian mekosuchine was only about 2m long (6’6″), just slightly bigger than the modern dwarf crocodile. It was much more terrestrial than living crocs, spending most of its time on land, and it had teeth in the back of its jaws that were specialized for crushing, suggesting it mainly preyed on hard-shelled invertebrates such as snails and crabs.

Based on its limbs anatomy it may also have been able to climb trees. Although this idea was ridiculed when it was originally suggested back in the 1990s, a more recent discovery has shown that modern crocs can actually climb trees too.

Like with many other Holocene island species, the extinction of Mekosuchus inexpectatus seems to be directly linked to the arrival of humans, who reached New Caledonia around 1500 BCE.

Bones in archaeological kitchen waste sites show that the settlers actively hunted and ate Mekosuchus, but dating of the last known remains is uncertain. The most generous estimate is actually as recent as about 300 CE, so much like Sylviornis these small land crocs may have persisted for some time before finally going extinct.

Tarsomordeo

While modern crocodilians are all semi-aquatic, their Mesozoic ancestors (known as neosuchians) started off fully terrestrial, only really moving into their familiar water-based ecological niches around the mid-Jurassic when the dinosaurs were dominating on land.

But on multiple occasions members of the neosuchian croc lineage independently went back to fully terrestrial habits, and Tarsomordeo winkleri here is one of the most recently discovered examples.

Living about 113 million years ago in the Early Cretaceous of central Texas, USA, Tarsomordeo was surprisingly small, only about 60cm long (2′) – the size of an average cat. Its tiny size even ended up inspiring its name, which translates to “ankle biter”.

It had long slender limbs held in an upright posture, suggesting it was a swift and agile runner capable of chasing after fast-moving prey. Since it lived in a semi-arid environment that seems to have been a major nesting site for the herbivorous Convolosaurus, their hatchlings probably also made up a large part of its diet during the breeding season.

Voay

Did you know some crocodiles have “horns”?

Formed from the corners of the squamosal bone at the back of their skulls, just above their ears, these structures are seen in living crocs like the Cuban crocodile and the Siamese crocodile, as well as some fossil species.

But perhaps the most impressively-horned croc was Voay robustus here.

Voay lived on the island of Madagascar during the Late Pleistocene and Holocene, between about 100,000 and 2,000 years ago. At about 5m long (16′5″) it was similar in size and build to a large male Nile crocodile – but despite this resemblance its closest living relative is actually the much smaller dwarf crocodile.

It had a fairly short and deep snout and chunky limbs, adaptations associated with a more terrestrial lifestyle that suggest it was specialized for hunting its prey on land rather than just at the water’s edge.

Much like modern horned crocodiles its particularly prominent horns were probably used for territorial displays, and may have been a sexually dimorphic feature with big mature males having the largest examples.

Voay’s disappearance just a couple of thousand years ago may have been the result of the arrival of human settlers on the island, either from being directly hunted or due to the large native species it preyed on also going extinct around the same time.

Almost-Living Fossils Month #22 – Some Marine Crocs

First appearing in the Middle Jurassic, about 175 million years ago, the tethysuchians were a group of neosuchian crocodilians – part of the same lineage that includes all living crocs, although they were probably more closely related to the highly marine thalattosuchians than to modern forms.

Their fossil remains have been found almost globally, except for in Antarctica and Australia, and they appear to have been highly aquatic animals living in both freshwater and marine environments. Most members of the group had very long and slender gharial-like snouts, indicating they were specialized for fish-eating, but some (like the enormous Sarcosuchus) developed broader or shorter snout shapes that suggest more generalized diets of whatever they could catch.

One lineage of tethysuchians known as the dyrosaurids evolved around the mid-Cretaceous (~100-90 mya) and quickly spread around most of the world. These crocs mainly inhabited coastal marine waters, with a few species also living full-time in estuaries or rivers.

They had tall vertebrae around their shoulders, giving them a slightly hump-backed appearance and anchoring large neck muscles that allowed them to quickly whip their jaws around to catch fast-moving fish. Their deep vertically-flattened tails were capable of an even more powerful swimming stroke than those of modern crocs, and their well-muscled limbs probably made them strong walkers when on land.

The dyrosaurids were some of the few marine reptiles to survive through the end-Cretaceous extinction (~66 mya) relatively unscathed, and several species are known from both sides of the K-Pg boundary. This may be because the marine dyrosaurids are thought to have seasonally migrated inland to breed in freshwater environments, with juveniles spending their early lives in rivers and only returning to the coasts as adults – and since freshwater ecosystems were much less affected by the mass extinction than marine ones, this allowed them to continue on while groups like the mosasaurs and plesiosaurs died out.

Dyrosaurus maghribensis here lived during the Late Paleocene and Early Eocene of Morocco (~56-48 mya). It was similar in size to the largest living crocs, around 6m long (19′8″), but had thinner and less extensive bony osteoderm armor.

During the Eocene the dyrosaurids began to disappear, and by the Late Eocene (~37 mya) the last known species were found only in northern Africa. It’s not entirely clear why these once-successful tethysuchians began to decline, but they may have been struggling to deal with the cooling climate trends at the time. If they managed to persist until the end of the Eocene, sudden temperature and sea level drops during the Eocene-Oligocene extinction (~33 mya) probably finished them off entirely.

Almost-Living Fossils Month #10 – Big Land Crocs

First appearing in the Middle Jurassic, about 167 million years ago, the sebecosuchians were a group of terrestrial crocodilians that are known from South America, Europe, North Africa, Madagascar, and India.

They were very closely related to (or possibly descended from) the diverse and often very weird notosuchian crocs. But while notosuchians were generally small and had specialized mammal-like heterodont teeth, the sebecosuchians were much larger (around 3-4m long / 9′10″-13′) and had blade-like serrated teeth convergently similar to those of some theropod dinosaurs. Their teeth were so incredibily dinosaur-like, in fact, that Cenozoic specimens have occasionally been mistaken for evidence of late-surviving non-avian dinos.

With deep narrow snouts, powerful jaws, and upright limbs, these crocs were clearly fast active predators, and must have been directly competing with similarly-sized theropods during the Mesozoic. They were obviously doing well enough to survive alongside their distant dinosaur relatives for many millions of years, right up until the end-Cretaceous extinction – but the surprising part is how the sebecosuchians seemed to survive the extinction just fine across most of their range, while the non-avian theropods obviously didn’t. Something about these particular large terrestrial predators allowed them to pull through relatively unscathed, although whether it was to do with their metabolisms or something else is still unknown.

By the end of the Eocene the sebecosuchians outside South America seem to have died off (coinciding with the rise of placental carnivorans), but the isolated South American forms continued their success for most of the rest of the Cenozoic.

One of the best-known species is Sebecus icaeorhinus from western South America, ranging from Colombia to Patagonia, with various fossils dating from the Early Paleocene all the way through to the mid-Miocene (~66-11 mya). About 3m long (9′10″), it was named after the ancient Egyptian crocodile god Sobek and was one the first sebecosuchians to be discovered, lending its name to the entire group.

The last definite fossils of Sebecus and its relatives come from the mid-Miocene, but they may possibly have survived up until at least the Miocene-Pliocene boundary about 5 million years ago. It’s not clear exactly why these big land crocs finally went extinct, but it was likely due to a combination of factors such as the influx of placental predators from North and Central America, along with climate changes from the continuing rise of the Andes mountains and the formation of the Isthmus of Panama.

Tarjadia

Tarjadia ruthae from the Middle Triassic of Argentina (~242-235 mya).

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.

Skull of *Tarjadia*, from Fig 2 in Ezcurra, M. D., et al (2017). Deep faunistic turnovers preceded the rise of dinosaurs in southwestern Pangaea. *Nature ecology & evolution*, 1(10), 1477. doi: 10.1038/s41559-017-0305-5

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.

Boverisuchus

Boverisuchus magnifrons*, a crocodilian from the early Eocene of Germany (~50-40 mya). Reaching about 3m long (9′10″) it was much more heavily armored than its modern cousins, with an interlocking “exoskeleton” of bony osteoderms covering its body and limbs – leading to it being given the nickname “panzer croc”.

It was adapted for walking and running on land, with relatively long legs and surprisingly hoof-like claws. It may even have carried its weight directly on these hooves similar to mammalian ungulates.

And if that’s not unusual enough, its hind leg musculature suggests it also might have been capable of short bursts of bipedal sprinting.

[ * Originally known as Pristichampsus rollinatii before being reassigned in 2013.]

Pakasuchus

Pakasuchus kapilimai, a notosuchian crocodyliform from the mid-Cretaceous of Tanzania (~105 mya). This 50cm long animal (1′8″) had an elongated body and relatively long limbs, and would have been an active terrestrial predator chasing after fast-moving small prey like insects.

The bony osteoderms on its body were much smaller and sparser than those found on most of its relatives – except for its tail, which was still heavily armored.

It also had some of the most complex teeth of all known crocodilians, with surprisingly mammal-like ‘canines’ and ‘molars’ that gave it the ability to chew its food.

Montealtosuchus

Montealtosuchus arrudacamposi, a crocodyliform from the Late Cretaceous of Brazil (~93-83 mya). About 1.8m in length (6′), it had slightly forward-facing eyes, giving it binocular vision, and long upright limbs – adaptations for active hunting on land.

Basically, it would have looked a little like a “crocodile dog”.