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 strangle 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.

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.