croc-line archosaur – Nix Illustration

Rhacheosaurus

Metriorhynchids were a group of fully marine crocodyliforms known from the mid-Jurassic to the early Cretaceous of Europe and the Americas. They were the most aquatic-adapted of all known archosaurs, with streamlined bodies, smooth scaleless skin, small front flippers, larger hind flippers, and shark-like tail flukes. They may also have been endothermic, and might even have given live birth at sea rather than laying eggs.

Rhacheosaurus gracilis here was a metriorhynchid that lived in warm shallow waters around what is now Germany during the late Jurassic, about 150 million years ago. Around 1.5m long (~5′), its long narrow snout lined with delicate pointed teeth suggests it fed on small soft-bodied prey, a niche partitioning specialization that allowed it to coexist with several other metriorhynchid species in the same habitat.

Unlike most other marine reptiles metriorhynchids didn’t have particularly retracted nostrils, which may have had a limiting effect on their efficiency as sustained swimmers since higher-set nostrils make it much easier to breathe without having to lift the whole head above the surface. The lack of such an adaptation in this group may be due to their ancestors having a single nasal opening formed entirely within the premaxilla bones at the tip of the snout, uniquely limiting how far it could easily shift backwards – other marine reptiles had nostrils bound by the edges of multiple different bones, giving them much more flexibility to move the openings around.

(By the early Cretaceous a close relative of Rhacheosaurus did actually evolve nostrils bound by both the premaxilla and the maxilla, and appeared to have started more significant retraction, but unfortunately this only happened shortly before the group’s extinction.)

Metriorhynchids also had well-developed salt glands in front of their eyes, but the large sinuses that accommodated these glands may have made their skulls ill-suited to deep diving, being more susceptible to serious damage from pressure changes and restricting their swimming to near-surface waters only.

Preserved skin impressions in some metriorhynchid fossils show several unusual “irregularities”, including curl shapes, small bumps, and cratering. It’s unknown what exactly caused these marks, but they may represent scarring from external parasites such as lampreys and barnacles.

It Came From The Wastebasket #15: Rauisuchian Revolution

Pseudosuchians, or “croc-line archosaurs”, are one of the two major lineages of archosaur reptiles, alongside the avemetatarsalians (pterosaurs and dinosaurs). Although today they’re represented only by crocodilians, they were especially successful and diverse back in the Triassic – and it was only after a mass extinction took out most of them that the dinosaurs were able to rise to prominence for the rest of the Mesozoic Era.

A grouping of pseudosuchians traditionally known as “rauisuchians” had upright limbs in a distinctive “pillar-erect” hip arrangement. Many of these croc-relatives were large quadrupedal predators, but others developed bipedal theropod-like postures, with some so remarkably convergent that they were initially misidentified as ornithomimosaurs.

The first rauisuchians were discovered in the 1930s, represented only by fragmentary remains, and while they were initially recognized as being pseudosuchians their exact evolutionary relationships within that group were poorly understood for a long time. Over the next several decades they were classified with aetosaurs (early armored pseudosuchians), then ornithosuchids (even earlier pseudosuchians), and then erythrosuchids (not even pseudosuchians but an earlier type of archosauriform).

More complete fossil discoveries and better cladistic analysis methods in the 1980s led to them being classified as being very closely related to crocodylomorphs, with three main lineages recognized: the prestosuchids, the rauisuchids, and the poposauroids.

An illustration of three different "rauisuchians", extinct relatives of modern crocodiles. At the top is Prestosuchus, a quadrupedal reptile with a boxy theropod-dinosaur-like head, platigrade bear-like legs, and a long tapering tail. It's colored yellow-brown with lighter underbelly and cat-like darker spots-and-stripes. In the middle of Postosuchus, a bipedal reptile that convergently resembles a tyrannosaur, with a boxy head, small arms, plantigrade legs, and a long counterbalancing tail. it's colored brown on top and white underneath, with irregular splotches of black and white across its body. At the bottom is Effigia, a bipedal reptile that convergently resembles a featherless ornithomimosaur, with a beaked bird-like head, a long neck, small arms, bird-like legs, and a counterbalancing tail. It's dark-colored with faint reddish and yellowish stripes and a paler underside. — The “prestosuchid” *Prestosuchus chiniquensis*, the rauisuchid *Postosuchus kirkpatricki*, & the poposauroid *Effigia okeeffeae* (not to scale)

But even by the end of the 20^th century “Rauisuchia” had never actually gotten a formal definition, and it had very much become a wastebasket taxon for a variety of paracrocodylomorph pseudosuchians that didn’t easily fit into any other major lineages.

In the 2000s renewed interest in rauisuchians’ anatomy and evolutionary relationships led to increasing recognition that they weren’t even a single defined group, with various species instead falling into different points along an “evolutionary grade“. The poposauroids and rauisuchids still seem to be distinct lineages, but the “prestosuchids” were found to be polyphyletic, with some forming a grade between the other two “rauisuchid” groups and others turning out to not even be paracrocodylomorphs.

A cladogram showing the classification of poposauroids, Prestosuchus, and rauisuchids within the group Pseudosuchia. They're shown as three separate lineages branching off between aetosaurs and the ancestors of modern crocodilians. A bracket marking indicates that all three traditionally used to be classified as "rauisuchians".

And although the taxonomic concept of “Rauisuchia” as a distinct group has now been abandoned, the term “rauisuchians” does still remain in common use as an informal name for these animals – probably because it’s much more concise than saying “non-crocodylomorph paracrocodylomorphs”.

Eons Roundup 12 (& Published Art!)

It’s been a while since I last showed off some of these, but here’s some more commission work I’ve done for PBS Eons:

The metriorhynchid marine crocodilians Aggiosaurus and Cricosaurus, from “When Crocs Thrived in the Seas”
https://www.youtube.com/watch?v=vgqs_9BBX10

And… what’s this?

A familiar Scutellosaurus makes an appearance in a recently-published children’s dinosaur book!

Eons Roundup 8

Once again it’s a PBS Eons commission roundup day!

An illustration of an extinct baleen whale. It's looks very similar to modern rorquals, with pleated throat grooves. — Cerro Ballena rorqual

An illustration of an extinct seal. it has an unusually long neck. — Cerro Ballena rorqual

An unnamed Cerro Ballena rorqual whale and the long-necked seal Acrophoca, from “How the Andes Mountains Might Have Killed a Bunch of Whales”
https://www.youtube.com/watch?v=iNk6r5WljGc

An illustration of an extinct pseudosuchian, a relative of modern crocodiles. It resembles a dinosaur much more than a croc, standing bipedally, with a long neck, tiny arms, and a long counter-balancing tail. — Shuvosaurus inexpectatus

A skeletal diagram of an extinct pseudosuchian, a relative of modern crocodiles. It resembles a dinosaur much more than a croc, standing bipedally, with a long neck, tiny arms, and a long counter-balancing tail. — Shuvosaurus inexpectatus

The poposauroid pseudosuchians Shuvosaurus (life restoration) and Effigia (skeletal) from “When Dinosaur Look-Alikes Ruled the Earth”
https://www.youtube.com/watch?v=QsmV34Co32c

Ceratosuchus

“Horns” seem to have convergently evolved multiple times in crocodiles over the last few million years, including in a couple of living species. These triangular crests are formed from the squamosal bone, just above their ears, and tend to be a sexually dimorphic feature used in territorial displays between males, serving to make them look bigger when they arch their necks.

But there’s another horned crocodilian known from much earlier in the Cenozoic – and this one was an alligator!

Ceratosuchus burdoshi lived in Colorado and Wyoming in the western United States during the late Paleocene and early Eocene, about 57-56 million years ago. It was a fairly small alligator, around 1.7m long (5’6″), with a broad snout featuring sharp teeth at the front and blunter teeth further back – an arrangement that suggests it was a generalist predator eating a variety of small prey, using those teeth to first grab and then crush whatever it managed to catch.

It also had large blade-like osteoderm armor on the back of its neck, which may have been arranged in line with its “horns” to make its visual displays look even spikier.

Weird Heads Month #28: Pig-Nosed Tanks

There’s already been quite a few Triassic weirdos in this series, so it’s probably not much of a surprise that we’ve got one more before the end of the month.

Desmatosuchus spurensis here was part of a group called aetosaurs, a lineage of heavily-armored herbivorous archosaurs which convergently resembled the later ankylosaurs but were more closely related to modern crocodilians.

Living in the Southwestern and South Central United States during the late Triassic, about 221-210 million years ago, Desmatosuchus measured around 4.5m long (14’9″) and was covered in thick interlocking bony osteoderms that protected its back, sides, belly, and tail, with longer spines over its neck and shoulders.

It had a triangular skull with a few blunt teeth at the back of its jaws and a toothless snout at the front. Its pointed lower jaw probably had a keratinous beak, while its upper jaw had an odd upturned flared tip. What exactly was going on with that snoot is uncertain, but it may have anchored a shovel-shaped upper keratinous beak – or, since there was a little bit of flexibility between its snout bones, possibly even a pig-like nose!

It probably mostly ate soft vegetation, using its shovel-like snout to dig up roots and tubers, although similarities with the skulls of modern armadillos suggest it may also have fed on insect grubs.

Weird Heads Month #22: Flat Headed Crocs

The heads of modern crocodilians are already pretty amazing, with their high-set eyes and nostrils, moveable ear flaps, numerous dermal pressure receptors, and a distinctive chaotic “scaly” surface texture that’s actually formed from cracks in thick stiff skin.

And back during the Late Cretaceous of West Africa, about 95 million years ago, there was a huge variety of odd-looking crocdyliformes all sharing a river delta environment and specializing in different ecological niches from terrestrial to aquatic. There were species with nicknames like “duck croc“, “boar croc“, and “pancake croc” – but one of the most intriguing of them all was Aegisuchus witmeri, the “shieldcroc”.

Known only from the back end of its skull, Aegisuchus seems to have had a very wide and flat head, possibly similar in shape to those of the “pancake crocs” which it may have been closely related to. From the sheer size of the known remains it must have been rather big, with a skull at least 2m long (6’6″) and a total length of around 10m (32’10”).

But its weirdest feature was a raised circular bony boss in the middle of its forehead. Unlike any other known croc, the bone around this area shows evidence of deep blood vessel channels, suggesting it was anchoring a more extensive keratinous “shield”. Much like the “horns” seen on some crocodilian species this was probably used for territorial and mating displays, but its extensive blood supply may have also allowed it to play a role in body temperature regulation.

Aegisuchus would have had a fairly weak bite, and may have fed more like a pelican than a modern croc, snapping up fish and other small animals with its gaping mouth. Its jaw mechanics also resembled those of the Triassic amphibian Gerrothorax’s “toilet seat head”, so it could have had a similar hunting strategy, laying motionless on riverbeds with its mouth wide open, waiting for prey to swim close enough to catch.

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.

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.

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.