crocodylomorph – 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.

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!

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.

Eons Roundup 7

It’s another PBS Eons commission roundup day!

An illustration of an extinct mammal related to marsupials, climbing on a tree branch. It's an opposum-like animal with protruding fangs and a long furred tail. — Pucadelphys andinus

An illustration of an extinct mammal related to marsupials. It's a mouse-like animal with large ears and a long naked tail. — Pucadelphys andinus

The metatherian mammals Pucadelphysand Khasia, and lineart of the sparassodont Paraborhyaena, from “How South America Made the Marsupials”
https://www.youtube.com/watch?v=l5doyrUWFbE

An illustration of an extinct relative of modern crocodiles. It has a long gharial-like snout, a slightly hunchbacked body, four paddle-like legs, and a long vertically flattened tail. — Acherontisuchus guajiraensis

An illustration of an extinct swimming turtle. It has a very wide and flat carapace, almost circular in shape. — Acherontisuchus guajiraensis

The dyrosaurid crocodyliform Acherontisuchus and the bothremydid turtle Puentemys, from “How a Hot Planet Created the World’s Biggest Snake”
https://www.youtube.com/watch?v=T-hDNbM-WLk

An illustration of an extinct early penguin, in both sitting and swimming poses. It has a long pointed beak and loon-like wings. — Waimanu manneringi

An illustration of an extinct giant penguin. It has a long pointed beak. — Waimanu manneringi

The early penguin Waimanu and the giant penguin Anthropornis, from “When Penguins Went From The Sky To The Sea”
https://www.youtube.com/watch?v=HMArjGQwLvY

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.

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 #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.

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.