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Saurosuchus

Saurosuchus was a 20-foot pseudosuchian (a distant ancestor of crocodiles) which lived in Argentina during the middle Triassic Period about 230 million years ago. It was the largest carnivorous animal in its environment in terms of both length and weight, and it was likely the top predator within the Ischigualasto Formation, but that was before the dinosaurs arrived. Saurosuchus lived side-by-side with the meat-eating dinosaurs, but eventually carnivorous dinosaurs like Herrerasaurus, which were faster, more agile, and possibly more intelligent, drove these creatures to extinction.

Skull of Saurosuchus. Drawn in Crayola black marker. © Jason R. Abdale. March 11, 2021.

 

Fruitachampsa, the crocodile-bear-cat of the Morrison Formation

Meet the Jurassic Period’s analog of the common house cat. This is Fruitachampsa callisoni, a prehistoric reptile which inhabited western North America during the late Jurassic Period. However, this was not a dinosaur. In fact, Fruitachampsa was a distant relative of crocodiles.

The fossils of this animal were discovered by James M. Clark and George Callison near Fruita, Colorado during the middle and late 1970s within the rocks of the Morrison Formation dated to about 150 million years ago (MYA). By the late 1980s, this creature was unofficially known by the name “Fruitachampsa”, but since it had not been officially named or described in any scientific research article, this name could not yet be used. It wasn’t until 2011 that the animal was officially classified under the name Fruitachampsa callisoni, “George Callison’s Crocodile from Fruita”.

Clark, James M. “A new shartegosuchid crocodyliform from the Upper Jurassic Morrison Formation of western Colorado”. Zoological Journal of the Linnean Society, volume 163, issue supplement 1 (December 2011): S152–S172.

Fruitachampsa belonged to a group of reptiles which were related to crocodiles known as the “shartegosuchids”. These reptiles are known from the late Jurassic and early Cretaceous Periods, and all known specimens have been found in North America, Europe, and Asia. Shartegosuchids have distinctive skull features, including:

  1. A lack of anteorbital fenestrae (the hole between the nostril and the eye socket) in the upper jaw.
  2. Within the upper jaw’s palate, the chonae (the holes that connect the nostril to the inside of the mouth) are set within a deep depression in the center of the palate.
  3. The palatal bones, which form most of the inside of the mouth of the upper jaw, are joined together medially.
  4. The teeth in the lower jaw never extend posteriorly past the mandibular fenestrae.
  5. The edges of the teeth in both the upper and lower jaws are ridged with serrations – quite unlike the smooth cone-shaped teeth that are often associated with crocodilians.

The shartegosuchids are visibly similar to earlier primitive crocodyliforms such as Protosuchus, and have even been ascribed to the same family as that genus. However, they appear to be slightly more advanced than Protosuchus and other members of Protosuchidae, and may represent the next evolutionary development of crocodilians.

Fruitachampsa measured three feet long, and its body was more-or-less about the same size as a cat. Like a cat, it also had large eyes, and was therefore possibly nocturnal, preying upon the small rodent-like mammals which inhabited the Morrison Formation.

Fruitachampsa also possessed unusually long legs in proportion with the rest of its body. However, like a crocodile, it walked in a “plantigrade” style, walking on the flats of its feet like a human or a bear, rather than walking “digitigrade”, on its toes, like a cat. So perhaps we should think of Fruitachampsa less like a cat and more like a pygmy-sized long-legged bear.

Fruitachampsa possessed a double-row of rectangular osteoderms which ran down the middle of its back, in which the row in front slightly overlapped the row behind, like roof shingles or a ancient Roman legionnaire’s body armor.

Fruitachampsa callisoni. © Jason R. Abdale. December 19, 2020.

Keep your pencils sharp, everybody.

 

Champsosaurus: The Croc-Lizard of the Cretaceous

When most people hear the words “aquatic reptile”, they usually think of two things: turtles and crocodilians. Some clever people might mention sea snakes, and others might mention marine iguanas. Those who are keen on impressing you may bring up some obscure species like the water monitor, the basilisk lizard, and other species of snakes which venture into water.

In prehistoric times, the list of options that you could choose from was much more expansive. In fact, there were animals around then which aren’t around today which fit into this category. One such group of prehistoric water-going reptiles was known as the “choristoderans” (pronounced as Kore-RISS-toe-DEER-rans).

The choristoderans were a group of semi-aquatic reptiles which lived during the Mesozoic Era. Although not as well-known as other non-dinosaurian reptiles of the Mesozoic such as pterosaurs and ichthyosaurs, they nevertheless shared their environments with dinosaurs for a span of approximately 110 million years and even survived the dinosaur extinction. Choristoderans first appeared during the middle of the Jurassic Period about 175 MYA. The oldest-known genus which is recognizably a choristoderan was Cteniogenys, which measured just one and a half feet long and was very lizard-like in appearance. In life, it probably resembled a small monitor lizard and it likely filled a similar ecological niche. However, the heyday for the choristoderans occurred during the early Cretaceous Period from about 144 to 100 MYA, after which they went into decline. They were fortunate to survive the K-T Extinction, but they were always second fiddle to their crocodile neighbors. Most of the surviving species went extinct about 50 MYA, with the remainder just barely hanging on. The last of the choristoderans completely went extinct around 20 MYA.

The choristoderans belonged to a group of vertebrates called the “diapsids”, meaning that they had two holes in their skull behind each eye socket. Lizards, snakes, crocodilians, pterosaurs, dinosaurs, and birds are all classified as diapsids.

At first glance, choristoderans might be mistaken for crocodiles. However, despite their crocodile-like appearance, they are more closely related to lizards than to crocodiles, at least according to a study made by Mike Lee in 2013 (“Turtle origins: insights from phylogenetic retrofitting and molecular scaffolds”). Their placement in the reptile tree is primarily based upon the structure and arrangement of their ear bones, which is more advanced than those seen in lizards but not as advanced as those seen in crocodilians and birds. Also, the skulls of choristoderans are structurally more lizard-like than crocodilian.

The order Choristodera is divided into four families: Champsosauridae, Hyphalosauridae, Monjurosuchidae, and Simoedosauridae. The more primitive the species, the more lizard-like it is in form. The more derived, then the more crocodilian it is in appearance. The most primitive choristoderans were the monjurosuchids, which looked similar to the modern-day Water Monitor Lizard (Varanus salvator). Even at this early stage in their development, there is fossil evidence that some species like Monjurosuchus possessed webbed fingers and toes. Already, they were adapted to living a semi-aquatic lifestyle.


Skeleton of Monjurosuchus splendens, a primitive choristoderan from China. Photograph by Jonathan Chen (June 13, 2019). Creative Commons Attribution-Share Alike 4.0 International license. https://commons.wikimedia.org/wiki/File:Monjurosuchus-Beijing_Museum_of_Natural_History.jpg

Even more advanced were the hyphalosaurids, which bear a remarkable resemblance to the earlier nothosaurs and thalattosaurs of the Triassic Period. Form tends to follow function in evolution, and these creatures almost certainly led a similar lifestyle. The act of species from completely different groups evolving into more-or-less the same shape is called “convergent evolution”.

The champsosaurids and the simoedosaurids are the most crocodile-like in appearance, and together they form the super-family Neochoristodera. Like crocodiles, these creatures were almost certainly living as shallow-water ambush predators, fitted with long slender jaws lined with small conical teeth. Like modern-day gharials, they may have been primarily or even exclusively fish-eaters.

Probably the most famous choristoderan genus was Champsosaurus (pronounced as CHAMP-so-SORE-us). It first appeared about 90 MYA during the Turonian Stage of the late Cretaceous Period, persisted through the K-T Extinction, and finally went extinct during the Paleocene Epoch of the Tertiary Period about 56 MYA. Impressive. Most genera don’t last that long.

Champsosaurus was named by the famed paleontologist Edward D. Cope in the year 1877. Despite not having an easily-recognizable name (most members of the general public have likely never heard of it), it has been rigorously studied by paleontologists ever since then. For example, three academic articles were published about it just in the year 2010, and another article was recently published in April 2020. So, from an academic standpoint, interest in this animal has been pretty consistent.

There are seven species which have been ascribed to the genus Champsosaurus. Most of them measured 5 feet long or thereabouts, but the largest, which was appropriately named Champsosaurus gigas, reached 10 feet long. Most Champsosaurus fossils have been found in south-central Canada and the north-central United States within rocks dated to the late Cretaceous Period from 90 to 66 MYA, but a few have also been found in Belgium and northern France in rocks dated to the Tertiary Period.

Champsosaurus skeleton from Montana, USA on display in the Royal Ontario Museum. Photograph by Daderot (November 21, 2011). Public domain image, Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Champsosaurus_sp.,_Montana,_USA,_Late_Cretaceous_-_Royal_Ontario_Museum_-_DSC00088.JPG.

 

Upper jaw of Champsosaurus, above view (left) and underside view (right). The skull’s length measures about 13 inches. Illustration by Samuel W. Williston. From The Osteology of the Reptiles (1925). Public domain image, Wikimedia Commons. https://commons.wikimedia.org/wiki/File:The_Osteology_of_the_Reptiles_p76.png.

Champsosaurus appears to have been able to tolerate both freshwater and saltwater environments. Fossils of a species called Champsosaurus laramiensis have been found in rocks from the Fox Hills Formation, a geological layer which represents a coastal or estuary environment on the edge of the Western Interior Sea. Fossils of mosasaurs and dinosaurs including Tyrannosaurus have also been found in these rocks.

Preserved skin impressions show that, unlike many lizards, choristoderans like Champsosaurus did NOT have overlapping scales. Instead, the skin consisted of tiny non-overlapping scales, with no crocodile-like dorsal scutes, giving it a very smooth-skinned appearance when seen from a distance.

Unlike crocodiles, which have their nostrils on the top of their upper jaw, Champsosaurus had its nostrils on the front tip of its upper jaw. Perhaps they would use their long nose like a snorkel, sticking just the tip out of the water’s surface in order to stay as concealed as possible.

Champsosaurus had a pair of long thin gharial-like jaws lined with tiny conical teeth. Because of its close affinity towards lizards than to crocodiles, it is highly likely that Champsosaurus had lips and a fully enclosed mouth. But that’s just speculation based upon phylogenic relationships to other reptiles. In terms of hard physical evidence, the teeth themselves are quite small, and are inset from the edge of the jawline rather than standing on the rim of the jaw like a crocodile. This suggests that Champsosaurus had lips covering its teeth like a lizard, unlike crocodiles which don’t have lips.

Compared with crocodilians, the eye sockets of choristoderans are positioned much further forwards on the skull, located halfway or two-thirds of the way back from the tip of the snout. This provides more space for jaw muscles, and the temporal fenestrae (the holes in the back of the skull that accommodate the jaw muscles) were very large in proportion with skull size. Champsosaurus, in particular, had very large temporal fenestrae, which indicates that it had strong jaw muscles and could quickly snap its mouth shut within a fraction of a second – an important adaptation if your diet consists primarily of small fish.

Unlike lizards, Champsosaurus might not have had external ears. Analysis of its skull structure shows that Champsosaurus had internal ears, similar to turtles. This is an important adaptation if you are spending much of your life in the water. Therefore, you would not have seen a pair of ear holes on a Champsosaurus head. Instead, there likely just would have been a slight depression (or maybe not even that) on the side of the head marking where the tympanum (the part of the ear that vibrates in order to make a sound) would have been.

If you spend much of your life in the water, walking really isn’t an issue. Therefore, the limbs of choristoderans are not well-developed. In fact, the more “advanced” the species, the weaker its limb bones appear to be. Champsosaurus is no exception to this – its legs are downright puny in comparison with its body. The bones that make up the arms and legs are short and stumpy, and the hands and feet are small, although the feet are noticeably bigger than the hands. The fingers and toes are thin and end with very tiny claws. This was an animal that would have had a hard time pushing itself onto land. However, there is some evidence that females had more robustly-built limbs than the males due to the need to haul themselves onto land in order to lay their eggs.

The tail of Champsosaurus was flattened, and looked more like that of a crocodile or even a mosasaur than to a lizard. Even so, this animal was definitely not a power-swimmer. If it was, then one would expect the tail to be both longer and broader. Instead, the tail seems to be peculiarly under-developed. Keep in mind, though, that this was likely not an animal that was actively chasing after its prey. If all it was doing was hunkering down on the bottom of a lake or river and waiting motionless for fish to carelessly swim by, then it doesn’t need a well-built tail that’s designed for plowing through the water.

Skeleton of Champsosaurus laramiensis. From “The Osteology of Champsosaurus”, by Barnum Brown (1905). Memoirs of the American Museum of Natural History, volume 9, part 1. Public domain image. http://commons.wikimedia.org/wiki/File:Large_williston_champsosaurus.jpg.

Below is a drawing made of Champsosaurus laramiensis drifting about in a murky pond or stream somewhere in Montana during the late Cretaceous Period. This five-foot-long piscivore would have shared this environment with alligators, crocodiles, turtles, large freshwater fish like gars, sturgeons, and bowfins, and of course dinosaurs like Triceratops and Tyrannosaurus. The drawing was made with No.2 pencil on printer paper.

Anyways, keep your pencils sharp.

Hallopus

This is Hallopus victor, a 3-foot long reptile that lived in the Morrison Formation of western North America during the late Jurassic Period approximately 150 million years ago. Hallopus belonged to a group of reptiles called the “sphenosuchians” a group that was closely related to crocodiles. Another example of a Jurassic sphenosuchian is the 5-foot-long Macelognathus, which you can read about here.

Hallopus is only known from fragmentary remains. However, we know that it had a thin build and it also had unusually long back legs. This suggests that Hallopus was a physically active predator that ran after its prey, and would have been capable of running on only their hind legs like a theropod dinosaur. Hallopus had a narrower pointier skull compared to its much larger relative Macelognathus, and would have looked similar in many respects to Terrestrisuchus, a 3-foot-long sphenosuchian from Europe. However, like Macelognathus, Hallopus had no teeth in the front end of its lower jaw; Terrestrisuchus, by contrast, had a full set of teeth. Thus suggests that Hallopus may have been an egg-eater.

Sphenosuchians like Hallopus, Macelognathus, and Terrestrisuchus have traditionally been depicted in paleo-art as being quadrupedal. However, many of the renditions of these animals walking quadrupedally look downright awkward. The back is shown as being strongly arched to the point where it looks as if its spine is being curved beyond its natural state, the joint between the cervical vertebrae and the dorsal vertebrae is cranked into a very uncomfortable position, and due to its long back legs, its rear end is so high up in the air that it almost looks as if its “presenting” to its mate.

Below is a drawing depicting Hallopus in the traditional quadrupedal manner. You can see some of the inherent problems in reconstructing an animal like this.

It’s more likely that these long-legged crocodile relatives were bipedal. As to how often they were bipedal, that’s a subject for debate. An “obligate biped” means that it was bipedal all or most of the time; moving on two legs was its default setting. A “facultative biped” means that it was usually quadrupedal, but it could rear up on its back legs if it wanted to. So, was Hallopus an obligate biped or a facultative biped? With its unusually long back legs, I am more than inclined to believe that Hallopus spent most of its time on two legs.

Below is a revised drawing showing Hallopus standing upright on two legs. This is probably the more accurate way of depicting this animal.

I hope that you found this interesting, and I also hope that it leads you to challenge the common traditional images that have dominated aspects of paleo-art for so long. Keep your pencils sharp, everyone.

 

Macelognathus

This is Macelognathus vagans. This animal lived in the Morrison Formation of western North America during the late Jurassic Period approximately 150 million years ago.

Macelognathus belongs to a group of animals called the “sphenosuchians”, which is a group that’s very closely related to crocodiles. They even possessed a double-row of crocodilian-like scutes running down the middle of their back. However, unlike crocodiles, sphenosuchians were 100% terrestrial animals, and likely occupied the same ecological niche that foxes and wildcats do today. Most sphenosuchians were small, measuring 3 feet long or less, but Macelognathus was unusually large, measuring 5 feet long (this size measurement is only an estimate, since a complete skeleton has never been found). The anatomy of sphenosuchians strongly suggests that they were physically active animals that were built for the chase. Many species possessed a long lithe body, short front legs and long back legs, and a long thin tail. Sphenosuchians were possibly “obligate bipeds”, meaning that they always walked around only on two legs, but they were almost certainly “facultative bipeds”, meaning that they could sometimes move on two legs if they wanted to.

Macelognathus is known from only fragmentary remains, including the front half of its lower jaw. What is most distinctive about this animal is that the front of its lower jaw was toothless, forming a flat palate.  So far, we have not found an upper jaw, so we don’t know if the front of both the upper and lower jaws were toothless, but it seems highly probable. Based upon its jaw structure, it’s likely that Macelognathus was a creature that had a particular preference for eating eggs. The image that comes to mind of this animal, therefore, is a nest-raider, a scavenger, or an ambush predator. I liken it as a Jurassic analog of a large monitor lizard.

There is some evidence which suggests that Macelognathus and another sphenosuchian named Hallopus are in fact the same animal. However, since both Macelognathus and Hallopus are known from only fragmentary remains, a definite answer cannot be given until more remains of both species are discovered and can be compared.

The drawing below was made with No. 2 pencil on printer paper.

Disclaimer: Even though the back legs are noticeably longer than the front legs, it’s still possible that I made the back legs too short in this drawing.

Keep your pencils sharp, everyone.

Some Quickie Drawings of Late Triassic Life

Hi everybody. As many of you already know, I occasionally volunteer at the Garvies Point Museum in Nassau County, New York. One day, I decided to hash out some drawings of Late Triassic creatures when I had a few moments of spare time, and I stuck them on the wall over the bulletin board. Recently, I went back to the museum for their annual Native American Feast, and to tell you the truth, I had completely forgotten about these pictures. I decided to take some photos of them while I was there. I’m hoping that the museum staff uses them for coloring activities with the children that visit the museum every week.