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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.


More photos of Allosaurus from the AMNH

Greetings friends. In an earlier post from 2014, I put up some photographs which I took of the two Allosaurus skeletons that are on public display in the American Museum of Natural History (or AMNH for short) in New York City. I’ve recently uncovered some other photos which I took during a visit there in March 2019, and so I’m posting them here. Enjoy!




This is Caturus, a prehistoric fish which swam in the oceans during the Mesozoic Era. Fossils of this saltwater fish have been found in North America, Europe, northern Africa, and as far as China within rocks spanning from the beginning of the Triassic Period about 250 MYA up to the middle of the Cretaceous Period, about 100 MYA. However, most fossils have been found in Europe in rock layers dated to the middle and late Jurassic Period, about 170-150 MYA.

Despite a superficial resemblance to a salmon, Caturus was actually more closely related to a bowfin (Amia calva), which is a rather primitive ray-finned fish.

So far, paleontologists have identified fourteen species of Caturus. The largest species, Caturus furcatus, which lived in the shallow sea that covered much of Europe during the late Jurassic period about 150 MYA, reached three feet long; other species were much smaller. One species, Caturus dartoni, is known from North America in rocks dated to the middle Jurassic, about 165 MYA. Only two skeletons of this particular species have been found, the largest measuring 15 inches long.

Caturus. © Jason R. Abdale. September 5, 2020.

This drawing was made on printer paper with No.2 pencil, No.3 pencil, Crayola colored pencils, Prismacolor colored pencils, and Artist’s Loft colored pencils.


Hello everyone. Several years ago, I wrote a short article for Prehistoric Times magazine about the Sundance Sea. This was a shallow saltwater sea which covered much of central North America during the middle to late Jurassic Period. One of the illustrations that I made which accompanied that article was a drawing of a pair of plesiosaurs chasing after a school of belemnites. The plesiosaurs in question were called Pantosaurus. This species was a member of the “cryptoclidid” family of plesiosaurs, and measured about 20 feet long.

I thought that I had posted that drawing onto this website at that time, but looking through my portfolio, it appears that I neglected to do so. Well, better late than never! Unlike many of my other drawings, which are mostly made with No.2 pencil or colored pencils, this one was made with a black ballpoint pen and a black marker.

Pantosaurus with belemnites. © Jason R. Abdale. November 18, 2014.


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This is Baptanodon, an ichthyosaur which lived during the middle and late parts of the Jurassic Period about 165-155 million years ago. During this time, the central part of North America was covered with a body of saltwater known as the Sundance Sea, and Baptanodon was one of the creatures that swam in this inland ocean. It measured 20 feet long, it had freakishly huge eyes, and, as far as I have been able to learn so far, it had small teeth only in the front half of its mouth while the rear half was completely toothless. The presence of grooves running along the sides of its jaws indicate that it probably had lips and the teeth would not have been visible when the mouth was closed.

Baptanodon was closely related to the European ichthyosaur Ophthalmosaurus. In fact, for a while it was believed that Ophthalmosaurus and Baptanodon might be the same animal. However, phylogenic studies indicate that they are indeed separate.

Baptanodon shared its habitat with numerous other forms of marine life including oysters, ammonites, belemnites, hybodont sharks, as well as the 20 foot long plesiosaur Pantosaurus and the 25 foot long pliosaur Megalneusaurus.

This drawing was made on printer paper with No.2 pencil, Crayola colored pencils, and Prismacolor colored pencils.

Baptanodon. © Jason R. Abdale. August 12, 2020.



There are over a thousand species of dinosaurs that are known to science today. Kids, it seems, are more disposed to remember these names than adults, and I have encountered several examples of children trying to impress people by rattling off as many dinosaur names as possible. In fact, it embarrasses me to state that I used to be one of these pint-sized paleontological know-it-alls. Of all of these names, there are about twenty or so that nearly everybody knows straight off the top of their heads, and Stegosaurus is unquestionably one of them.

Stegosaurus is one of the most well-known and easily-recognized dinosaurs out there. It is the definitive Jurassic armored plant-eater that everybody knows and loves. It has been consistently featured in nearly every children’s dinosaur book going back as far as the 1950s and it is a favorite subject of paleo-artists. Ask practically anybody what a Stegosaurus is, and they can describe what one looks like for you: four legs, plates on its back, spikes on its tail, and a brain the size of a walnut.

However, there are a lot of misconceptions about this iconic Jurassic armored tank, not only regarding its intelligence but also its appearance. Paleo-artists have regularly portrayed Stegosaurus as a massive hulking brute, but new science suggests that this animal was much slimmer and elegant than how it’s commonly portrayed.



The first Stegosaurus fossils were discovered in Colorado during the 1870s as part of the “Bone Wars”, an intense scientific feud between Prof. Edward D. Cope of the Philadelphia Academy of Natural Sciences and Prof. Othniel Charles Marsh of Yale University in their quest to become THE paleontologist of the late 19th Century by discovering and naming more species than the other guy. When the fossils were first uncovered, Marsh looked at the large back plates and thought that they were pieces of an enormous turtle shell; it wasn’t until later that he realized that they actually came from a dinosaur. In 1877, the fossils were officially given the name Stegosaurus armatus “the armed roof lizard”, because the back plates reminded Marsh of roof shingles.

As the Bone Wars continued, more specimens of Stegosaurus were discovered. However, O. C. Marsh was not working with complete specimens – only with partial skeletons or fragments. Therefore, whenever he found a specimen that did not look EXACTLY like something that he had already seen, he automatically assumed that it was a different species. Consequently, numerous species were ascribed to Stegosaurus such as S. armatus, S. affinis, S. duplex, S. laticeps, S. sulcatus, S. ungulatus, and probably the most well-known of all of them, S. stenops. A few of these were later determined to by synonymous. However, after a long and thorough examination of the finds, it appears that there were indeed three or maybe four distinct species.

Of all of the species that have been named, Stegosaurus stenops is probably the most widely recognized simply because more skeletons have been found of this particular species than any other. Stegosaurus stenops, therefore, might have been the most common species of its genus. However, prehistoric population percentages are extremely difficult to determine because the studies tend to be very subjective rather than objective. There might also be preservation biases in fossilization which would lead to some species being more likely to fossilize than others. The number of fossils, therefore, should not always be automatically correlated to population numbers.

Illustration of the skull of Stegosaurus stenops. Illustration from The Dinosaurs of North America by Othniel Charles Marsh. Washington D.C.: U.S. Government Printing Office, 1896.

Stegosaurus stenops might be the most well-known Stegosaurus species due to the sheer number of fossil specimens that have been found, but it wasn’t the largest member of its kind. Stegosaurus stenops reached about 25 feet long, while another species named Stegosaurus ungulatus was slightly larger, reaching 30 feet long. In fact, S. ungulatus is the largest stegosaur species that we know of in the entire world. However, a stegosaur from Europe named Dacentrurus may challenge that title. The problem is that this animal is known only from partial remains, so its total length is difficult to determine. Most sources that I have seen identify it as a medium-sized animal measuring 15 to 20 feet long, and there are only a handful of finds which hint that it might have grown larger. So, for the time being, S. ungulatus still holds the world record of “biggest stegosaur ever”.



Stegosaurus has been intensely studied ever since its discovery, partly due to its novel appearance. Even so, there are a lot of misconceptions about how this animal looked which have been perpetuated over the years.

Previous restorations have shown Stegosaurus as having a short compressed body with a highly arched back, short front legs, freakishly tall back legs, and a tail that’s substantially shorter than what you would expect. This image has been copied for decades and it has become so engrained into our consciousness that we automatically assume that this is how a Stegosaurus is supposed to look. One of the things that needs to be considered is that this image was completely contrary to the majority of other stegosaur species found elsewhere in Europe, Africa, India, and China, which had longer necks, shorter legs, smoothly-curving backs, and long tails. However, we just assumed that Stegosaurus was weird and didn’t fit with the majority of stegosaur anatomy, until some new discoveries were made in the 2000s.

While a complete specimen of Stegosaurus has never been found, a skeleton of a sub-adult Stegosaurus which was discovered in 2003 in Wyoming helped to substantially change our perceptions of this animal. Named “Sophie”, this 18-foot-long skeleton was 80% complete, making it the most complete Stegosaurus skeleton ever found. It took many years to clean the skeleton up, measure it, and mount it for public display in 2014. When all of the work was done, Sophie had some noteworthy aspects to her anatomy which did not fit with the traditional image, and this compelled scientists to update their reconstructions of how Stegosaurus was supposed to look. The revised image showed this animal as having much shorter back legs, a lower back, a longer stretched-out neck, and a longer tail. The resulting image is much more sinuous and streamlined than the previous image of the brooding bruising hulk that’s been around for ages.

Below is a rough sketch that I had made sometime during the late 2000s of Stegosaurus stenops based upon the information that I had at the time. This shows how Stegosaurus was believed to appear since at least the 1980s, with its conspicuously high-arched hump back, very long rear legs, and a rather short tail.

Now, here is an updated version of how Stegosaurus stenops would have looked based upon our current understanding of this animal’s anatomy. The neck is slightly longer because this creature had more cervical vertebrae than we had previously thought. The back legs aren’t as tall as we once thought they were, and this makes the back much lower and less strongly arched. Finally, the tail is noticeably longer. The resulting image is much more in-line with what we know about other stegosaur species and doesn’t make Stegosaurus appear as freakish as it once was. This drawing was made with No.2 pencil on printer paper and was made in 1:20 scale. From the tip of its nose to the tip of its tail, this drawing measures precisely 15 inches long, which would make it 25 feet long in real life.

Stegosaurus is instantly recognizable due to its back plates and tail spikes. These physical features are, anatomically-speaking, highly transformed osteoderms. The word “osteoderm” literally means “skin bone”, and it refers to any bone object which is embedded within the skin or is visible on the body’s exterior rather than forming a part of the structural skeleton. Technically, a stegosaur’s plates and spikes are osteoderms because they are attached onto the body rather than being incorporated as part of its skeleton.

While the plates and spikes may be the most obvious features to Stegosaurus’ anatomy, there were other, more subtle aspects that provided it with a certain measure of protection. Notably, there existed a series of marble-like osteoderms covering the underside of the neck where the neck connects to the skull and extending backwards for about half of the neck’s length. This almost certainly evolved as a means to protect the carotid artery and jugular vein from being torn open by a predator, yet it’s perplexing that it would only extend halfway down the neck rather than covering the entire neck. This pebbly structure forms the equivalent of a chain-mail pixane, a type of armored throat protector which was worn by Medieval knights. My gracious thanks to Mr. Ian LaSpina for his wonderful video series on Medieval armor which let me know of the existence of such an object. Please check out his website on Medieval clothing, armor, and weapons here or his YouTube channel here.

Medieval armor researcher Ian LaSpina wearing a pixane (also called a pisan or a standard), a chain-mail collar meant to protect the throat. Image courtesy of Ian LaSpina (2014), used with permission.

As stated earlier, Stegosaurus was a genus composed of three or four species, and each of them had a slightly different appearance not only in terms of their overall size but also in their body proportions, including the size and shape of the dorsal plates. The plates of Stegosaurus ungulatus are much smaller and narrower than those of Stegosaurus stenops, and they come to a pronounced sharp point at the tip. By contrast, the plates of Stegosaurus stenops were large, wide-based, and they have somewhat rounded ends.

The number of plates that Stegosaurus possessed is difficult to determine. Various sources give numbers ranging from seventeen to twenty-two plates in total. This probably has to do with the fact that most sources lump all species of Stegosaurus together, not taking into account that different species have different appearances, including different numbers of plates running along the back. It also might be partially to do with the fact that a 100% complete specimen of Stegosaurus has never been found, and therefore we cannot be entirely certain of how many plates it indeed had. The finished drawing of Stegosaurus stenops which you see above has a total of nineteen plates.

One of the topics which has generated a sizeable amount of academic debate is how the plates were arranged on the back. The earliest reconstruction of this animal shows the plates lying down on the back like overlapping fish scales. Some artists depicted this animal as having a double row of plates with the plates arranged in pairs. For much of the late 1800s and into the early 1900s, Stegosaurus was shown with the plates arranged in a single line running down the middle of the back. However, the most common arrangement that you will see nowadays is a double line of staggered alternating plates. How far apart were these two rows from each other? That, also, is a subject of conjecture. Some reconstructions show them butted up against each other along the top of the animal’s spine, forming a V-shape when seen from the front. Other artists put a gap in between the two rows, with the wideness being largely personal interpretation.


Depiction of Stegosaurus ungulatus made in 1896 showing it with eight tail spikes and a single line of back plates. Public domain image, Wikimedia Commons.


Depiction of Stegosaurus ungulatus by Charles Knight (1901) showing it with a double row of paired plates. Public domain image, Wikimedia Commons.


Depiction of Stegosaurus ungulatus by G. E. Roberts (1901) showing it with a double row of alternating plates. Public domain image, Wikimedia Commons.


One of the things that you’ll notice in my drawing is that the plates are non-symmetrical. Not only are they arranged in a staggered formation along the back instead of being arranged in pairs, but also the plates on one side are of a different size and shape to the plates on the opposite side; no two plates on any Stegosaurus’ back look exactly alike to each other.

While some older sources on dinosaurs claimed that Stegosaurus’ plates were used in defense, this idea is false. Defensive armament would be better served if the plates were lying flat upon the body like overlapping armor plates, and being substantially thickened. However, the plates stand erect upon the animal’s back, leaving the sides completely unprotected. The plates are also very thin in cross-section and they would have been easily broken if they were impacted by something. Rather, the plates were almost certainly used for display. The plates extending upwards from the animal’s spine also would have made the animal look far larger than it actually was, likely as a means to deter predators.

In life, the back plates would not have been exposed bone. Instead, they would have been covered with a protective layer of keratin – the same stuff that your fingernails are made out of. Based upon the texture of the plate’s surface, it seems highly probable that these plates were not covered in scaly skin.

In addition to the dorsal plates, another distinctive feature of Stegosaurus are the four spikes on the end of its tail. While there is no official anatomical term for this feature, this weaponized tail is nowadays commonly referred to as a “thagomizer”. The name is based upon a 1982 comic from The Far Side by Gary Larson in which the tail was named in honor of a caveman named Thag Simmons who met his maker by it. Since then, it has gained popularity within the scientific community and is now an unofficial anatomical vocabulary term. It was even referenced in the fourth episode of the 2011 BBC documentary series Planet Dinosaur.

Similar to the academic debate concerning the placement and arrangement of Stegosaurus’ dorsal plates, there has likewise been an argument concerning the placement of the tail spikes. Based upon the shape of the base of these spikes, nearly all people can agree that they were angled backwards, pointing towards the tip of the tail rather than pointing forwards or directly sideways. Unfortunately, there’s not much else that we know about the spikes’ position on the body, and this has led to a lot of varying interpretations over the years. Some reconstructions and paleo-art show the spikes sticking virtually straight up, while others show them positioned outwards horizontally; this latter position has become somewhat trendy recently. However, the vast majority of 2D and 3D reconstructions show the left and right spikes positioned in a V-shape at varying degrees, with the angle being either narrower or wider depending upon the supervising museum curator, fossil preparator, or artist. So far, nobody has been able to definitively say how the tail spikes ought to be positioned. Perhaps the only way in which this debate may be settled is if a mummy is found or if a Stegosaurus specimen is found preserved in three dimensions similar to the infamous “Dueling Dinos” find.

While no skin impressions have been found in association with Stegosaurus fossils, they have been found with a related species called Hesperosaurus. It’s based upon this find that we can make inferences about what the skin of Stegosaurus would have looked like.

In 1985, the remains of a stegosaur skeleton were discovered in north-central Wyoming in rocks dated to approximately 156 MYA, in a rock layer that marks the lowest and oldest layer of the Morrison Formation. Upon careful examination of the skeleton, it was determined that this did not belong to any known species of Stegosaurus, but was instead a previously unknown genus. In 2001, it was named Hesperosaurus, “the western lizard”. Hesperosaurus differed from Stegosaurus in that it was slightly smaller (20 feet long instead of 25-30 feet) and its plates were smaller and a bit more rounded in shape. Hesperosaurus might have been the direct ancestor of the more famous Stegosaurus, but more evidence is needed before this claim can be definitively proven.

In 1995, another stegosaur skeleton was discovered in northern Wyoming in rocks dated to approximately 155-150 MYA. This skeleton was remarkable not only due to the fact that it was nearly complete, but it also contained one spot on its body with preserved skin, located on the animal’s right side in between its front right and back right legs. It wasn’t until September of 2010, fifteen years after the skeleton was discovered, that a description of this specimen was published. It was identified as belonging to Hesperosaurus.

The skin impression from Hesperosaurus consists of small non-overlapping scales which are either round, oval, or polygonal in shape. The further up the back you go, the larger the animal’s scales become, with some of the scales becoming large, oval-shaped, and surrounded by a ring of smaller scales. Most of the body’s hexagonal scales measured 2-7mm in diameter, but the oval scales higher up on the flanks are much larger than that. One rosette measured 8x10mm in area, and another further up on the back measured 10x20mm in area. These larger scales are noticeably more rounded in texture, forming distinctive “lumps”, arranged in rows lengthwise down the body. Technically these are not true osteoderms because they do not have a bony core. Instead, they could be considered as “dermal scutes”, which are nothing more than scales, like other body scales, which just happen to be unusually large and thick compared with other scales on the body. Although it cannot be proven, it’s possible that Stegosaurus had a similar skin texture to its relative Hesperosaurus.


Color Patterns

While skin texture can be speculated upon with a certain degree of accuracy, skin color is something that falls entirely into the realm of guesswork. To date, no preserved pigment cells have been discovered in any stegosaur fossil. Traditionally, Stegosaurus has been depicted as being green with the back plates colored in red, orange, or pink. This color scheme has been around since the 1950s, and it has been copied so many times that many people automatically think of this image whenever they hear the word “Stegosaurus”. This contrasting color scheme of green plus some color on the red end of the spectrum is visually striking and appealing to the eye, and may be the reason why it is so commonly seen to the point of it being considered a “paleo meme” to use Darren Naish’s term. But how probable is it that Stegosaurus was colored in this way? There’s really no way to tell.

Below is a colorized rendition of my updated Stegosaurus drawing showing it garbed in a traditional color scheme consisting of a mottled green with reddish plates.

One argument can be made that Stegosaurus was probably colored in more muted tones given it lived in an environment which was dry and arid for much of the year. Such a color scheme can be seen in Fred Wierum’s artwork in which he gives his Stegosaurus a distinctly desert-themed coloration of tan and brown. Unfortunately, I was not able to gain permission to use his work on this website; you can see his painting here.

Paleo-artist and children’s author Patricia Bujard has also liveried her Stegosaurus in various desert-themed color patterns. Below are a series of Stegosaurus illustrations that she has made dated, left to right: November 9, 2016. August 2, 2017. January 4, 2018. All images © Patricia Bujard. All images are used with permission. Please check out her wonderful website, Pete’s Paleo Petshop, to view more of her lovely illustrations.


It has also been proposed by Patricia Bujard that Stegosaurus, and possibly all stegosaurs, might have been decked out with bright color patterns that are similar to venomous snakes, poison arrow frogs, or poisonous insects. Such colors would loudly advertise that it is a dangerous animal and it would serve as a warning to potential predators to back off. A color scheme which evokes this idea is a painting of Tuojiangosaurus, a stegosaur from China, made by Brian Franczak during either the late 1980s or early 1990s. In this painting, the animal is vividly portrayed in contrasting colors of black and yellowish-orange.

Here is another colorized version of my Stegosaurus drawing portraying it in a much more un-orthodox color scheme of bright black and orange stripes with a bold yellow underside, and with plates that are patterned with red, a black edge, and bright yellow “eye spots” in the center, and with black-and-yellow striped tail spikes. The message here – Stay away from me! The stripes on the body are formed by the lines of dermal scutes that are arranged on the animal’s sides. Since we only have a small patch of preserved skin from one Hesperosaurus specimen, we cannot know how extensive these scutes were on the animal’s body or if they were arranged in any kind of pattern. However, if they were arranged in a series of horizontal lines, or at least lines that more-or-less followed the body’s contours in a front-to-back arrangement, then it’s possible that these lines of scutes might have demarcated different color areas on the body. It’s just a thought. The resulting coloration is remarkably reminiscent of Brian Franczak’s painting, even though it wasn’t intended to be.  My gracious thanks to Madame Bujard for helping me with this.

Finally, here is a colorized rendition of a Stegosaurus showing a combination of the two color patterns which you see above. It has the stereotypical mottled green body, but it has the more vibrantly-colored plates seen in the second drawing. Personally, I like this one the best.

It has come to my attention that I have quite a few illustrations portraying dinosaurs patterned in broad longitudinal black stripes, including Dryosaurus and Camptosaurus. Hmmm. I don’t like making my dinosaurs look too similar to each other, but honestly, I cannot imagine these two species looking any other way. After all, both Dryosaurus and Camptosaurus are supposed to be related to each other, being primitive iguanodonts. However, the similarity of the stripes on Stegosaurus with the previously-mentioned species was entirely coincidental.

I hope you enjoyed this article. Please like, comment, and subscribe, and as always, keep your pencils sharp.


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.

Allosaurus, revised

Greetings, all. For those who regularly visit this website, you will know that this post has been a long time coming. Years ago, I mentioned that I was planning on re-doing my Allosaurus drawing so that it would be more accurate. However, that project always seemed to be shuffled onto the back-burner in place of other things that I was working on. Well not anymore. I recently completed a detailed drawing of an Allosaurus head (another one of the projects on my to-do list that I never seemed to get around to doing) which you can look at here, and I’m happy to state that after a long delay, I’ve finally completed my updated full-body Allosaurus.

Below is an Allosaurus drawing which I made in July of 2013 and which I posted to this website at that time. This portrays Allosaurus in a color scheme based upon that seen in the 1999 BBC series Walking With Dinosaurs. I must state that, as flawed as this illustration is, this piece was actually itself an updated version of a drawing that I had made a couple of years earlier. Even so, upon reflection, while it was an improvement on my previous work, it still needed more improvement.

And here is my revised Allosaurus drawing, made in July of 2020. This drawing was made in 1:20 scale, which is my preferred scale for illustrating prehistoric animals. From the tip of its nose to the tip of its tail, this drawing measures precisely 21 inches long, which would make the real-life animal 35 feet long; this measurement is regularly given as the maximum size for Allosaurus fragilis. This drawing was made with No.2 pencil on printer paper.

Finally, here is a colorized version of the new drawing. Again, the color scheme is based upon that seen in Walking With Dinosaurs, but the coloration and the color patterns differ slightly from the original image seen at the top. The picture was colored using Crayola colored pencils and No.2 pencil for re-shading.

Nearly everything about my previous drawing was altered in order to make this present artwork. This includes:

  1. The head was changed to be more accurate in appearance. Designing the head took most of the research time.
  2. The shape of the eye’s pupil was changed from a sort of oval slit to being a circle.
  3. The neck was made thicker, more muscular, and not as strongly S-curved.
  4. The body was made deeper.
  5. The arms were slightly enlarged and the hands were changed to be more anatomically accurate.
  6. The legs were thickened to provide extra weight support.
  7. The orientation of the hip bones was shifted.
  8. The tail was thickened to provide better balance to the front of the body. The previous drawing was conspicuously front-heavy.
  9. The tail was slightly elongated.

As you can see, one of the major changes to this drawing was the addition of dermal scutes along its back and sides. Unlike osteoderms, dermal scutes are scales which are enlarged and unusually thick compared to other scales on the rest of the body. There is evidence from preserved skin impressions from stegosaurs and ceratopsians that their skins possessed patterns of dermal scutes, sometimes arranged in lines, and it is therefore possible that theropods had such a feature to their outward appearance as well. It also gives this particular Allosaurus a distinctly reptilian look to it. I decided not to include any type of feathering or some other filamentous structures to the skin.

I also chose to portray this animal in a walking pose rather than running. I think that too many of my drawings of bipedal dinosaurs portray them running Gregory Paul-style, and I wanted to show something more natural. Also, unlike Scott Hartman’s illustrations, the legs are not splayed so widely apart from each other that they’re halfway to performing a split. Mostly, a normal walking stride is about three times the length of the foot. In fact, I actually practiced walking back and forth in front of a mirror, bending my legs theropod-style, in order to get a rough idea of how the leg position on this drawing ought to look.

Keep your pencils sharp, everyone.

Allosaurus head

Hello everyone. Here is a drawing of the head of Allosaurus fragilis, the top predator of the Morrison Formation of Late Jurassic North America. This drawing has been on my to-do list for quite some time, and I’m happy that it’s finally finished. The drawing was made with No. 2 pencil on printer paper.

Some Morrison Formation Sauropods: Apatosaurus, Barosaurus, and Diplodocus

Hello everyone. Here are some simple sketches of three Late Jurassic sauropod dinosaurs from the Morrison Formation of western North America: Apatosaurus, Barosaurus, and Diplodocus. All three of these sauropods are members of the family Diplodocidae, which includes the eponymous Diplodocus and any other sauropod that’s more closely related to Diplodocus than to any other sauropod group. The “diplodocids”, as these species are sometimes called, are distinctive for having long peg-like teeth in the fronts of their jaws (good for raking and stripping, but not well-suited for biting), a nares (the hole in the skull that contains your nostril openings) that’s located on the top of the skull, and long tapering whip-like tails.

The first is Apatosaurus louisae, which measured around 75 feet long. Like all diplodocid sauropods, Apatosaurus had a long whip-like tail, but it also had a massive thickly-built neck. Some paleontologists hypothesize that Apatosaurus used its neck in whacking contests during the mating season like modern-day giraffes. You can read more about that here.

Apatosaurus louisae. © Jason R. Abdale. May 11, 2020.


Next is Barosaurus lentus, which measured around 85 feet long. This animal was made famous by the impressive display in the entrance hall of the American Museum of Natural History in New York City. Notice that the neck and the tail are almost the same size; the tail is only slightly longer.

Barosaurus lentus. © Jason R. Abdale. May 11, 2020.


Finally is Diplodocus carnegii, which measured around 90 feet long. For a long time, this animal held the record as the longest dinosaur ever, until it was challenged by Supersaurus, Seismosaurus (which is almost certainly another species of Diplodocus), and various titanosaurid sauropods from South America. Of all of the diplodocid sauropods, Diplodocus itself had the longest tail. Some have speculated that the long ribbon-like tails of Diplodocus and its kind were used like whips, and it was even calculated that they could be cracked like a modern-day bull-whip. In the early 1990s, a partial skeleton of a Diplodocus-like dinosaur was found in Howe Quarry, Wyoming which had preserved skin impressions, including a series of keratin spikes similar to those seen on the back of an iguana lizard. An article was published about this discovery in 1992, which you can read here, although it wasn’t expressly stated within the report that the creature in question was indeed a Diplodocus. However, many paleo-artists ran with the idea anyway, and it was even incorporated into the 1999 BBC television series Walking With Dinosaurs. Since this is the prevailing trend, I decided to outfit my Diplodocus razorback-style as well.

Diplodocus carnegii. © Jason R. Abdale. May 11, 2020.


Next is an image showing a size comparison between Apatosaurus (75 feet), Barosaurus (85 feet), and Diplodocus (90 feet). For some people, it can be difficult to mentally grasp the size and the anatomical differences of these animals just by looking at numbers on a page. Perhaps by looking at this picture, you can truly appreciate the differences in the size proportions. Apatosaurus is a muscular beast. Barosaurus looks like a see-saw with legs. Diplodocus‘ tail measures three-fifths of its whole body length. So, as you can see, not all sauropods are the same.

A size comparison between Apatosaurus (75 feet), Barosaurus (85 feet), and Diplodocus (90 feet). © Jason R. Abdale. May 11, 2020.

Keep your pencils sharp, everyone.