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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.

Evidence of Therizinosaurs in North America during the Late Cretaceous Period

Introduction

For many years, paleontologists have known about the presence of therizinosaurs (formerly classified as segnosaurs) in Asia, especially within what’s now Mongolia and China. However, Asia and North America were linked during a considerable portion of the Cretaceous Period, and this resulted in an interchange of faunas between the two continents, notably ceratopsians, pachycephalosaurs, tyrannosaurs, and maniraptorans. Could therizinosaurs, which had hitherto been exclusively Asian, have lived in North America as well?

A pair of Tarbosaurus attacking a herd of Therizinosaurus somewhere in Mongolia, approximately 80 million years ago. © Gregory S. Paul (1988). Image used with permission.

 

During the early 2000s, that question was answered with a definitive “yes”. Two genera of therizinosaurs have been described from North America, named Falcarius and Nothronychus. Falcarius represents possibly the earliest stage in therizinosaur evolution, dated to the early Cretaceous Period, while Nothronychus is much larger and more advanced and is dated to the middle Cretaceous. The presence of these two creatures clearly shows that therizinosaurs existed in North America, but so far they have only been found in rocks dated to the early and middle parts of the Cretaceous Period. One wonders if therizinosaurs managed to stay in North America right up until the end of the Mesozoic, 66 million years ago. Would they have kept evolving, becoming larger and more advanced? Would they have lived alongside Triceratops and Tyrannosaurus? (1)

It just so happens that there are a few pieces of evidence here and there which suggest that therizinosaurs did survive past the middle Cretaceous within North America, and that they kept living in North America up to the end of the Cretaceous Period.

 

The Evidence

The idea that there were therizinosaurs in late Cretaceous North America was first proposed by the German paleontologist Hans Sues in 1978. Specifically, he was writing about a particular specimen that had been uncovered in the Dinosaur Park Formation, located in Alberta, Canada, in rocks dated to the Campanian Stage of the Cretaceous Period. The specimen in question was a single “frontal” bone, which forms part of the skull. Today, this specimen is in the collections of the Carnegie Museum of Natural History, categorized as “CMN 12355” (NOT 12349 as you’ll sometimes see in internet searches). In his paper, Sues thought that this frontal bone belonged to a “raptor” dinosaur, and listed it as “gen. et sp. indet.”, which is an abbreviated Latin way of saying “genus and species undetermined” (2).

“CMN 12355”: A frontal bone which may belong to a therizinosaur. Left top: ventral view. Right top: dorsal view. Left bottom: lateral view. Right bottom: medial view. © Tracy Ford. Image from Paleofile.com. Used with permission. http://www.paleofile.com/Dinosaurs/Theropods/Segnosaurincertae.asp

 

Saying that this bone belonged to a raptor is understandable, since the dromaeosaurs and the therizinosaurs are related to each other. Both groups are located in a clade called the “maniraptorans”, which includes the ornithomimids, the oviraptorosaurs, the therizinosaurs, and famously, the dromaeosaurs and troodontids – the so-called “raptors” with their famous killing claws.

The second piece of evidence came in the early to mid 1980s. A single bone called an “astragalus”, which forms part of the ankle, was found in the Hell Creek Formation in rocks dated to the very end of the Cretaceous Period. In 1984, the Canadian paleontologist Dale Russell listed this single peculiar find in a long list of specimens uncovered in the Hell Creek Formation during the middle 1980s. However, this particular specimen has never been analyzed or described in a publication exclusively devoted to this bone. It is simply listed as “therizinosaurid indet.”. In 1992, Kenneth Carpenter looked at this bone, and concluded that it actually belonged to Tyrannosaurus, not a therizinosaur (3).

In 1987, the Canadian paleontologist Philip Currie, who is widely acknowledged as the world’s expert on meat-eating dinosaurs, took a second look at the frontal bone which Sues had examined in the late 1970s, and concluded that Hans Sues had made a mistake. It wasn’t a raptor, but was instead a “segnosaur”, which was the way therizinosaurs were called back then. Currie stated that the bone looked similar to the frontal bone of an Asian therizinosaur called Erlikosaurus, and so he reclassified the bone as “cf. Erlikosaurus” (4).

In 1992, Philip Currie did a more thorough examination of possible therizinosaur finds in Canada. He again wrote about the frontal bone which was initially described in 1978, but he also added two more specimens to the discussion table, both of which were housed in the collection of the Royal Tyrell Museum of Paleontology (RTMP). These specimens were given the identification codes “RTMP 81.16.231” (again, Currie classified this specimen as “cf. Erlikosaurus”) and “RTMP 79.15.1” (a “pedal ungual”, or foot claw, which was classified as “cf. therizinosaurid”) (5).

“RTMP 79.15.1”: A foot claw which may belong to a therizinosaur. © Tracy Ford. Image from Paleofile.com. Used with permission. http://www.paleofile.com/Dinosaurs/Theropods/Segnosaurincertae.asp

 

In 2001, Michael Ryan and Anthony Russell conducted their own analysis of North American therizinosaur finds. They confirmed Currie’s claim that the frontal bone found in 1978 did indeed come from a therizinosaur. They also wrote about a neck vertebra found in the Scollard Formation (specimen identification code is “RTMP 86.207.17”), which dates to the very end of the Cretaceous Period, and which they classified as “Therizinosauridae indet.” (6).

Body fossils of therizinosaurs may be rare in North America, but footprints which may belong to therizinosaurs are more abundant. The first footprints were discovered in the 1990s in the Harebell Formation of northwestern Wyoming. According to an article published in 1996, these footprints were unique because they looked like theropod prints except that they had four toes instead of three – unique among theropods, therizinosaurs have four main toes. The authors postulated that the footprints belonged to an animal whose physical remains had not yet been discovered (7).

In 2011, a single therizinosaur footprint was discovered in Denali National Park, Alaska. The rock that the footprint was found in was part of the Cantwell Formation, which spans 80-65 MYA, and the footprint was placed in a layer dated to about 71-69 MYA. Depending upon which source that you read concerning geological dating, this date of 71-69 MYA either marks the boundary between the where the Campanian Stage ends and the Maastrichtian Stage begins, or else it is the earliest phase of the Maastrichtian Stage. In 2012, Anthony R. Fiorillo of the Perot Museum of Nature and Science (located in Dallas, Texas) published an article concerning this peculiar footprint (8). You can see a photo of it here.

In 2013 and 2014, Anthony R. Fiorillo and a team of other researchers returned to the site in Denali National Park and found a total of thirty-one therizinosaur footprints, along with numerous hadrosaur footprints as well. Like the first footprint that had been found in 2011, all of the other footprints were in rock dated to 71-69 MYA. The fact that footprint trackways of both hadrosaurs and therizinosaurs were found together might indicate that these animals traveled together, possibly for mutual protection. An article was published in August 2018 detailing these discoveries (9).

 

Species Identification

As we have seen in the previous section, there is some evidence in the way of footprints and a handful of isolated bones which suggests that therizinosaurs inhabited North America during the late Campanian or early Maastrichtian Stages of the Cretaceous Period. However, is there any way that we can identify which particular genus or species that these fossils belong to?

The subject of identification has been especially contentious concerning the footprints that were found in Wyoming and Alaska. So far, footprints form the majority of finds that are attributed to late Cretaceous therizinosaurs within North America. The problem is that it is difficult to identify a particular genus or species based solely on footprints, unless the shape of the footprint is extremely distinctive. Another problem is that while footprints are abundant, very few body fossils have been found, and none of them are highly diagnostic. Most researchers who examined them determined vaguely that the creature was a therizinosaur, but they couldn’t be more specific than that, with the exception of Philip Currie who proposed that they might belong to Erlikosaurus or a creature very similar to it.

Because it is so difficult to match a footprint with a particular animal, paleontologists often ascribe footprints their own genus and species names. This is what is referred to as an “ichnogenus”, which is a genus of animal known only from trace fossils, such as footprints, rather than actual physical body fossils.

In the 1996 article which discussed the unusual footprints found in Wyoming, the footprints were ascribed to the ichnogenus Exallopus (pronounced as Ex-ALLO-pus, meaning “from different foot” due to its unusual shape) and its species name was given as Exallopus lovei. The type specimen is identified as “DMNH 5989”, and it was identified as a coelurosaur. According to the website Fossilworks, “Its type locality is Whetstone Creek tracksite, which is in a Maastrichtian terrestrial sandstone in the Harebell Formation of Wyoming” (10). The following year in 1997, the genus name was changed from Exallopus to Saurexallopus (SORE-ex-ALLO-pus), because the name Exallopus was already taken by a species of marine worm (11). Another species, Saurexallopus zerbsti, was named in a 2003 article. The type specimen is identified as “CUMWC 224.2”. According to Fossilworks, “Its type locality is Zerbst Ranch Tracksite, which is in a Lancian fluvial sandstone/sandstone in the Lance Formation of Wyoming” (12). In 2014, a third species was named called Saurexallopus cordata based upon a single footprint fount in British Columbia, Canada, and dated to the Wapiti Formation of the late Cretaceous Period (13).

While all of the scientific articles concerning Saurexallopus identify it as a theropod, there has been some dispute as to what particular type of theropod it is. The original article which was written in 1996 identified it as a coelurosaur. In 2012, Anthony Fiorillo and Thomas Adams identified Saurexallopus as a therizinosaur (14). In an article written in 2015, Saurexallopus was identified as an oviraptorid (15). In an article written in 2018, Saurexallopus was simply identified as a theropod without any specific affinity (16). The website Fossilworks identifies Saurexallopus as a therizinosaur (17).

 

Reconstructing Saurexallopus

During the late 1980s and early 1990s, Philip Currie made comparisons between the various finds in North America with the Asian species Erlikosaurus. According to a phylogenic analysis of therizinosaur genera which was conducted in 2019, Erlikosaurus was closely related to Nothronychus, a therizinosaur which lived in North America during the middle Cretaceous Period. Since Saurexallopus is believed to be physically similar to Erlikosaurus, it is likely that it was genetically related as well, and as such would have been genetically related to Nothronychus. It is therefore quite possible that Erlikosaurus, Nothronychus, and Saurexallopus would have been similar in appearance (18).

Erlikosaurus skull and foot.jpg

Upper jaw and right foot of the Asian therizinosaur Erlikosaurus. Saurexallopus was probably similar in appearance to this genus. Illustration from Rinchen Barsbold and Altangerel Perle (1980) “Segnosauria, a new infraorder of carnivorous dinosaurs”. Acta Palaeontologica Polonica, 25 (2): pages 187-195. https://www.app.pan.pl/article/item/app25-187.html. Creative Commons Attribution License.

 

We can guess that Saurexallopus reached a similar length to Erlikosaurus, measuring about fifteen to twenty feet long (Holtz claims that Erlikosaurus was smaller than other authors do, although his estimate of Nothronychus is in fitting with the size bracket mentioned above) (19). Unlike the eponymous Therizinosaurus, which possessed long scythe-like finger claws (hence its name, which translates to “scythe lizard”), Nothronychus possessed shorter hook-shaped claws, which looked very similar to the stereotypical talons that are seen on carnivorous dinosaurs like Allosaurus and Torvosaurus. These claws were only one-third the size of the claws of Therizinosaurus, but they were well-suited for pulling down branches, for digging (if they could pronate their hands, but that’s a whole other argument), and for smacking the daylights out of any would-be predator. Thomas R. Holtz Jr. has compared therizinosaurs to the large ground sloths of the Cenozoic Era, and the analogy has some merit (20). Saurexallopus and other therizinosaurs likely lived a similar lifestyle and occupied a similar ecological niche, with the possible exception of Falcarius, which may have had a more cursorial lifestyle similar to early coelurosaurs like Ornitholestes.

Based upon their place within the dinosaur family tree, as well as from fossil finds, we are fairly certain that therizinosaurs were feathered. Therefore, it is almost certain that Saurexallopus would have had some form of feather covering as well, although whether it was over the entire body or only partially cannot be determined.

Below is a drawing that I made of Saurexallopus, based upon Erlikosaurus and Nothronychus. The erect mane running down the middle of its neck, back, and tail are just artistic conjecture.

Saurexallopus. © Jason R. Abdale. May 7, 2020.

 

Conclusions

So where does all of this information lead us? So far, there is some evidence which suggests that therizinosaurs were living in Alberta, Canada and Alaska, USA during the late Campanian Stage or early Maastrichtian Stage of the late Cretaceous Period up until about 70 MYA or thereabouts. As such, they would have lived side-by-side with creatures such as Albertosaurus, Edmontosaurus, and Hypacrosaurus. There is only one piece of evidence, a single neck vertebra, which suggests that therizinosaurs existed in North America during the Maastrichtian Stage of the Late Cretaceous. However, no specimens that can be definitely and unquestionably identified as belonging to a therizinosaur have been found in the Hell Creek Formation. Therefore, as far as our current evidence goes, it is unlikely that therizinosaurs lived side-by-side with Triceratops and Tyrannosaurus. However, this may change in the future if more body fossils are discovered.

 

Sources

  1. Utah’s Dino Graveyard; When Dinosaurs Roamed America.
  2. Lindsay Elizabeth Zanno. A Taxonomic and Phylogenetic Reevaluation of Therizinosauria (Dinosauria: Theropoda): Implications for the Evolution of Maniraptora. PhD dissertation, submitted to the University of Utah. December 2008. Page 172.
  3. Lindsay Elizabeth Zanno. A Taxonomic and Phylogenetic Reevaluation of Therizinosauria (Dinosauria: Theropoda): Implications for the Evolution of Maniraptora. PhD dissertation, submitted to the University of Utah. December 2008. Page 172; Dinosaur Mailing List. “Re: Yet even more questions (and I’m sure there’ll be more…)”, by Mickey Mortimer (June 22, 2002). http://dml.cmnh.org/2002Jun/msg00369.html; Theropod Database. “Therizinosauroidea”. http://theropoddatabase.com/Therizinosauroidea.htm.
  4. Lindsay Elizabeth Zanno. A Taxonomic and Phylogenetic Reevaluation of Therizinosauria (Dinosauria: Theropoda): Implications for the Evolution of Maniraptora. PhD dissertation, submitted to the University of Utah. December 2008. Page 172.
  5. Lindsay Elizabeth Zanno. A Taxonomic and Phylogenetic Reevaluation of Therizinosauria (Dinosauria: Theropoda): Implications for the Evolution of Maniraptora. PhD dissertation, submitted to the University of Utah. December 2008. Page 172; Dinosaur Mailing List. “Re: Yet even more questions (and I’m sure there’ll be more…)”, by Mickey Mortimer (June 22, 2002). http://dml.cmnh.org/2002Jun/msg00369.html.
  6. Lindsay Elizabeth Zanno. A Taxonomic and Phylogenetic Reevaluation of Therizinosauria (Dinosauria: Theropoda): Implications for the Evolution of Maniraptora. PhD dissertation, submitted to the University of Utah. December 2008. Page 172.
  7. J. D. Harris, K. R. Johnson, J. Hicks and L. Tauxe (1996). “Four-toed theropod footprints and a paleomagnetic age from the Whetstone Falls Member of the Harebell Formation (Upper Cretaceous: Maastrichtian), northwestern Wyoming”. Cretaceous Research, 17: 381-401.
  8. Anthony R. Fiorello and Thomas L. Adams (2012). “A therizinosaur track from the Lower Cantwell Formation (Upper Cretaceous) of Denali National Park, Alaska”. Palaios, 27: 395-400.
  9. Anthony R. Fiorello and Thomas L. Adams (2012). “A therizinosaur track from the Lower Cantwell Formation (Upper Cretaceous) of Denali National Park, Alaska”. Palaios, 27: 395-400; “The Lower Cantwell Formation and Its Fossils”; “Therizinosaur: prehistoric predator set standard for ‘weird’ in Alaska”; “First North American co-occurrence of Hadrosaur and Therizinosaur tracks found in Alaska”.
  10. Fossilworks. “Saurexallopus lovei”. http://fossilworks.org/bridge.pl?a=taxonInfo&taxon_no=65844.
  11. J. D. Harris, K. R. Johnson, J. Hicks and L. Tauxe (1996). “Four-toed theropod footprints and a paleomagnetic age from the Whetstone Falls Member of the Harebell Formation (Upper Cretaceous: Maastrichtian), northwestern Wyoming”. Cretaceous Research, 17: 381-401; J. D. Harris (1997). “Four-toed theropod footprints and a paleomagnetic age from the Whetstone Falls Member of the Harebell Formation (Upper Cretaceous: Maastrichtian), northwestern Wyoming: a correction”. Cretaceous Research, 18: 139.
  12. Martin G. Lockley, G. Nadon, and Philip J. Currie. (2003). “A diverse dinosaur-bird footprint assemblage from the Lance Formation, Upper Cretaceous, eastern Wyoming; implications for ichnotaxonomy”. Ichnos, 11: 229-249; Fossilworks. “Saurexallopus zerbsti”. http://fossilworks.org/bridge.pl?a=taxonInfo&taxon_no=81011.
  13. R. T. McCrea, L. G. Buckley, A. G. Plint, Philip J. Currie, J. W. Haggart, C. W. Helm, and S. G. Pemberton (2014). “A review of vertebrate track-bearing formations from the Mesozoic and earliest Cenozoic of western Canada with a description of a new theropod ichnospecies and reassignment of an avian ichnogenus”. In Lockley Martin G.; Lucas, Spencer G., eds. New Mexico Museum of Natural History & Science. Bulletin 62: Fossil Footprints of Western North America. Albuquerque: New Mexico Museum of Natural History & Science, 2014. Page 87.
  14. Anthony R. Fiorello and Thomas L. Adams (2012). “A therizinosaur track from the Lower Cantwell Formation (Upper Cretaceous) of Denali National Park, Alaska”. Palaios, 27: 395-400.
  15. R. T. McCrea, D. H. Tanke, L. G. Buckley, M. G. Lockley, J. O. Farlow, L. Xing, N. A. Matthews, C. W. Helm, S. G. Pemberton and B. H. Breithaupt (2015). “Vertebrate ichnopathology: pathologies inferred from dinosaur tracks and trackways from the Mesozoic”. Ichnos, 22 (3–4): 235-260.
  16. Martin Lockley, Gerard Gierlinski, Lidia Adach, Bruce Schumacher, and Ken Cart (2018). “Newly Discovered Tetrapod Ichnotaxa from the Upper Cretaceous Blackhawk Formation, Utah”. In Spencer G. Lucas and Robert M. Sullivan, eds. New Mexico Museum of Natural History and Science. Fossil Record 6, Volume 2: Bulletin 79. Albuquerque: New Mexico Museum of Natural History and Science, 2018. Pages 469-480.
  17. Fossilworks. “Saurexallopus”. http://fossilworks.org/bridge.pl?a=taxonInfo&taxon_no=65843.
  18. Scott Hartman, Mickey Mortimer, William R. Wahl, Dean R. Lomax, Jessica Lippincott, and David M. Lovelace (2019). “A new paravian dinosaur from the Late Jurassic of North America supports a late acquisition of avian flight”. PeerJ, 7: e7247. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6626525/.
  19. David Lambert, The Dinosaur Data Book (New York: Avon Books, 1990), page 61; Don Lessem and Donald F. Glut, The Dinosaur Society Dinosaur Encyclopedia (New York: Random House, 1993), page 184; Peter Dodson, The Age of Dinosaurs (Lincolnwood: Publications International Ltd., 1993), page 142; Thomas R. Holtz Jr, Dinosaurs: The Most Complete, Up-To-Date Encyclopedia for Dinosaur Lovers of All Ages (New York: Random House, 2007), page 382.
  20. Thomas R. Holtz Jr, Dinosaurs: The Most Complete, Up-To-Date Encyclopedia for Dinosaur Lovers of All Ages (New York: Random House, 2007), page 147.

 

Bibliography

Books:

  • Dodson, Peter. The Age of Dinosaurs. Lincolnwood: Publications International Ltd., 1993.
  • Holtz Jr., Thomas R. Dinosaurs: The Most Complete, Up-To-Date Encyclopedia for Dinosaur Lovers of All Ages. New York: Random House, 2007.
  • Lambert, David. The Dinosaur Data Book. New York: Avon Books, 1990.
  • Lessem, Don; Glut, Donald F. The Dinosaur Society Dinosaur Encyclopedia. New York: Random House, 1993.

Articles:

  • Fiorello Anthony R.; Adams Thomas L. (2012). “A therizinosaur track from the Lower Cantwell Formation (Upper Cretaceous) of Denali National Park, Alaska”. Palaios, 27: 395-400.
  • Fiorillo, Anthony R.; McCarthy, Paul J.; Kobayashi, Yoshitsugu; Tomsich, Carla S.; Tykoski, Ronald S.; Lee, Yuong-Nam; Tanaka, Tomonori; Noto Christopher R. (August 3, 2018). “An unusual association of hadrosaur and therizinosaur tracks within Late Cretaceous rocks of Denali National Park, Alaska”. Scientific Reports, 2018; 8 (1) DOI: 10.1038/s41598-018-30110-8. https://www.nature.com/articles/s41598-018-30110-8.
  • Harris, J. D.; Johnson, K. R.; Hicks, J.; Tauxe; L. (1996). “Four-toed theropod footprints and a paleomagnetic age from the Whetstone Falls Member of the Harebell Formation (Upper Cretaceous: Maastrichtian), northwestern Wyoming”. Cretaceous Research, 17: 381-401.
  • Harris, J. D. (1997). “Four-toed theropod footprints and a paleomagnetic age from the Whetstone Falls Member of the Harebell Formation (Upper Cretaceous: Maastrichtian), northwestern Wyoming: a correction”. Cretaceous Research, 18: 139.
  • Hartman, Scott; Mortimer, Mickey; Wahl, William R.; Lomax, Dean R.; Lippincott, Jessica; Lovelace, David M. (2019). “A new paravian dinosaur from the Late Jurassic of North America supports a late acquisition of avian flight”. PeerJ, 7: e7247. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6626525/.
  • Lockley, Martin G.; Nadon, G.; Currie, Philip J. (2003). “A diverse dinosaur-bird footprint assemblage from the Lance Formation, Upper Cretaceous, eastern Wyoming; implications for ichnotaxonomy”. Ichnos, 11: 229-249.
  • Lockley, Martin; Gierlinski, Gerard; Adach, Lidia; Schumacher, Bruce; Cart, Ken (2018). “Newly Discovered Tetrapod Ichnotaxa from the Upper Cretaceous Blackhawk Formation, Utah”. In Spencer G. Lucas and Robert M. Sullivan, eds. New Mexico Museum of Natural History and Science. Fossil Record 6, Volume 2: Bulletin 79. Albuquerque: New Mexico Museum of Natural History and Science, 2018. Pages 469-480.
  • McCrea, R. T.; Buckley, L. G.; Plint, A. G.; Currie, Philip J.; Haggart, J. W.; Helm, C. W.; Pemberton, S. G. (2014). “A review of vertebrate track-bearing formations from the Mesozoic and earliest Cenozoic of western Canada with a description of a new theropod ichnospecies and reassignment of an avian ichnogenus”. In Lockley Martin G.; Lucas, Spencer G., eds. New Mexico Museum of Natural History & Science. Bulletin 62: Fossil Footprints of Western North America. Albuquerque: New Mexico Museum of Natural History & Science, 2014. Pages 5-94.
  • McCrea, R. T.; Tanke, D. H.; Buckley, L. G.; Lockley, Martin G.; Farlow, James O.; Xing, L.; Matthews, N. A.; Helm, C. W.; Pemberton, S. G.; Breithaupt, B. H. (2015). “Vertebrate ichnopathology: pathologies inferred from dinosaur tracks and trackways from the Mesozoic”. Ichnos, 22 (3–4): 235-260.
  • Zanno, Lindsay Elizabeth. A Taxonomic and Phylogenetic Reevaluation of Therizinosauria (Dinosauria: Theropoda): Implications for the Evolution of Maniraptora. PhD dissertation, submitted to the University of Utah. December 2008.

Websites:

Videos:

  • Utah’s Dino Graveyard. The Discovery Channel, 2005.
  • When Dinosaurs Roamed America. The Discovery Channel, 2001.

 

Tyrannosaurus rex juvenile, two years old

Hello everyone. This is  drawing which I made of a juvenile Tyrannosaurus rex, two years old. The anatomy is based upon the skeletons of juvenile Tarbosaurus (a tyrannosaur from Asia which is closely related to Tyrannosaurus) as well as from bones found in North America which may/may not belong to T. rex. There is a theory that the young were covered in a full or partial coating of feathery fuzz, and gradually lost it as they aged. Therefore, I have shown this 2 year old T. rex with a mottled camouflage coloring similar to that seen on wild boar piglets and some species of birds. This drawing was made with No. 2 pencil on printer paper.

Revising my Troodon drawing

Hello everyone. Time to kick off the new year with some much belated paleo-art. One of the projects on my to-do list was to re-do my old and very out-dated 2012 drawing of Troodon. Seven years ago, this drawing was my first attempt at making modern up-to-date paleo-art by featuring theropods with feathers, something that I’d never done before. However, I soon realized that my illustration was utterly pitiful, and I needed to make a new one that was not only more scientifically accurate but also more artistically pleasing. Below is my revised drawing that I made recently. I hope that you’ll agree it’s a distinct improvement. The old drawing has been deleted, partly because I was embarrassed by it, and partly because I don’t what people to get incorrect ideas about what Troodon would have looked like.

Dakotaraptor

Hi everybody. Here is my latest Hell Creek paleo-art. Say hello to Dakotaraptor steini, a large dromaeosaurid raptor that lived in South Dakota at the end of the Cretaceous Period. How large? We don’t have an exact measurement because this animal is known only from partial remains. However, enough was recovered to give a ballpark estimate that the creature measured somewhere around 15 to 20 feet long. Not as big as Utahraptor, but still pretty impressive.

Dakotaraptor steini. © Jason R. Abdale. May 26, 2018.

This drawing was made with No. 2 pencil, Crayola and Prismacolor colored pencils, a black felt-tipped marker, and A LOT of touch up work on the computer in order to make the scanned image as bold and vivid as it is in real life.

Caenagnathus, or Chirostenotes, or…um…something…

During the early 1920s, Charles W. Gilmore, a paleontologist from the Smithsonian Institute in Washington, DC, was prospecting for fossils in Alberta, Canada. While on this trip, he would discover several new species of dinosaurs, including a strange creature known only from a pair of incomplete hands. These hands had long slender fingers, which was highly unusual for theropods known at the time. He officially named and described them as Chirostenotes pergracilis in 1924.

The hands of Chirostenotes pergracilis. Illustration by Tracy Ford. From The Dinosaur Society Dinosaur Encyclopedia, written by Don Lessem and Donald F. Glut. New York: Random House, Inc., 1993. Page 109. Image used with permission.

Chirostenotes was originally believed to be a member of the family Elmisauridae. This is an enigmatic group of dinosaurs, whose members consist of only one genus, Elmisaurus. This animal lived in Mongolia during the late Cretaceous Period about 80 MYA, and the only evidence that we have of its existence is one incomplete foot and a hand found in 1970. Scientists recognized that the hands of Chirostenotes and Elmisaurus looked similar, and so Chirostenotes was placed into that family. By 1990, Elmisauridae was recognized as an invalid family name, and it was discarded.

Chirostenotes is now classified as a member of the family Caenagnathidae, named after the genus Caenagnathus, which might actually be the same animal as Chirostenotes (as early as 1990, scientists suspected that these two might actually be the same animal). The caenagnathids were a group of bird-like theropod dinosaurs who belonged to a much larger group called the oviraptorosaurs, who are well-known from Asia. Their presence in North America only adds further proof to a faunal exchange between Asia and North America. Caenagnathids are distinguished from oviraptorids by their feet, which look more like those of the ornithomimids, more commonly-known as “ostrich dinosaurs”. This suggests that the oviraptorosaurs evolved from the ornithomimids. According to current phylogenic analysis, the ornithomimids are more primitive than the oviraptorosaurs, so this hypothesis might be plausible.

The lower jaw of Caenagnathus collinsi, with a hypothetical upper jaw. Illustration by Tracy Ford. From The Dinosaur Society Dinosaur Encyclopedia, written by Don Lessem and Donald F. Glut. New York: Random House, Inc., 1993. Page 79. Image used with permission.

Because Caenagnathus and Chirostenotes are known from incomplete specimens, nobody can make up their minds as to whether or not they’re two separate genera or if they’re the same animal. Some paleontologists firmly believe the former, while others firmly believe the latter. Because of their incompleteness, we are also not 100% sure what the animal looked like. It’s reasonably certain that it bore a strong resemblance to Oviraptor, Citipati, or Anzu, but any recreation of what the entire animal looked like is guesswork. During the 1980s and 1990s, there were a wide range of images crafted by various paleo-artists which took a stab at what the whole animal would look like if it were fleshed out. Ever since the discovery of Anzu, which is both the largest and most well-known caenagnathid, the diversity of images has largely disappeared. Now, modern depictions of both Caenagnathus and Chirostenotes, if you can find them, are really nothing more than clone copies of Anzu, which I disagree with not only as a paleontology buff but also as an artist.

Below is my own rendition of what I think Caenagnathus, or possibly Chirostenotes, or both, would have looked like. Since no complete skull of either species has been found, the design for the head is based upon a hypothetical skull drawing made by Tracy Ford. My drawing was made on printer paper with No. 2 pencil, Crayola and Prismacolor colored pencils, and a black felt-tipped marker. Since my scanner has a tendency to wash out a lot of the detailing, I had to do a bit of touching-up on my computer to replicate how the image looks in real life. Hope you enjoy, and keep your pencils sharp.

Caenagnathus collinsi. © Jason R. Abdale. May 11, 2018.

UPDATE: In the year 2020, a research paper was published by Gregory F. Funston and Philip Currie which stated that new fossils of Chirostenotes had been discovered in Alberta, Canada. These fossils were distinct enough from those of Caenagnathus to support the idea that Caenagnathus and Chirostenotes ought to be considered as two separate genera.

For more information, please look at the following sources:

  • Funston, Gregory F.; Currie, Philip J. “New material of Chirostenotes pergracilis (Theropoda, Oviraptorosauria) from the Campanian Dinosaur Park Formation of Alberta, Canada”.  Historical Biology: An International Journal of Paleobiology (February 2020). DOI: 10.1080/08912963.2020.1726908. Published online.
  • Funston, Gregory (July 27, 2020). “Caenagnathids of the Dinosaur Park Formation (Campanian) of Alberta, Canada: anatomy, osteohistology, taxonomy, and evolution”. Vertebrate Anatomy Morphology Paleontology, volume 8 (1): Pages 105-153. https://journals.library.ualberta.ca/vamp/index.php/VAMP/article/view/29362.
  • Greg Funston Paleontology. “The Caenagnathids of Dinosaur Park” by Gregory F. Funston (July 27, 2020). https://gregfunston.com/2020/07/27/the-caenagnathids-of-dinosaur-park/

 

Habrosaurus, a Late Cretaceous siren amphibian from the Hell Creek Formation

The Hell Creek Formation of the north-central United States is famous for its dinosaur fossils, notably those of Tyrannosaurus, Triceratops, and others whose names are well-known to children and adults. However, this fossilized environment was home to more than just dinosaurs. The Hell Creek Formation was home to a wide range of fish, amphibians, reptiles, and mammals. One of the animals that called this landscape home during the late Cretaceous was Habrosaurus.

Despite its name, Habrosaurus was not a dinosaur, and it wasn’t even a reptile. It was, in fact, an amphibian, and a large one at that. Habrosaurus dilatus was a three-foot long siren, a type of salamander that bears more of a resemblance to an eel than the lizard-like forms that we associate salamanders with. Unlike most salamanders, sirens are fully-aquatic amphibians that retain gills throughout their whole lives, unlike other amphibian species that possess gills only in the early development stages of their lives. Sirens also possess small rudimentary lungs, and are able to breath air. There are four species of sirens that are alive today, and all of them are found within North America. Depending upon the species, they can have one to three gill slits on each side of the head. They have completely lost their hind limbs, and their front limbs have shrunk considerably, with three or four short stubby fingers on each hand. Sirens have tiny eyes and no eyelids, and possess a long tail reminiscent of an eel or a sea snake – ideal for swimming. Sirens prefer to live in slow-moving or static bodies of water with lots of underwater vegetation and muddy bottoms. They might occasionally come onto land during the night if the ground is wet or if it’s raining.

Habrosaurus is, to date, the oldest-known siren genus. So far, there are two species known: H. prodilatus, which was found in Alberta, Canada in rocks dating to the Campanian Phase (83-70 MYA) of the late Cretaceous, and H. dilatus, which is much more widespread in the western United States, being found in Montana and Wyoming (with more specimens being found in Wyoming) and dating to the Maastrichtian Stage (70-65 MYA) of the late Cretaceous, as well as being found in the early Paleocene Epoch of the Tertiary Period. This means that H. dilatus was one of several species to survive the K-T Extinction, if only for a short while. It may be possible that H. dilatus is simply the evolved form of H. prodilatus.

Habrosaurus dilatus was named by the eminent paleontologist Charles W. Gilmore in 1928. To my knowledge, six specimens have been found of this animal, and all of them have been found in stream channel deposits. The presence of this type of animal, as well as its impressive size of three feet in length, indicates the presence of large bodies of fresh water, such as slow-moving rivers or ponds. However, the possibility of a dry year was ever-present, and for a fully-aquatic or mostly-aquatic animal like Habrosaurus, that could spell doom. During dry periods or droughts, modern-day sirens are able to dig burrows into the mud and encase themselves in a cocoon, like a lungfish, and Habrosaurus might have adopted the same strategy.

Habrosaurus had rows of blunt teeth arranged in the roof of its upper jaw, which indicates that these jaws were designed for crushing rather than grabbing. Presumably, it fed upon tiny mollusks and arthropods, such as snails and shrimp. Modern-day sirens feed mainly upon worms, aquatic snails, shrimp, and occasionally algae. Like fish, sirens possess lateral lines to find prey by indicating differences in water pressure and underwater vibrations.

An appropriate modern-day analog for the three-foot long Habrosaurus dilatus is the Greater Siren (Siren lacertina), which also grows to three feet long and is the largest siren species in the world today.

Below is a simple drawing of a Habrosaurus that I made with a felt-tipped marker. This style is a considerable departure from my usual style of highly-detailed pencil drawings, but I wanted to do some artistic experimenting.

Ornithomimus, Before and After

Hello all. I’ve recently finished an important writing project that I’ve been laboring upon for months. Now that it’s finished, I have a little breathing room to do art, and this is what I’ve done so far. I decided to do an updated version of an old illustration that I had made of an Ornithomimus. While the general color scheme was what I had in mind, I was never truly happy with the end-product. This latest version is much more in line with what I was imagining the “Bird Mimic” would look like.

Here is the “before” picture, made in 2013.

 

And here is the “after”, made today.

 

You’ll notice several differences right away, the most noteable of them being the re-shaping of its wing feathers. While Ornithomimus, or perhaps ornithomimids in general, had pennaceous feathers, I don’t think that they had primaries, because those would have been attached onto the wrist and the hand. This would have been difficult for ornithomimids because, unlike “raptor” dinosaurs (dromaeosaurids and troodontids), ornithomimids could not flex their hands backwards. I also increased the size of its tail feathers, made the neck thicker, changed the shape of the skull so that it was more anatomically accurate, and added Secretary Bird-style feathers to the back of its head. So much for form. In terms of color, I made it more vibrant, with deeper richer yellows and oranges and a lot more black patches. I changed the color of its bare skin from pink to a mixture of tan and black. I made its beak black, I changed its eye from yellow to blood red, and gave it black feet.

I can definitely see this character rushing about on the plains of the Hell Creek Formation. This shows that artists should never be stagnant. They must always strive to improve their work, and in so doing, improve their skill.

This drawing was made on computer printer paper with a No. 2 pencil, Prismacolor colored pencils, markers, and a black felt-tiped pen. The size of the drawing, from the tip of its nose to the tip of its tail feathers, measures 10.75 inches long, which is almost 1/12 scale, as the real animal possibly measured 12 feet long with its neck and tail fully stretched out.

Keep your pencils sharp.

Tyrannosaurus rex with scales

Tyrannosaurus full body with scales

Behold my masterpiece.

This is the fifth T. rex drawing that I’ve posted to this blog, and it is the hardest drawing that I have ever had to make. Every individual scale was done by hand, one by one. This drawing took me months to finish. To give you a better idea about the utterly insane amount of detail, the actual drawing of the dinosaur itself from the tip of its nose to the tip of its tail measures precisely 24 inches. Most of the drawn scales measure at only one millimeter in diameter.

As you can see, it is done in the same pose as my previous two full-body T. rex drawings, but I made some noteable improvements:

  1. Slightly changing the shape of the skull – my original one looked a little too much like Tarbosaurus rather than Tyrannosaurus.
  2. Not making the face as shrink-wrapped as the original head drawing was.
  3. Making the neck more detailed and fuller.
  4. Changing the position of the hands to be more anatomically correct.
  5. Making its body fatter – the original was too skinny.
  6. Making the tail thicker and fatter to properly counter-balance the now-heavier front half of the body.
  7. Changing the shape of the feet.

This drawing was made on several sheets of 8.5 x 11 printer paper, with just an ordinary No. 2 pencil…and a whole lot of patience.

Chasmosaurus

Chasmosaurus

Chasmosaurus was a common genus of ceratopsian dinosaur found in North America, especially Alberta, Canada circa 75 MYA. This creature is so recognizable due to its rectangle-shaped frill that it has given its name to a whole slew of other ceratopsians that are related to it – the “chasmosaurine” ceratopsians.

Made with regular No. 2 pencil on plain white printing paper. The actual drawing of the creature from the tip of the beak to the tip of the tail measures just a smidge over seven inches. Scanned at 600DPI to show as much of the detail as possible.

Keep your pencils sharp, everybody.