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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.