Martharaptor was a therizinosaur theropod dinosaur which lived in Utah during the early Cretaceous Period approximately 135-132 million years ago.

Therizinosaurs are known mostly from the middle and later parts of the Cretaceous Period. Fossils dating to the early Cretaceous are far rarer. The best example of a basal therizinosaur is Falcarius utahensis, which, as the name suggests, was found in Utah. The fossils of this animal, of which there are many belonging to hundreds of individuals, were recovered from the rock layers which form the lower part of the Yellow Cat Member of the Cedar Mountain Formation. It lived during the early Cretaceous Period about 139-136 MYA (note: It was originally believed that the Yellow Cat Member of the Cedar Mountain was much younger, but these dates have since been revised) and measured approximately 12 feet long (Kirkland et al 2005, pp. 84-87). It’s possible that there may be a second species belonging to this genus (Zanno et al 2014, p. 260), but it hasn’t been named or described yet.

Falcarius was officially named and described in 2005, but then a second therizinosaur genus was found within the Cedar Mountain Formation a little while later. Approximately 8 miles southeast of the town of Green River, Utah, a team of paleontologists discovered fragmentary remains belonging to a theropod dinosaur. The fossils include pieces of one left scapula, pieces of arm and leg bones, several finger bones and finger claws, one ischium, pieces of several metatarsals, two foot claws, and several vertebrae. Upon closer study, scientists came to the conclusion that this creature was likely a therizinosaur, although the few fossil remains which were found indicated that it was different from Falcarius. The creature was officially named Martharaptor greenriverensis in 2012. The genus name is in honor of Martha Hayden who was one of the members of the dig team that discovered the site where the fossils were found. The species name is in reference to the town of Green River. The holotype is UMNH VP 21400 (Senter et al 2012: e43911).

There are a few differences between Falcarius and Martharaptor. First, Falcarius dates to a slightly older time. Fossils of Falcarius were found within the lower Yellow Cat Member of the Cedar Mountain Formation, 139-136 MYA, while Martharaptor was found within the upper Yellow Cat Member 135-132 MYA. Secondly, the hand claws were a different shape. The hand claws (or “manual unguals”, as is the appropriate biological term) of Martharaptor were short, had a tall base, but were strongly curved. These claws were almost the same size. Of these, the first claw was the largest, the middle claw was slightly smaller, and the third claw was just slightly smaller than that. Third, the fourth metatarsal was a different shape. Please note that slightly differently-shaped claws and a slightly differently shaped metatarsal are the only anatomical differences noted within the article that describes it. That doesn’t seem to be enough to differentiate this animal as a new genus. However, considering that Falcarius is known from literally hundreds of skeletons which can be easily cross-referenced to Martharaptor’s bones, there may have been more differences between the two than were implied (Senter et al 2012: e43911).

It has been proposed that Martharaptor might actually be a basal oviraptorosaur (Kirkland et al 2016, p. 137). This is because a similarity was noted concerning the third metatarsals of Martharaptor, Falcarius, and Chirostenotes (Senter et al 2012: e43911). However, the fact that this similarity is shared with both a caenagnathid oviraptorosaur AND a basal therizinosaur, and the fact that the majority of Marthraptor’s anatomy is clearly therizinosaur in its morphology, indicates that this hypothesis doesn’t hold much water.

According to the 2012 article which officially named and described Martharaptor, aspects of its anatomy indicate that Martharaptor was an intermediate species within Therizinosauria – more advanced than basal members such as Falcarius, but still not as advanced as the more derived species such as Therizinosaurus itself. Senter and his colleagues commented that Martharaptor seems to be the closest to Alxasaurus, a therizinosaur from the middle Cretaceous of Mongolia (Senter et al 2012: e43911). Both Falcarius and Alxasaurus measured around the same length (12-13 feet), but Alxasaurus was stockier, it had a larger body, and had a shorter neck and tail. It’s probable that, in terms of its outward appearance, Martharaptor would have looked “in between” Falcarius and Alxasaurus, but until more specimens are found, this is just a guess. Since Martharaptor has been compared to both Falcarius and Alxasaurus, and since both of these genera measured around 12 feet long, Martharaptor also likely measured 12 feet long. However, until a complete adult specimen is found, this measurement is just a guess.

I liken Martharaptor and most other therizinosaurs as dinosaurian analogs of anteaters or giant ground sloths (several paleontologists and other paleo-artists have also made this comparison). I like to think of creatures like these breaking through termite mounds and slurping up the little goodies inside (keep in mind that termite mounds haven’t been found so far within the Cedar Mountain Formation, but they have been found within the Morrison Formation which occurred earlier). The large claws would have also made formidable weapons in self-defense. However, I don’t think that they would have been much good against the much larger and much more formidable 20 foot long Utahraptor which Martharaptor shared its world with.

Below is my artistic interpretation of how Martharaptor may have looked. As I hypothesized above, I decided to draw it combining aspects of the anatomy of Falcarius and Alxasaurus. The coloration is vaguely reminiscent of the modern-day Giant Anteater of South America. The drawing was made with No.2 pencil, No.3 pencil on printer paper, and a lot of post-production with Microsoft Paint.

Martharaptor greenriverensis. © Jason R. Abdale (February 7, 2023).


Kirkland, James I.; Suarez, Marina; Suarez, Celina; Hunt-Foster, ReBecca (2016). “The Lower Cretaceous in East-Central Utah—The Cedar Mountain Formation and its Bounding Strata”. Geology of the Intermountain West, volume 3 (October 2016). Pages 101-228.

Kirkland, James I.; Zanno, Lindsay E.; Sampson, Scott D.; Clark, James M.; DeBlieux, Donald D. (2005). “A primitive therizinosauroid dinosaur from the Early Cretaceous of Utah”. Nature, volume 435, issue 7038 (2005), pages 84-87.

Senter, Phil; Kirkland, James I.; DeBlieux, Donald D. (2012). “Martharaptor greenriverensis, a New Theropod Dinosaur from the Lower Cretaceous of Utah”. PLOS ONE, volume 7, issue 8 (August 29, 2012): e43911.

Zanno, Lindsay E.; Kirkland, James I.; Herzog, Lisa L. (2014). “Second therizinosaurian mass death locality in the Lower Cretaceous Cedar Mountain Formation yields a new taxon”. Journal of Vertebrate Paleontology, Abstracts and Programs, 74th Annual Meeting (Berlin, Germany) (November 5-8, 2014), page 260.

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