The Skull of Erlikosaurus andrewsi and Understanding Therizinosaurs

Article by: J. D. Dixon
Consultation: Stephan Lautenschlager
Edited by: Lauren Malin and Jack Wood

Erlikosaurus andrewsi illustration by Jack Wood.

Erlikosaurus andrewsi is a therizinosaur – a strange group of theropod dinosaurs with long necks, blunt teeth, and large sickle-shaped claws on their hands. Erlikosaurus was first described by Altangerel Perle and Rinchen Barsbold in 1980 after its fossils were recovered from Late Cretaceous (Cenomanian – Santonian; approximately 100 – 80 Ma) sediments in the Bayanshiree Svita of Mongolia. The species is named after Erlik, the Mongolian king of the dead, and is estimated to have had a body mass of 160-174 kg. The remains of Erlikosaurus, while lacking in material, are truly special due to the extremely well-preserved three dimensional skull. This is one of the only examples of a therizinosaur skull, especially at such a level of preservation. It has since been used to estimate potential skull morphologies and functionalities for various other species in this bizarre clade.

Some of the most infamous dinosaur species, such as Tyrannosaurus Rex and Velociraptor mongoliensis, are theropods. Therizinosaurs also belong to this suborder, but unlike most theropods, they are non-carnivorous. Hypotheses of an insectivorous, piscivorous, or herbivorous diet have been put forward for this group, with the latter most likely. The vegetarian diet resulted in a number of anatomical changes in the species. The skull of Erlikosaurus andrewsi is elongated, and the teeth are symmetrical, particularly on the upper jaw. The species is thought to have approximately 31 teeth in the mouth, with those at the cheeks lanceolate in shape (a narrow oval coming to a point at the end, like a leaf). The therizinosaur clade exhibits a reduction of teeth as a result of adapting to herbivory, with the tip of the jaw taking a shape similar to a bird’s beak, even exhibiting a rhamphotheca (keratin protein sheath). Nevertheless, total tooth loss was never accomplished within the group.

Erlikosaurus andrewsi 3D digital skull restoration. Available at: https://sketchfab.com/3d-models/erlikosaurus-andrewsi-4172d3c1bab5474fbf6fa5fe4f49a68a.

The skull of Erlikosaurus was CT scanned, and digital technology was used to reconstruct the muscle attachments in order to measure potential bite force of the animal. The bite force of Erlikosaurus was calculated in three different positions, and was found to be considerably lower than that of other theropods. Erlikosaurus only has a bite force of 200 N, while a Tyrannosaurus Rex has a bite force of 50,000 N. This again indicates a higher likelihood of a branch stripping or plant cropping method of eating rather than the active chewing required for carnivory. The bite force of this species is constrained by the cranial anatomy, and is also decreased in other derived therizinosaurs, additionally demonstrating a shift from carnivory in this unique clade of dinosaurs.

The Erlikosaurus andrewsi skull, along with the partial braincases from specimens of the therizinosaurid Nothronychus mckinleyi and the basal therizinosaur Falcarius utahensis have also been CT scanned. These scans were then used to describe the endocranial anatomy (inside the skull) of therizinosaurs, giving insight into the sensory abilities of the group. The results indicated that Erlikosaurus had considerable development and distinction of the olfactory system (required for smell). Calculations applied to the skulls also suggest a well-developed sense of hearing, with a best frequency of hearing range of 630-1630 Hz and a high frequency hearing limit between 2200-4000 Hz. The visual competences of therizinosaurians could not be estimated as accurately due to the lack of apparent optic lobes in the casts, however the large orbital socket length implies the presence of a large eye. These highly acute senses could have developed due to the change to herbivory, and may have served as useful for foraging, predator evasion, or social behaviours. Still, these senses may actually reflect the retention of ancestral adaptations, and so therizinosaurs may have exploited these for their newfound diet.

In conclusion, this remarkable skull has provided the key into understanding the diet, morphology and sensory abilities of the therizinosaur clade as a whole, and without its discovery, there would be a great lack of information regarding this strange group of dinosaurs.

Special thank you to Stephan Lautenschlager, whose insight and advice helped make this article possible.

Image References
[1] Erlikosaurus andrewsi illustration by Jack Wood.
[2] Erlikosaurus andrewsi 3D digital skull restoration. Available at: https://sketchfab.com/3d-models/erlikosaurus-andrewsi-4172d3c1bab5474fbf6fa5fe4f49a68a.

Information References and Further Sources
[1] Barsbold, R., and Perle, A. (1980). Segnosauria, A New Infraorder of Carnivorous Dinosaurs, Acta Palaeontologica Polonica, 25 (2), pp. 187-195. Available at: http://agro.icm.edu.pl/agro/element/bwmeta1.element.agro-84678ce2-a499-45ed-a29c-6f34e28fa28e/c/app25-187.pdf. Accessed 20th July 2019.
[2] Clark, J. M., Maryańska, T., and Barsbold, R. (2004). ‘Therizinosauroidea’, in Weishampel, D. B., Dodson, P., and Osmólska, H (2nd ed.) The Dinosauria. University of California Press. pp. 152
[3] Clark, J. M., Perle, A., and Norell, M. A. (1994). The Skull of Erlicosaurus andrewsi, a Late Cretaceus “Segnosaur” (Theropoda: Therizinosauridae) from Mongolia, American Museum Novitates, 3115 (39), pp. 1-39. Accessed 19th August 2019.
[4] Hedrick, B. P., Zanno, L. E., Wolfe, D. G., and Dodson, P. (2015). The Slothful Claw: Osteology and Taphonomy of Nothronychus mckinleyi and N. graffami (Dinosauria: Theropoda) and Anatomical Considerations for Derived Therizinosaurids, PLOS ONE. doi: https://doi.org/10.1371/journal.pone.0129449. Available at: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0129449. Accessed 20th July 2019.
[5] Lautenschlager, S. (2013). Cranial myology and bite force performance of Erlikosaurus andrewsi: a novel approach for digital muscle reconstructions, Journal of Anatomy, 222 (2), pp. 260–272. doi: https://doi.org/10.1111/joa.12000. Available at: https://onlinelibrary.wiley.com/doi/full/10.1111/joa.12000. Accessed 18th August 2019.
[6] Lautenschlager, S., Rayfield, E. J., Perle, A., Zanno, L. E., and Witmer, L. M. (2012). The Endocranial Anatomy of Therizinosauria and Its Implications for Sensory and Cognitive Function, PLOS ONE. doi: https://doi.org/10.1371/journal.pone.0052289. Available at: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0052289. Accessed 19th August 2019.
[7] Lautenschlager, S., Witmer, L. M., Perle, A., Zanno, L. E., and Rayfield, E. J. (2014). Cranial anatomy of Erlikosaurus andrewsi (Dinosauria, Therizinosauria): new insights based on digital reconstruction, Journal of Vertebrate Paleontology, 34 (6), pp. 1263-1291. doi: https://doi.org/10.1080/02724634.2014.874529. Available at: https://www.tandfonline.com/doi/full/10.1080/02724634.2014.874529. Accessed 20th July 2019.
[8] Zanno, L. E., and Makovicky, P. J. (2011). Herbivorous ecomorphology and specialization patterns in theropod dinosaur evolution, PNAS, 108 (1), pp. 232-237. doi: https://doi.org/10.1073/pnas.1011924108 Available at: https://www.pnas.org/content/108/1/232.short. Accessed 19th July 2019.