Nothronychus

Credit: PaleoNeolitic on the Wikimedia Commons.

*Nothronychus* is an extinct genus of therizinosaurid that lived in North Americs during the Late Cretaceous.

## History

*Nothronychus* was discovered by paleontologists in the Zuni Basin, Haystack Butte site, Moreno Hill Formation, New Mexico. An ischium was originally misidentified as a squamosal, part of the newly-identified *Zuniceratops*. However, the bone was soon identified and more therizinosaur elements were discovered. Published in the Journal of Vertebrate Paleontology on the 22th of August, 2001, the New Mexican team, led by Jim Kirkland and Doug Wolfe, published and named the remains. However, The Arizona Republic first announced the name on June 19, 2001 based on a column written by R.E. Molnar. *Nothronychus* derives from the Greek νωθρός (nothros), "slothful" and ὄνυξ (onyx), "claw". "*Mckinleyi*" refers to Bobby McKinley, a rancher whose land brought the *Nothronychus* remains. MSM P2106, the holotype, consists of a partial skull, a braincase, several vertebrae, parts of the shoulder girdle, forelimbs, a pelvis and hindlimbs.

UMNH VP 16420, the second and more complete specimen, was discovered from the Tropic Shale Formation (Early Turonian), southern Utah by Merle Graffam in 2000. This area had been a subject of several expeditions from the Museum of Northern Arizona, known for its abundance of marine reptiles, especially plesiosaurs. This area had been submerged in a shallow sea (the Western Interior Seaway) during the Late Cretaceous. Graffam had found a single large toe bone, which was surprising to colleagues, since it was clearly of a large terrestrial dinosaur. However, at that time, the bone would have been ~100 kilometers from the shoreline. The MNA crew excavated more of the area and found more of the skeleton, the first therizinosaur from the Americas. *N. graffami* was erected, being the most complete, but lacking a skull.

The MNA specimen was closely related to *N. mckinleyi*, but it was heavier and ~0.5 million years older. It was first announced in 2002 during the 54th Rocky Mountain Geological Society of America meeting, later discussed in an issue of Arizona Geology. It was referenced as distinct, but not assigned a name. Lindsay Zanno et al. named *N. graffami*, and was put on display at MNA in September 2007. Hedrick et al. (2015) conducted a large osteological revision of *Nothronychus*, concluding the genus was the most complete therizinosaurid.

## Description

The genus has several autapomorphies: a prominent subcircular obturator formation, an elongate obturator foramen, the pubis and ischium constrict at the upper obturator, deep-cutting towards the lower facet of the obturator process and the the frontal ischial shaft. Both species are similarly-sized, having similar humeri (41.5-42.4 centimeters, 415-424 millimeters respectively). Nothronychus is pot-bellied, with long necks, stocky 4-toed hindlimbs and robust arms tipped with three 30 centimeter (300 millimeter) long scythe-like claws and dexterous hands. The tail is shortened, but more flexible, and in N. mckinleyi is more robust in the tail, less vertebrae detail and a more bent ulna than N. graffami.

## Classification

*Nothronychus* is a coelurosaur, and further a maniraptoran. Though most coelurosaurs are carnivorous, some maniraptors evolved omnivory, and therizinosaurids herbivory. *N. mckinleyi* was assigned Therizinosauridae in 2001 based on derived features. Lindsey Zanno (2010) performed a large, comprehensive analysis of Therizinosauria, recovering both species as a sister grouping. Most of the data Zanno used was also used by Hartman et al. (2019) when analyzing coelurosaurs. *Nothronychus* was a therizinosaurid with both species assuming a derived position. The results:

## Paleobiology

In 2018, the holotype's braincase was reassessed by Smith et al., noting it had a large pneumatic chambers at the sensorial locations, which would cause increased tympanic systems and, in return, could receive low frequency sounds, likely infrasound. This suggests a complex social life. The increased size in the cochlea and pneumatic chambers also support this observation. They also determined the average frequency of hearing in *Nothronychus* to be between 1100 and 1450 Hz, and upper estimates being from 3000 to 3700 Hz. However, they plainly stated that these estimates could be over exaggerated. *N. mckinleyi* possesses semicircular canals in their ears, which is usually a feature in active predators. This means that all therizinosaurid may have kept their ancestry hearing configuration, as suggested in *Erlikosaurus*. *N. mckinleyi* may have had a relatively horizontal head posture, which is associated with binocular vision and overlapping visual fields. Binocular vision can be determined via horizontal semicircular canals relative to the horizontal orientation of the occipital condyle.

## References

https://en.wikipedia.org/wiki/Nothronychus