Nothronychus (meaning "slothful claw") is a genus of therizinosaurid theropod dinosaurs that lived in North America during the Late Cretaceous period.

It was recovered near New Mexico's border with Arizona, in an area known as the Zuni Basin, from rocks assigned to the Moreno Hill Formation, dating to the Turonian stage of the Late Cretaceous.

Nothronychus were bulky herbivorous theropods with wide, sloth-like hip (resembling that of the non-related ornithischians), four-toed feet with all four toes facing forward, elongated necks and prominent arms with sharp claws.

The first fossil evidence later attributed to Nothronychus was discovered by a team of paleontologists working in the Zuni Basin of New Mexico at the Haystack Butte site, Moreno Hill Formation.

The New Mexico team, led by paleontologists Jim Kirkland and Doug Wolfe, published their find in the Journal of Vertebrate Paleontology on 22 August 2001, making it the type specimen of the new species Nothronychus mckinleyi.

The holotype, specimen MSM P2106, consists of very sparse skull fragments, a braincase, some vertebrae and parts of the shoulder girdle, forelimbs, pelvis and hindlimbs.

The area around Big Water had been subject to several expeditions by teams from the Museum of Northern Arizona (MNA), and was known for its abundance of marine reptile fossils, especially plesiosaurs.

[3] During part of the late Cretaceous period, the region had been submerged under a shallow sea, the Western Interior Seaway, and preserves extensive marine deposits.

[4][5] The Utah specimen studied by the MNA team was found to be closely related to N. mckinleyi, though it differed in build (being heavier) and age (about half a million years older).

[3] The specimen was classified and named as the new species Nothronychus graffami by Lindsay Zanno and colleagues in the journal Proceedings of the Royal Society B on 15 July 2009.

[3] In 2015, Hedrick and colleagues conducted a large osteological revision of both species and their respective specimens concluding that Nothronychus was one of the most complete and well-known therizinosaurids.

Erlikosaurus Nothronychus mckinleyi Nothronychus graffami In 2009, Zanno and colleagues stated therizinosaurs were the most-widely regarded candidates for herbivory among theropods based on the small, densely packed, coarse serrations; lance-shaped teeth with a relatively low replacement rate; a well-developed keratinous beak; long neck for browsing; relatively small skulls; a very large gut capacity as indicated by the rib circumference at the trunk and the outwards flaring processes of the ilia; and the notable lack of cursorial adaptations in the hind limbs.

However, he could neither confirm nor disregard that the hand claws could have been fully used for sexual display, self-defense, intraspecific competition, mate-gripping during mating or grasping stabilization when foraging.

[17] In 2012, paleontologist Stephan Lautenschlager and team examined the endocasts (brain cavity) of several therizinosaurs (including N. mckinleyi), concluding that most members had well developed senses of smell, hearing, and balance, mostly useful for foraging.

These senses were also well-developed in earlier coelurosaurs and other theropods, indicating that therizinosaurs may have inherited many of these traits from their carnivorous ancestors and used them for their different and specialized dietary purposes.

They noted that the braincase has particularly large pneumatic chambers on the sensorial areas, suggesting that the increased tympanic systems would result in optimal low frequency sound reception, possibly infrasound, and in complex social behavior.

With lungs attached to the dorsal vertebrae, Nothronychus might have had a respiratory system similar to that of birds, probably having an "avian-style unidirectional airflow with cross-current blood/oxygen exchange."

Unlike birds, however, the clavicular air sac of Nothronychus might have been reduced or just absent, evidenced by the lack of pneumatic furcula or appendicular elements.

[20] Specimens of Nothronychus are known from the Moreno Hill Formation which documents a time of tectonic upheaval, volcanic activities, humid paleoclimate, and North American coastal margin shifts.