Nothronychus (meaning "slothful claw") is a genus of therizinosaurid theropod dinosaurs that lived in North America during the Late Cretaceous period. The type species, Nothronychus mckinleyi, was described by James Kirkland and Douglas G. Wolfe in 2001. 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. A second specimen, described in 2009 as a second species, Nothronychus graffami, was found in the Tropic Shale of Utah, dating to between one million and 500,000 years older than N. mckinleyi. Like other therizinosaurids, Nothronychus was likely a bulky herbivore with wide, sloth-like hips (resembling those of the unrelated ornithischians), feet with forward-facing four toes, elongated necks, and prominent arms with sharp claws. Both species were similar in dimensions, but N. graffami was slightly more robust than N. mckinleyi.
History of discovery
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. A therizinosaur ischium (a hip bone) had originally been mistaken for a squamosal, a part of the skull crest of the newly discovered ceratopsian Zuniceratops. However, closer examination revealed the true identity of the bone, and soon more parts of the skeleton were found. 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 Arizona Republic newspaper, however, was first to announce the name on 19 June 2001, in a column by R.E. Molnar. The generic name, Nothronychus, is derived from Greek νωθρός (nothros, meaning slothful) and ὄνυξ (onyx, meaning claw). The specific name, mckinleyi, honours rancher Bobby McKinley on whose land the fossil findings were made. 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.
A second, more complete specimen, UMNH VP 16420, was discovered from the Tropic Shale Formation (dating to the early Turonian stage) of southern Utah in 2000 by Merle Graffam, a resident of Big Water, Utah. 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. 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. Graffam's initial discovery (a large, isolated toe bone) came as a surprise to colleagues, as it clearly belonged to a land-dwelling dinosaur, rather than a plesiosaur. However, the location of the bone at the time would have been nearly 100 kilometres (62 miles) from the Cretaceous shoreline. An excavation of the area by an MNA crew revealed more of the skeleton, and the scientists found that it was a therizinosaur, and the first example of that group to be found in the Americas. All previous therizinosaur fossils had come from China and Mongolia. Between both species, N. graffami is the most complete but lacks the skull.
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). The MNA specimen was first announced in two 2002 talks during the 54th meeting of the Rocky Mountain Geological Society of America. It was later discussed in an issue of Arizona Geology as a distinct species from N. mckinleyi, but not named. 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. N. graffami was named for Graffam, who discovered the original specimens. A reconstructed skeleton of N. graffami went on display at the MNA in September 2007. 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.
Description
Nothronychus is a medium-sized therizinosaur, measuring 4.2–5.3 m (14–17 ft) long and weighing 800–1,200 kg (1,800–2,600 lb). Nothronychus had large "pot-bellied" abdomens, long necks, and stocky hindlimbs with four-toed feet. Its arms were relatively large with dexterous hands equipped with up to 30 cm (12 in) long curved and sharply-pointed claws on their fingers. In addition, the tail was reduced in length but more flexible. The genus Nothronychus can be distinguished from other therizinosaur taxa based on several features: a distinctly subcircular obturator process, an elongated obturator foramen towards the bottom, the contact area between pubis and ischium restricted to the upper half of the obturator projection, and deep cut between towards the bottom facet of the obturator process and frontal ischial shaft.
Both species, N. mckinleyi and N. graffami, are known from mature specimens of similar size, as evidenced by the fusion of the neural spines and scapulocoracoid. They have roughly equal sized humeri (upper arm), measuring 41.5 cm (16.3 in) and 42.4 cm (16.7 in) in length respectively, though N. mckinleyi was different from N. graffami in being slightly less robust as well as details of the tail vertebrae, and a more bent ulna (lower arm bone).
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