Nothronychus mckinleyi

Nothronychus mckinleyi was a specialized therizinosaurid from the middle Turonian Moreno Hill Formation of New Mexico. Although descended from predatory theropods, it combined a small head, long neck, broad gut, backward-directed pubis and coarse leaf-shaped teeth suited to a strongly plant-based or omnivorous diet. Its holotype, MSM P2117, is a partially disarticulated skeleton preserving teeth, a partial braincase, vertebrae and substantial parts of all four limbs and the pelvis. It became the first definitive therizinosaur recognized in North America and demonstrated that this predominantly Asian lineage had crossed into western North America by about 92 million years ago.

Its fossils occur between approximately 92 and 91 million years ago. Values shown here are approximate and reflect the current curated seed dataset.

Nothronychus had a small head on a long neck, a wide torso and a shortened tail. The preserved teeth are tall, narrow, leaf-shaped and coarsely serrated. Long arms ended in three strongly curved claws, while expanded hips and a backward-pointing pubis created room for a large digestive system. Both feet carried four functional forward-facing toes rather than the three-toed weight-bearing pattern of most theropods. The partial braincase is heavily air-filled and preserves canals for cranial nerves and the inner ear. A shaggy feather covering is plausible from related therizinosaurs but is not directly preserved with MSM P2117.

• Long arms ending in greatly enlarged, curved hand claws
• Tall, narrow and coarsely serrated leaf-shaped teeth
• Long neck and small head reconstructed from preserved and related anatomy
• Broad pelvis with a backward-directed pubis and deep abdominal space
• Four functional forward-facing toes on each foot
• Strongly pneumatic braincase containing extensive air spaces
• Holotype preserving substantial parts of the limbs, pelvis and vertebral column

A field team led by James Kirkland and Douglas Wolfe excavated the specimen during the early to mid-1990s at Haystack Butte in the southern Zuni Basin. One unusual ischium was initially mistaken for a skull bone of Zuniceratops from the same bonebed; additional discoveries revealed a giant-clawed theropod instead. Kirkland and Wolfe named Nothronychus mckinleyi in 2001. The genus combines Greek words for slothful and claw, referring to its sloth-like appearance, and the species honours rancher Bobby McKinley. A detailed 2015 study redescribed the skeleton and showed that river currents had transported, sorted and concentrated its bones with logs and other animals.

Discovery credit: James I. Kirkland, Douglas G. Wolfe and field team.

Naming authors: James I. Kirkland, Douglas G. Wolfe.

Nothronychus lived on the humid river floodplains of the Moreno Hill Formation near the western shore of the Western Interior Seaway. Conifers, ferns and early flowering plants supported large herbivores, including the horned dinosaur Zuniceratops, while tyrannosauroids and other predators shared the ecosystem. The holotype accumulated in a current-sorted bonebed containing plant debris and bones from several animals. The map marker represents the generalized Haystack Butte region rather than a protected quarry coordinate.

Its broad torso, retroverted pubis and leaf-shaped teeth are consistent with a large fermentation chamber and a mainly plant-based diet, although small animals or insects may also have been eaten. The long claws could have drawn branches inward, defended the animal or served in display, but no preserved trace identifies one exclusive use. The inner ear and braincase have prompted hypotheses about posture and hearing; claims of specialized low-frequency communication remain tentative. There is no direct evidence for herding, nesting or social care.

• A hip bone was first mistaken for part of Zuniceratops' skull crest.
• It was the first definitive therizinosaur discovered in North America.
• Despite belonging to Theropoda, its skeleton records a major shift toward eating plants.
• The holotype's bones were transported and aligned by river currents before burial.
• Digital scans of many bones were published with the detailed 2015 description.

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1. 01
   First definitive therizinosaurid (Dinosauria; Theropoda) from North America

   James I. Kirkland, Douglas G. Wolfe · Journal of Vertebrate Paleontology 21(3), 410-414 · 2001

   Open source
2. 02
   The Slothful Claw: Osteology and Taphonomy of Nothronychus mckinleyi and N. graffami (Dinosauria: Theropoda) and Anatomical Considerations for Derived Therizinosaurids

   Brandon P. Hedrick, Lindsay E. Zanno, Douglas G. Wolfe, Peter Dodson · PLOS ONE 10(6):e0129449 · 2015

   Open source
3. 03
   A re-evaluation of the basicranial soft tissues and pneumaticity of the therizinosaurian Nothronychus mckinleyi (Theropoda; Maniraptora)

   David K. Smith, R. Kent Sanders, Douglas G. Wolfe · PLOS ONE 13(7):e0198155 · 2018

   Open source
4. 04
   Nothronychus dinosaur directory entry

   Natural History Museum, London

   Open source
5. 05
   Arizona Museum of Natural History research and collections

   Arizona Museum of Natural History

   Open source