Nothronychus Research Articles

Osteology of the derived Therizinosaur Nothronychus with evidence for convergence in dinosaurian evolution

Abstract Therizinosaurs were a taxon of unusual theropods from North America and Asia. The derived therizinosaur Nothronychus graffami possessed a synsacrum and hip convergent with extant birds. The osteology is figured and described in detail supplemented with material from Nothronychus mckinleyi. Both species exhibit traits convergent with extant birds, ornithischian dinosaurs, and titanosaurs. As preserved, the ilia and sacrum are minimally distorted and apomorphically modified into a synsacrum. The pubes are modified into a mesopubic condition, wherein the pubes are partially retroverted. The femoral shaft has an oval cross-section. As these characters are absent from the basal therizinosaur Falcarius utahensis, they evolved more than once within the maniraptoran lineage. An herbivorous ecology is supported. Both specimens were skeletally mature, based on fusion of the neural spines and scapulocoracoid.

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

The soft-tissue reconstruction and associated osteology of the North American therizinosaurian Nothronychus mckinleyi is updated. The cranial nerve topology is revised, bringing it more in line with coelurosaurs. The trunk of the trigeminal nerve is very short, with an incompletely intracranial trigeminal ganglion, an ophthalmic branch diverging anteriorly first, with later divergences of the maxillomandibular branches, following typical pathways. The facial nerve has been re-evaluated, resulting in a very typical configuration with an extracranial geniculate ganglion. The single foramen leading to the cochlea probably transmitted the vestibulocochlear nerve, along with some fibers of the facial. This configuration is reduced from the more standard three foramina (vestibular, cochlear, and facial) and may be apomorphic for therizinosaurs. Some alteration is proposed for the dorsiflexive musculature. The insertion point for m. transversospinalis capitis is partially changed to extend onto the parietal, along with a proposed functional difference in the moment arm. The expansion of the basicranial pneumatic system is limited to the paratympanic system, enhancing low frequency sound sensitivity. There is little expansion of the median pharyngeal and subcondylar sinuses. Ossification of the surrounding epithelium may provide some information on the embryology of the theropod skull. It may be associated with a reduced stress field, or the general similarity of the basicranium with anterior cervical vertebrae may reflect activation of a cervical vertebral (Hox) gene regulating ossification of the pneumatic sinuses. This might be a local, selectively neutral, fixed gene in the basicranium reflecting embryological regulation of cervical vertebrae development.

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

Nothronychus was the first definitive therizinosaurian discovered in North America and currently represents the most specialized North American therizinosaurian genus. It is known from two species, No. mckinleyi from the Moreno Hill Formation (middle Turonian) in west-central New Mexico, and No. graffami from the Tropic Shale (early Turonian) in south-central Utah. Both species are represented by partial to nearly complete skeletons that have helped elucidate evolutionary trends in Therizinosauria. In spite of the biogeographical and evolutionary importance of these two taxa, neither has received a detailed description. Here, we present comprehensive descriptions of No. mckinleyi and No. graffami, the latter of which represents the most complete therizinosaurid skeleton known to date. We amend previous preliminary descriptions of No. mckinleyi and No. graffami based on these new data and modify previous character states based on an in-depth morphological analysis. Additionally, we review the depositional history of both specimens of Nothronychus and compare their taphonomic modes. We demonstrate that the species were not only separated geographically, but also temporally. Based on ammonoid biozones, the species appear to have been separated by at least 1.5 million years and up to 3 million years. We then discuss the impacts of diagenetic deformation on morphology and reevaluate potentially diagnostic characters in light of these new data. For example, the ulna of No. mckinleyi is curved whereas the ulna of No. graffami was considered straight, a character originally separating the two species. However, here we present the difference as much more likely related to diagenetic compression in No. graffami rather than as a true biologic difference. Finally, we include copies of three-dimensional surface scans of all major bones for both taxa for reference.

Craniocervical myology and functional morphology of the small-headed therizinosaurian theropods Falcarius utahensis and Nothronychus mckinleyi.

Therizinosaurs represent a highly unusual clade of herbivorous theropods from the Cretaceous of North America and Asia. Following descriptions of the basicrania of the North American therizinosaurs Falcarius utahenisis and Nothronychus mckinleyi, the craniocervical musculature in both taxa is reconstructed using Tyrannosaurus, Allosaurus, and some extant birds as models. These muscles are subdivided into functional groups as dorsiflexors, lateroflexors, and ventroflexors. Lateroflexors and dorsiflexors in Nothronychus, but not Falcarius, are reduced, from the plesiomorphic theropod condition, but are still well developed. Attachments in both genera are favorable for an increase in ventroflexion in feeding, convergent with Allosaurus fragilis. Falcarius and Nothronychus are both characterized by a flat occipital condyle, followed by centra with shallow articular facets suggesting neck function very similar to that of an ostrich Struthio camelus. Neck movement was a combined result of minimal movement between the individual cervical vertebrae.

The braincase of the North American therizinosaurian Nothronychus mckinleyi (Dinosauria, Theropoda)

ABSTRACTThe description and comparison of the posterior braincase of Nothronychus mckinleyi (MSM P-2117) from the Upper Cretaceous (Turonian) Moreno Hills Formation of east-central New Mexico is updated. As a result of the enlarged basicranial pneumatic system, this region of the skeleton has undergone extensive rearrangement of the cranial nerves and blood vessels from the plesiomorphic theropod condition. Nothronychus and Falcarius possessed a number of avian-associated characters or avian-trending characters in the braincase and endocranium characteristic of highly derived coelurosaurs. These traits, in some cases hypothesized, include the posterior endocranium nearly filled with nervous tissue, a laterally directed enlarged optic tectum, an enlarged flocculus, an intracranial trigeminal ganglion, and the abducens canal entering the endocranium separate from the infundibulum with no associated cavernous sinus. The angle of the occipital plate with the basal plate of the basicranium appears to be perpendicular and, therefore, intermediate between more basal theropods and extant birds. However, the posterior endocranial cavity does not appear flexed, which would result in a posteroventral rotation of the endocranial cavity in Nothronychus, a character also seen in the Cretaceous hesperornithiform bird Enaliornis and in contrast to extant birds. Therefore, the evolution of the coelurosaurian brain and braincase was trending in an avian direction before the rest of the skeleton. The results presented here support the model that therizinosaurs possessed weak bite forces as compared to obligatory carnivorous theropods.

New information on the braincase of the North American therizinosaurian (Theropoda, Maniraptora) Falcarius utahensis

ABSTRACT Many disarticulated bones from multiple individuals of a primitive therizinosaurian, referred to Falcarius utahensis, were found in the paucispecific Crystal Geyser bonebed in the Lower Cretaceous Cedar Mountain Formation of eastern Utah. To date, more than 2000 specimens from this species have been excavated. Included in this collection are two partial braincases, one of which is designated the holotype. Here we describe the braincase morphology of Falcarius utahensis. These specimens help establish the primitive cranial condition for the Therizinosauria and further substantiate intraspecific and contralateral braincase pneumatic variation in theropods. When combined with new observations on the cranial remains of the therizinosaurid Nothronychus mckinleyi derived from computed tomographic (CT) scans, the braincase morphology of Falcarius clarifies several evolutionary trends within the Therizinosauria and establishes a suite of synapomorphies for the Therizinosauridae. Trends within the clade include increased basicranial pneumatization (the development of a basisphenoid bulla and loss of external subcondylar recesses), anterior deflection of the supraoccipital, and the reduction of points of origin of the craniocervical musculature, associated with the loss of discrete basipterygoid processes, probably due to incorporation of these structures into the expanded hyperpneumatic bone. Finally, CT scans reveal a complete, nearly avian, inner ear with bird-like semicircular canals and a long cochlea indicating broad frequency discrimination.