| Nothronychus | |
|---|---|
 | |
| Reconstructed skeletons ofN. mckinleyi (right) andN. graffami (left) | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Superfamily: | †Therizinosauroidea |
| Family: | †Therizinosauridae |
| Genus: | †Nothronychus Kirkland & Wolfe,2001 |
| Type species | |
| †Nothronychus mckinleyi Kirkland & Wolfe, 2001 | |
| Other species | |
Nothronychus (meaning "slothful claw") is agenus oftherizinosauridtheropoddinosaurs that lived inNorth America during theLate Cretaceous period. Thetype species,Nothronychus mckinleyi, was described byJames Kirkland and Douglas G. Wolfe in2001. It was recovered nearNew Mexico's border withArizona, in an area known as theZuni Basin, from rocks assigned to theMoreno Hill Formation, dating to theTuronian stage of theLate Cretaceous. A second specimen, described in 2009 as a second species,Nothronychus graffami, was found in theTropic Shale ofUtah, dating between one and a half million years older thanN. mckinleyi.
Like other therizinosaurids,Nothronychus was likely a bulkyherbivore with wide, sloth-like hips (resembling those of the unrelatedornithischians), feet with forward-facing four toes, elongated necks, and prominent arms with sharp claws. Both species were similar in dimensions, butN. graffami was slightly more robust thanN. mckinleyi.
The first fossil evidence later attributed toNothronychus was discovered by a team of paleontologists working in the Zuni Basin ofNew Mexico at the Haystack Butte site,Moreno Hill Formation. A therizinosaurischium (a hip bone) had originally been mistaken for asquamosal, a part of the skull crest of the newly discoveredceratopsianZuniceratops. 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 theJournal of Vertebrate Paleontology on 22 August 2001, making it thetype specimen of the new speciesNothronychus mckinleyi.[2] TheArizona Republic newspaper, however, was first to announce the name on 19 June 2001, in a column byR.E. Molnar. The generic name,Nothronychus, is derived fromGreek νωθρός (nothros, meaning slothful) and ὄνυξ (onyx, meaning claw). Thespecific name,mckinleyi, honours rancher Bobby McKinley on whose land the fossil findings were made. Theholotype, specimenMSM P2106, consists of very sparse skull fragments, a braincase, some vertebrae and parts of the shoulder girdle, forelimbs, pelvis and hindlimbs.[2]
A second, more complete specimen,UMNH VP 16420, was discovered from theTropic Shale Formation (dating to the early Turonian stage) of southern Utah in 2000 by Merle Graffam, a resident ofBig Water, Utah. The area around Big Water had been subject to several expeditions by teams from theMuseum of Northern Arizona (MNA), and was known for its abundance of marine reptile fossils, especiallyplesiosaurs.[3] During part of the late Cretaceous period, the region had been submerged under a shallow sea, theWestern 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 kilometers 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 fromChina andMongolia.[2] Between both species,N. graffami is the most complete but lacks the skull.[4][5]
The Utah specimen studied by the MNA team was found to be closely related toN. mckinleyi, though it differed in build (being heavier) and age (about half a million years older).[3] The MNA specimen was first announced in two 2002 talks during the 54th meeting of the Rocky Mountain Geological Society of America.[6][7] It was later discussed in an issue ofArizona Geology as a distinct species fromN. mckinleyi, but not named.[3] The specimen was classified and named as the new speciesNothronychus graffami byLindsay Zanno and colleagues in the journalProceedings of the Royal Society B on 15 July 2009.N. graffami was named for Graffam, who discovered the original specimens.[4] A reconstructed skeleton ofN. graffami went on display at the MNA in September 2007.[3] In 2015, Hedrick and colleagues conducted a large osteological revision of both species and their respective specimens concluding thatNothronychus was one of the most complete and well-known therizinosaurids.[8]
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).[9][10]Nothronychus had large "pot-bellied" abdomens, long necks, and stockyhindlimbs with four-toedfeet. 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.[8]
The genusNothronychus can be distinguished from other therizinosaur taxa based on several features: a distinctly subcircular obturator process, an elongated obturatorforamen towards the bottom, the contact area between pubis andischium 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.[8]
Both species,N. mckinleyi andN. graffami, are known from mature specimens of similar size, as evidenced by the fusion of the neural spines and scapulocoracoid.[11] They have roughly equal sizedhumeri (upper arm), measuring 41.5 cm (16.3 in) and 42.4 cm (16.7 in) in length respectively, thoughN. mckinleyi was different fromN. graffami in being slightly less robust as well as details of the tail vertebrae, and a more bentulna (lower arm bone).[8]
Nothronychus were members of theCoelurosauria, the theropod group composed mainly by carnivorous dinosaurs. However, more specifically,Nothronychus form part of the sub-groupManiraptora, theropods which evolved into partial omnivores and, in the case ofNothronychus and their family, plant-eaters.[4]Nothronychus mckinleyi was in 2001 assigned to theTherizinosauridae given the derived features of the genus.[2] In 2010,Lindsay E. Zanno performed a large and comprehensive analysis of theTherizinosauria. Thecladistic analysis performed recovered bothN. mckinleyi andN. graffami as asister group.[5] Most of the data provided by Zanno was used by Hartman and colleagues in 2019 during an extensive phylogenetic analysis for the Coelurosauria.Nothronychus was recovered as a therizinosaurid taxon with both species in derived positions. Below are the obtained results for theTherizinosauridae:[12]
| Therizinosauridae |
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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. All of these features suggest that members of this family feed on vegetation, as well as pre-processing it within their mouths to begin the breakdown of cellulose and lignin. This is perhaps even more so true for therizinosaurids, which seem to have further exploited these characters. One of the most notable adaptations in advanced therizinosaurids are the four-toed feet, which had a fully functional, weight-bearing first digit that was likely adapted to slow life-style. Zanno and colleagues found that Ornithomimosauria, Therizinosauria, and Oviraptorosauria had either direct or morphological evidence for herbivory, which would mean either this diet evolved independently multiple times in coelurosaurian theropods or that the primitive condition of the group was at least facultative herbivory with carnivory only emerging in more derived maniraptorans.[4]
In 2014, Lautenschlager tested thebiomechanical function of multiple therizinosaur claws. He noted that the hands of some therizinosaurids (such asNothronyhus orTherizinosaurus) were more effective when piercing or pulling down vegetation. The arms would have had to be able to extend the range of the animal to a point that could not be reached by the head if they were used for browsing and pulling down vegetation. In genera where both neck and forelimb elements are preserved, however, the necks were equal in length or longer than the forelimbs, so pulling vegetation would only be likely if lower parts of long branches were pulled down to access out-of-reach vegetation. Lautenschlager also found that therizinosaurid claws would not have been used for digging, which would have been done with the foot claws because, since as in other maniraptorans,feathers on the forelimbs would have interfered with this function. Additionally, this action leads to a higher stress tension on the dorsal area of the claw−this is more evident inTherizinosaurus. However, he could neither confirm nor disregard that the hand claws could have been fully used forsexual display, self-defense,intraspecific competition, mate-gripping duringmating or grasping stabilization whenforaging.[13] In 2024, Smith and Gillette claimed thatNothronychus is herbivorous on the basis of retroverted pubis, analogous with the postacetabular bar of ornithischians, and that many characters ofNothronychus including the pubic condition are absent in earlier therizinosaurs likeFalcarius, so such structures would have evolved multiple times throughout the maniraptoran lineage.[11]
In 2015, Smith studied the myology and functional morphology of the craniocervical junction ofN. mckinleyi. The craniocervical muscles can be subdivided into functional groups as dorsiflexors, lateroflexors, and ventroflexors. Lateroflexors and dorsiflexors inNothronychus are relatively reduced but are still well-developed, while ventroflexors allow increased ventroflexion. With small neural spines relative to the long S-shaped neck, its neck pattern was similar to that ofostriches andDiplodocus. Its individual centra didn't have high degree of freedom and likely had little apparent movement, meaning that the total neck movement would have only been resulted by the combined effect of all cervical vertebrae.[14]
In 2021, he also reconstructed the forelimb musculature of both species in the same year. As predicted for derived non-avian theropod dinosaurs, the forelimbs likely had no increase in range of motion within them. The distal humerus contained a well-developed fossa brachialis, and the proximal humerus may have had a fossa pneumotricipitales; the deflected epicleidium of the furcula is a development yet to be described in other theropods. Other forelimb musculature were similar todromaeosaurs, such as thepectoral girdle being similar to that ofvelociraptorines.[15]
In the same year, Smith reconstructed the hindlimb musculature ofN. graffami.Nothronychus had a pelvis that is incipiently opisthopubic, which Smith expects to be associated with the muscular system's extensive modification. His reconstruction of the muscular system revealed that the musculature ofNothronychus to a certain extent is convergent with birds because of the retroverted pubis that becomes fused with the ischium, and because of the synsacrum and pes similar to that of birds. It also showed that its musculature was not only convergent with birds, but also with ornithischians, indicating that some maniraptorans' retroverted pubis and the ornithischians' postacetabular bar are analogous structures.[16] A 2023 study by Smith and Gillette suggests that much of the walking motion forN. graffami took place at the knee when moving, with movement at the femur limited. This same study also suggests a waddling gait forNothronychus, and a digitigrade stance, though plantigrade walking could not entirely be ruled out.[17]
In2012, paleontologistStephan Lautenschlager and team examined the endocasts (brain cavity) of several therizinosaurs (includingN. mckinleyi), concluding that most members had well developed senses of smell, hearing, and balance, mostly useful forforaging. The enlarged forebrain of therizinosaurs may also have been useful in complexsocial behavior andpredator evasion. 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.[18]
In 2018, the holotype braincase ofNothronychus mckinleyi was re-examined by Smith and colleagues updating numerous basicranial and soft-tissues aspects. They noted that the braincase has particularly largepneumatic chambers on the sensorial areas, suggesting that the increasedtympanic systems would result in optimallow frequency sound reception, possiblyinfrasound, and in complex social behavior. The enlargedcochlea and presence of enlarged pneumatic chambers near the middle ear also supports this insight. Smith and colleagues established an average hearing frequency of 1100 to 1450Hz and upper limits of 3000 to 3700 Hz. They stated, however, that these estimates could be slightly exaggerated. Also,N. mckinleyi retained elongatedsemicircular canals in theear, which are more related to an active, predatory life-style. This trait indicates that most therizinosaurids (possibly all therizinosaurs) retained the ancestral, carnivorous ear configuration−previously suggested forErlikosaurus. Finally, based onErlikosaurus,N. mckinleyi may have had a relatively horizontal head posture, which is associated with overlapping visual fields andbinocular vision, given the orientation of the horizontal semicircular canal relative to the horizontal orientation of theoccipital condyle.[19]
In 2021, Smith and colleagues examined the pneumaticity of both species' specimens. Theaxial skeleton ("the basicranium, the presacralvertebrae, and the proximal caudal vertebrae") shows extensive pneumatization, meaning that this body part ofNothronychus had extensiveair sacs. Thesynsacrum andilium is not pneumatized, so the air sac may have bypassed the synsacrum. The researchers considered that the pneumatization in therizinosaurs wouldn't have been related to weight reduction orthermoregulation, but to an avian-stylerespiratory system. 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 ofNothronychus might have been reduced or just absent, evidenced by the lack of pneumatic furcula or appendicular elements. Its ribs also show no sign ofuncinate processes.[20]
Specimens ofNothronychus are known from theMoreno Hill Formation which documents a time of tectonic upheaval, volcanic activities, humid paleoclimate, and North American coastal margin shifts.[1] Other dinosaurs fossils recovered from this formation areSuskityrannus,Zuniceratops,Jeyawati, and undescribedankylosaur remains.[21] Three groups of turtle fossils have been reported: abaenidEdowa, ahelochelydridNaomichelys and an indeterminatetrionychid.[22] Other vertebrate fossils includecrocodyliform teeth,amiid teeth andgar scales.[23][22]
Text was copied from this source, which is available under aCreative Commons Attribution 4.0 International License.
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