Aardonyx

Aardonyx (Afrikaans aard, "earth" + Greek onux, "nail, claw") is a genus of basal sauropodomorph dinosaur. It is known from the type species Aardonyx celestae found from the Lower Jurassic Elliot Formation of South Africa. A. celestae was named after Celeste Yates, who prepared much of the first known fossil material of the species. It has arm features that are intermediate between prosauropods and sauropods.[1]

Based on the structure of the hind limbs and pelvic girdle of Aardonyx, the dinosaur normally moved bipedally but could drop to quadrupedal movement similar to Iguanodon. It shares some attributes with giant quadrupedal sauropods like Apatosaurus.[2] Australian[3] paleontologist Adam Yates and his team's discovery of the genus was published online before print in Proceedings of the Royal Society B in November 2009, and was scheduled to appear in the March 2010 issue.[2] British paleontologist Paul Barrett of the Natural History Museum, London, who was not involved in the research, commented that the discovery of Aardonyx "helps to fill a marked gap in our knowledge of sauropod evolution, showing how a primarily two-legged animal could start to acquire the specific features necessary for a life spent on all-fours".[2]

According to Dr. Matthew Bonnan, a co-author on the study, "We already knew that the earliest sauropods and near-sauropods would be bipeds. What Aardonyx shows us, however, is that walking quadrupedally and bearing weight on the inside of the foot is a trend that started very early in these dinosaurs, much earlier than previously hypothesized." Bonnan adds, "On a scientific level, it's really fulfilling to have a hypothesis on how you think dinosaurs got large, then to test that in the field and get back these kind of data — a new dinosaur — that really does start to fill in some of those anatomical gaps."

Aardonyx
Temporal range: Early Jurassic, 195 Ma
Aardonyx NT
Restoration
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Order: Saurischia
Suborder: Sauropodomorpha
Clade: Anchisauria
Genus: Aardonyx
Yates et al., 2010
Species:
A. celestae
Binomial name
Aardonyx celestae
Yates et al., 2010

Description

Fig 2 - Aardonyx life restoration by Matthew Bonnan
Aardonyx compared to a human in size

The genus is known from disarticulated bones belonging to two immature individuals. The material consists of cranial elements, vertebrae, dorsal and cervical ribs, gastralia, chevrons, elements of the pectoral and pelvic girdles, and bones of the fore and hind limbs, manus, and pes. The presence of these bones in a single dense accumulation in a localized channel fill suggests that they came from relatively complete carcasses.[4] Both individuals are thought to have been less than 10 years old at the time of their death because of the lack of peripheral rest lines in the cortices of sampled bones. Additional evidence for immaturity at the time of death includes calcified cartilage at the articular end of the scapula.[1]

Classification

Sauropodomorpha 

 Plateosauridae 

 Riojasauridae 

 Gryponyx 

 Massospondylidae 

 Yunnanosaurus 

 Jingshanosaurus 

 Anchisauria 

 Anchisaurus 

 Aardonyx 

 Melanorosaurus 

 Sauropoda 

 Antetonitrus 

 Lessemsaurus 

 Gongxianosaurus 

   

 Tazoudasaurus 

 Vulcanodon 

 Eusauropoda

Cladogram showing the position of Aardonyx within Sauropodomorpha
after Yates et al., in print.[1]

Aardonyx is thought to be the sister taxon of a sauropodomorph clade containing Melanorosaurus and sauropods, which are all obligatory quadrupeds, based on a phylogenetic analysis conducted along with the first description of the genus. Many features of the skeleton support this relationship. These include derived traits seen in the vertebrae (such as hyposphenes that are as deep as the neural canal and mid-cervical neural spines that are less than twice as long as high) as well as the appendicular skeleton (such as the position of the fourth trochanter over the midlength of the femur and an adult femur length exceeding 600 mm).[1]

Paleobiology

Feeding

Aardonyx shows a transition toward the bulk-browsing form of feeding characteristic of sauropods. The jaws of Aardonyx are narrow and V-shaped with a pointed symphysis, a plesiomorphic characteristic shared with other basal sauropodomorphs. In sauropods, the jaws are broad and U-shaped to allow for a wider bite. The absence of a lateral ridge at the caudal end of the dentary is indicative of a loss of fleshy cheeks. This is seen as an adaptation for a wider gape to facilitate in bulk browsing, and is observed in nearly all sauropods. The lateral neurovascular foramina of the maxilla of Aardonyx are smaller than those of other basal sauropodomorphs, and indicate that there was a reduction in blood supply to the buccal tissues and thus a loss of fleshy cheeks. The development of lateral plates along the alveolar margins of some bones of the skull would have helped brace the lingual sides of the teeth against bucco-lingual forces during foliage stripping.

The presence of plesiomorphic V-shaped jaws along with the absence of fleshy cheeks is an unusual characteristic of Aardonyx. Previously, it was thought that broader jaws evolved prior to the reduction and loss in fleshy cheeks as an adaptation toward bulk-browsing in sauropods. The sauropod Chinshakiangosaurus possessed jaws that were U-shaped, while still retaining fleshy cheeks, the opposite of the condition seen in Aardonyx.[5] Because Chinshakiangosaurus is a more derived sauropodomorph, this suggests that a wide, cheekless gape may have evolved twice in Sauropodomorpha: once with Aardonyx and again with sauropods more advanced than Chinshakiangosaurus.[1]

Locomotion

Characteristics of the limbs of Aardonyx suggest that it was habitually bipedal. Evidence for bipedalism can be seen in the forelimbs; the structure of the radius and ulna limited the degree to which the manus could be pronated, and the length of the humerus is only 72 percent that of the femur. However, characteristics found in both the fore and hind limbs of Aardonyx show a trend toward more habitual quadrupedalism that would eventually lead to the obligatory quadrupedalism seen in sauropods. The proximal end of the ulna possesses an incipient craniolateral process that gives the bone a y-shape similar to, although more subtle than, those of obligatory quadrupedal sauropodomorphs. The radius is shifted cranially, and a radial fossa allows for the ulna to cradle the radius craniolaterally. These characteristics suggest that there was a development towards greater quadrupedalism in Aardonyx. Although the hindlimbs of Aardonyx clearly show evidence for bipedalism (such as the retention of a convex proximal lateral profile of the femur and the position of the cranial trochanter far from the lateral margin of the femur), there is also evidence that indicates a shift toward quadrupedalism. Features of the femur suggests that the gait of Aardonyx was slower than that of more basal sauropodomorphs. The shaft of the femur is straighter and the fourth trochanter is more distally placed. The repositioning of the fourth trochanter to a more distal position causes the M. caudofemoralis longus muscle, the main femoral retractor muscle, to have greater leverage (more mechanical advantage) but conversely a decrease in the velocity of femoral retraction; consequently, Aardonyx was a powerful but slower walker than typical prosauropods.

Another characteristic that suggests a slower gait in Aardonyx is the robustness of metatarsal I in comparison with those of other basal sauropodomorphs. This is evidence of a more medial, or entaxonic, position of the weight bearing axis of the foot, as opposed to a more mesaxonic position where the weight bearing axis runs through digit III. The development of entaxony in Aardonyx provides further evidence for its reduced cursorial ability and wider gauge-gait, which is thought to have preceded obligatory quadrupedalism in sauropodomorphs. Previously, it was thought that entaxony developed after the divergence of Vulcanodon due to the presence of mesaxony in the genus.[6] However, the presence of mesaxony in Vulcanodon can be now considered an evolutionary reversal given the clear presence of entaxony in Aardonyx.[1]

In popular culture

The discovery of Aardonyx is presented in the second episode of the 2010 BBC documentary Museum of Life, "Digging up the Past",[7] where it is described by paleontologist Paul Barrett as a transitional form between bipedal prosauropods and the giant quadrupedal sauropods.

References

  1. ^ a b c d e f Yates, A. M.; Bonnan, M. F.; Neveling, J.; Chinsamy, A.; Blackbeard, M. G. (2010). "A new transitional sauropodomorph dinosaur from the Early Jurassic of South Africa and the evolution of sauropod feeding and quadrupedalism". Proceedings of the Royal Society B. 277 (1682): 787–794. doi:10.1098/rspb.2009.1440. PMC 2842739. PMID 19906674.
  2. ^ a b c Associated Press (November 11, 2009). Scientists: New dinosaur species found in South Africa. NPR.
  3. ^ Dixon, Robyn (2009-11-12). "New dinosaur a kind of missing link". Los Angeles Times. p. A24. Retrieved 2009-11-14. Also published in the Australian newspaper The Age as:
  4. ^ Yates, A. M.; Bonnan, M. F.; Neveling, J.; Chinsamy, A.; Blackbeard, M. G. (2009). "A new transitional sauropodomorph dinosaur from the Early Jurassic of South Africa and the evolution of sauropod feeding and quadrupedalism". Proceedings of the Royal Society B. 277 (1682): 787–794. doi:10.1098/rspb.2009.1440. PMC 2842739. PMID 19906674.
  5. ^ Upchurch, P.; Barrett, P. M.; Zhao, X.; Xu, X. (2007). "A re-evaluation of Chinshakiangosaurus chunghoensis Ye vide Dong 1992 (Dinosauria, Sauropodomorpha): implications for cranial evolution in basal sauropod dinosaurs". Geological Magazine. 144 (2): 247–262. doi:10.1017/S0016756806003062.
  6. ^ Carrano, M. T. (2005). The evolution of sauropod locomotion: morphological diversity of a secondarily quadrupedal radiation. In Curry Rogers, K. A. and Wilson J. A. (eds.) The Sauropods: Evolution and Paleobiology, pp. 229–251. Berkeley, CA: University of California Press.
  7. ^ Museum of Life - episode 2

External links

Anchisauria

The Anchisauria were a clade of sauropodomorph dinosaurs that lived during the Late Triassic and Early Jurassic. The name Anchisauria was first used by Galton and Upchurch in the second edition of The Dinosauria. Galton and Upchurch assigned two families of dinosaurs to the Anchisauria: the Anchisauridae and the Melanorosauridae. The more common prosauropods Plateosaurus and Massospondylus were placed in the sister clade Plateosauria.

However, recent research indicates that Anchisaurus is closer to sauropods than traditional prosauropods; thus, Anchisauria would also include Sauropoda.The following cladogram simplified after an analysis presented by Blair McPhee and colleagues in 2014.

Arcusaurus

Arcusaurus is an extinct genus of sauropodomorph dinosaur from the Early Jurassic (Hettangian to Sinemurian stages) of South Africa.

Arcusaurus was first named by Adam Yates, Matthew Bonnan and Johann Neveling in 2011 and the type species is Arcusaurus pereirabdalorum. The generic name is derived from Latin arcus, "rainbow", a reference to the Rainbow Nation. The specific epithet honours Lucille Pereira and Fernando Abdala who discovered the fossils .

Arcusaurus is known from two fragmentary skeletons collected in March 2006 at the Spion Kop Heelbo site from the upper Elliot Formation in Senekal in Free State. The holotype, BP/1/6235, consists of a partial skull. Some limb bones and vertebrae are included in the material. Both specimens represented juvenile individuals. From detailed features the describers concluded these were not the young of either Aardonyx or Massospondylus.

A phylogenetic study of Arcusaurus found it to be a basal sauropodomorph, placing it as the sister taxon of Efraasia and all of the more derived sauropodomorphs. Since Efraasia is known from the Norian stage of the Late Triassic, the close relationship with Arcusaurus implies that there was a 35-million-year ghost lineage of sauropodomorphs stretching from Late Triassic forms to Arcusaurus. However, Arcusaurus possesses many features unique to more advanced groups included in the clade Plateosauria, raising doubts about the results of the phylogenetic analysis.

Averostra

Averostra, or "bird snouts", is a clade that includes most theropod dinosaurs that have a promaxillary fenestra (fenestra promaxillaris), an extra opening in the front outer side of the maxilla, the bone that makes up the upper jaw. Two groups of averostrans, the Ceratosauria and the Orionides, survived into the Cretaceous period. When the Cretaceous–Paleogene extinction event occurred, ceratosaurians and two groups of orionideans within the clade Coelurosauria, the Tyrannosauroidea and Maniraptoriformes, were still extant. Only one subgroup of maniraptoriformes, Aves, survived the extinction event and persisted to the present day.

Avetheropoda

Avetheropoda, or "bird theropods", is a clade that includes carnosaurians and coelurosaurs to the exclusion of other dinosaurs.

Cerapoda

Cerapoda ("ceratopsians and ornithopods") is a clade of the dinosaur order Ornithischia.

Dinosauriformes

Dinosauriformes is a clade of archosaurian reptiles that include the dinosaurs and their most immediate relatives. All dinosauriformes are distinguished by several features, such as shortened forelimbs and a partially to fully perforated acetabulum, the hole in the hip socket traditionally used to define dinosaurs. The oldest known member is Asilisaurus, dating to about 245 million years ago in the Anisian age of the middle Triassic period.

Haya griva

Haya is an extinct genus of basal neornithischian dinosaur known from Mongolia.

Jingshanosaurus

Jingshanosaurus (meaning "Jingshan lizard") is a genus of sauropodomorph dinosaurs from the early Jurassic period.

Lessemsaurus

Lessemsaurus is an extinct genus of sauropod dinosaur belonging to Lessemsauridae.

Meroktenos

Meroktenos is a genus of basal sauropodomorph dinosaur that lived during the Late Triassic of Lesotho.

Mussaurus

Mussaurus (meaning "mouse lizard") is a genus of herbivorous sauropodomorph dinosaur that lived in southern Argentina during the Late Triassic, about 215 million years ago. It receives its name from the small size of the skeletons of juvenile and infant individuals, which were once the only known specimens of the genus. However, since Mussaurus is now known from adult specimens, the name is something of a misnomer; adults possibly reached 6 metres (20 ft) in length and weighed more than 1,000 kilograms (2,200 lb). Mussaurus possesses anatomical features suggesting a close, possibly transitional evolutionary relationship with true sauropods.

Neotheropoda

Neotheropoda (meaning "new theropods") is a clade that includes coelophysoids and more advanced theropod dinosaurs, and the only group of theropods who survived the Triassic–Jurassic extinction event. Yet all of the neotheropods became extinct during the early Jurassic period except for Averostra.

Orionides

Orionides is a clade of tetanuran theropod dinosaurs from the Middle Jurassic to the Present. The clade includes most theropod dinosaurs, including birds.

Orodrominae

Orodrominae is a subfamily of parksosaurid dinosaurs from the Cretaceous of North America and Asia.

Pulanesaura

Pulanesaura is an extinct genus of basal sauropod known from the Early Jurassic (late Hettangian to Sinemurian) Upper Elliot Formation of the Free State, South Africa. It contains a single species, Pulanesaura eocollum, known from partial remains of at least two subadult to adult individuals.

Sefapanosaurus

Sefapanosaurus was an early, herbivorous sauropodomorph dinosaur occurring in the southern regions of Gondwana some 200 million years ago in the Late Triassic or Early Jurassic. The sauropodomorphs were the dominant terrestrial herbivores throughout much of the Mesozoic Era, from their origins in the mid-Triassic (approximately 230 Ma) until their decline and fall at the end of the Cretaceous (approximately 66 Ma).

Unaysauridae

Unaysauridae is a family of basal sauropodomorphs from the Late Triassic of India and Brazil.

Xixiposaurus

Xixiposaurus is a genus of prosauropod dinosaur which existed in what is now Lower Lufeng Formation, China during the lower Jurassic period. It was first named by Sekiya Toru in 2010 and the type species is Xixiposaurus suni.

Yueosaurus

Yueosaurus is an extinct genus of basal ornithopod dinosaur known from Zhejiang Province, China.

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