| Aardonyx | |
|---|---|
 | |
| Reconstruction of the skull | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | †Sauropodomorpha |
| Clade: | †Anchisauria |
| Genus: | †Aardonyx Yateset al.,2010 |
| Species: | †A. celestae |
| Binomial name | |
| †Aardonyx celestae Yateset al., 2010 | |
Aardonyx (Afrikaansaard, "earth" +Greekonux, "nail, claw") is agenus ofbasalsauropodomorphdinosaur. It is known from thetype speciesAardonyx celestae found from theEarly JurassicElliot Formation ofSouth Africa.A. celestae was named afterCeleste Yates, who prepared much of the first known fossil material of the species. It has arm features that are intermediate between basalsauropodomorphs and more derivedsauropods.[1]
Based on the structure of the hind limbs and pelvic girdle ofAardonyx, the dinosaur normally moved bipedally but could drop to quadrupedal movement similar toIguanodon. It shares some attributes with giant quadrupedal sauropods likeApatosaurus.[2] Australian[3][4]paleontologistAdam Yates and his team's discovery of the genus was published online before print inProceedings of the Royal Society B in November 2009, and was scheduled to appear in the March 2010 issue.[2] British paleontologistPaul Barrett of theNatural History Museum, London, who was not involved in the research, commented that the discovery ofAardonyx "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 toMatthew Bonnan, a co-author of the study, "We already knew that the earliest sauropods and near-sauropods would be bipeds. WhatAardonyx 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."
The genus is known from disarticulated bones belonging to two immature individuals. The material consists of cranial elements,vertebrae, dorsal and cervicalribs,gastralia,chevrons, elements of thepectoral andpelvic girdles, and bones of the fore and hind limbs,manus, andpes. The presence of these bones in a single dense accumulation in a localized channel fill suggests that they came from relatively complete carcasses.[5] 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 thecortices of sampled bones. Additional evidence for immaturity at the time of death includes calcified cartilage at the articular end of thescapula.[1]
Aardonyx is thought to be thesister taxon of a sauropodomorph clade containingMelanorosaurus 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 theneural canal and mid-cervicalneural spines that are less than twice as long as high) as well as the appendicular skeleton (such as the position of thefourth trochanter over the midlength of thefemur and an adult femur length exceeding 600 mm).[1]
Cladogram showing the position ofAardonyx within Sauropodomorpha
after Yateset al., 2010:[1]
The following cladogram shows the position ofAardonyx within Massopoda, according to Oliver W. M. Rauhut and colleagues, 2020:[6]
Aardonyx shows a transition toward the bulk-browsing form of feeding characteristic of sauropods. The jaws ofAardonyx 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 thedentary is indicative of a loss of fleshy cheeks. This is seen as an adaptation for a wider gape to facilitate bulk browsing and is observed in nearly all sauropods. The lateral neurovascular foramina of themaxilla ofAardonyx 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 thelingual 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 ofAardonyx. Previously, it was thought that broader jaws evolved before the reduction and loss in fleshy cheeks as an adaptation toward bulk-browsing in sauropods. The sauropodChinshakiangosaurus possessed jaws that were U-shaped, while still retaining fleshy cheeks, the opposite of the condition seen inAardonyx.[7] BecauseChinshakiangosaurus is a more derived sauropodomorph, this suggests that a wide, cheekless gape may have evolved twice in Sauropodomorpha: once withAardonyx and again with sauropods more advanced thanChinshakiangosaurus.[1]
Characteristics of the limbs ofAardonyx 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 themanus could bepronated, and the length of the humerus is only 72 percent that of the femur. However, characteristics found in both the fore and hind limbs ofAardonyx 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 inAardonyx. Although the hindlimbs ofAardonyx 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 ofAardonyx 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 theM. 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 inAardonyx is the robustness ofmetatarsal 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 inAardonyx 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 ofVulcanodon due to the presence of mesaxony in the genus.[8] However, the presence of mesaxony inVulcanodon can be now considered an evolutionary reversal given the clear presence of entaxony inAardonyx.[1]
The osteohistology ofAardonyx shows that it grew rapidly, but that its bodily growth was seasonally interrupted. Evidence for this comes from greatly vascularized woven-parallel complexes that contain regular growth marks, with its early and mid-ontogenetic stages exhibiting fibrolamellar complexes.[9]
Media related toAardonyx at Wikimedia Commons
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