Aardoniks (lat.: Aardonix; from Afrikaans: aard/ “earth” + Greek.: onux/ “claw”; t.e. “claw from the earth”) the basal genus of herbivorous dinosaurs from the suborder Sauropodomorphs (lat.: Sauropodomorpha).
Until now, the remains of presumably only two individuals of the only species of this genus, Aardonyx celestae, found in the Lower Jurassic layers of the South African Elliot Formation are known to science.
Aardonyx was first scientifically described in 2010 by researchers led by Adam Yates. Although this dinosaur was presumably bipedal, it already had a number of features that are characteristic of four-legged sauropods. This genus, which is an important transitional form, helps to understand the evolution of tetrapods within the suborder Sauropodomorpha.
Aardonyx was a supposedly long-necked, bipedal herbivorous dinosaur with a small skull. The length of the skulls of the found individuals was presumably about 36 cm. In plan view, the muzzle of this dinosaur was narrow and tapering towards the end.
Aardonyx differed from other genera close to it, in particular, in large nasal openings, which most often had the same size as the orbits, as well as the presence of 5 teeth on the premaxilla, and the same feature independently developed in Plateosaurus (lat.: Plateosaurus).
There were about 18 teeth on each side of the upper jaw, while there were more than 19 on each side of the lower jaw. The cervical vertebrae of the dinosaur were oblong in cross section and low.
The limb bones indicate that Aardonyx was a “two-legged” dinosaur, but at the same time, they are already adapted to move on 4 legs, as in later sauropods. So, the bones of the feet of aardonyx were quite short and powerful, and the first metatarsal bone was especially massive.
Location of the find and its description
This sauropodomorph is known from dissected (anatomically disjointed) bones believed to have belonged to two juveniles. Fossilized remains were discovered in the city of Senecal, Free State Province, South Africa, in an old riverbed covered with sand and silt.
The found bone material includes elements of the skulls, vertebrae, dorsal and cervical ribs, abdominal ribs, chevrons, elements of the shoulder and pelvic girdle, as well as bones of the fore and hind limbs, hands and feet.
Before being buried under the sediment layer, the bodies of these two Aardonyx celestae apparently remained relatively intact and were not carried too far by the current: this is evidenced by the fact that not only the massive bones of the skeletons were preserved, but also the fragile bones of the skulls, that is, the bone material was not carried by the flow of water.
At the time of death, the age of both individuals was presumably less than 10 years old. On transverse sections of dinosaur bones, concentric growth zones are visible, alternating with rings of weak growth. Since none of the studied bones ends on the outside with such a ring of weak growth, it can be concluded that at the time of death both animals were still in the phase of active growth.
Fossils were found in the developed upper layers of the Elliot Formation, which is the site of an accumulation of a large number of fossil remains of dinosaurs and located in the territories of South Africa and Lesotho. The Elliot Formation is part of the Stromberg Formation.: Stromberg Group), belonging to the Karoo Supergroup.
Aardonyx shared a common habitat with a more primitive representative of the Sauropodomorph suborder Massospondylus (lat.: Massospondylus) and early sauropods (lat.: Sauropoda). Other dinosaurs whose bones are contained in the upper layers of the Elliot Formation were representatives of the family Heterodontosaurids (lat.: Heterodontosauridae) heterodontosaurus (Heterodontosaurus), abrictosaurus (Abrictosaurus) and lycorine (Lycorhinus), as well as belonging to the infraorder Ceratosaurus (Ceratosauria) megapnosaurus (Megapnosaurus).
Aardonyx was the basal member of the Anchisaurus family (lat.: Anchisauria) within the suborder Sauropodomorphs. Previously, it was attributed to the classic prosauropods (lat.: Prosauropoda), that is, basal sauropodomorphs that did not belong to the infraorder Sauropoda. Since the prosauropods were not a natural (paraphyletic) group, this name (“prosauropods”) is almost never used today.
According to a 2010 analysis by a team of researchers led by Adam Yates, Aardonyx is classified as the sister taxon of the clade that includes Melanorosaurus (lat.: Melanorosaurus) and sauropods. All representatives of this clade necessarily moved on four limbs.
Thus, aardonyx was the closest relative of sauropods, but at the same time he himself was still “two-legged”. This genus was a transitional form between the bipedal basal sauropodomorphs and the four-legged sauropods.
Basal sauropodomorphs had narrow, V-shaped jaws and probably fleshy cheeks. At the same time, sauropods, on the contrary, had adaptations that allowed them to consume larger amounts of food faster: for example, their jaws were wide and U-shaped, which provided a wider bite without large cheeks. Sauropod tooth plates were internally reinforced with palatal plates that aided the teeth in scraping foliage from plant branches.
Aardonyx had a unique combination of these traits: although his jaws tapered towards the end, he did not appear to have fleshy cheeks. Unlike Aardonyx, the basal sauropod Hinshakiangosaurus (lat.: Chinshakiangosaurus) seems to have had the opposite combination: this sauropod had fleshy cheeks, although its jaws were U-shaped.
The researchers suggest that the evolution of U-shaped jaws in sauropods did not occur in a straight line, but through the independent emergence of individual features (homoplasia).
Way to travel
Features of the bones of the limbs of aardonyx indicate that this dinosaur moved on two limbs. Thus, the head of the radius had an ovoid shape, which limited the rotation of this bone around the ulna. Thus, the dinosaur was not able to turn its forearms inward, as a result of which its palms could not be turned down. In addition, the length of the humerus of aardonyx was only 72% of the length of its femur.
However, other features of the bones of its fore and hind limbs are considered by researchers as adaptations for movement on four “legs”. For example, the upper end of the ulna has a craniolateral crest, giving it a Y-shape in horizontal projection. A similar crest is also found in four-legged sauropods, but in them it is more pronounced.
In addition, the cavity in the radius provided the possibility of craniolateral movement of the ulna.
The lower (hind) limbs also had devices for movement on 4 limbs. Thus, the body of the femur was more straight, while the fourth trochanter (a process of the femur that serves as a muscle attachment site) was located lower than in basal sauropodomorphs.
This provided the most important traction muscle of his thigh, the tail-femur, with a larger arm of force, but at the same time made its movements slower. Therefore, Aardonyx moved more slowly than typical basal sauropodomorphs.
Another feature that indicates a slower movement of this dinosaur is the first metatarsal bone, which is more massive compared to other basal sauropodomorphs. This feature indicates that the axis of the center of gravity of the body of Aardonyx did not fall on the middle toes, as in basal sauropodomorphs, but was shifted further to the middle of the feet.
In addition, this shift indicates that the dinosaur’s hind limbs were more outward than those of basal sauropodomorphs, which was an adaptation that presumably preceded the development of obligatory quadrupedalism during evolution.
Before the discovery of aardonyx, it was assumed that the axis of the center of gravity of the dinosaur body shifted only when the dinosaurs had already become tetrapods. So, the foot of the early sauropod Vulcanodon (lat.: Vulcanodon), although it moved exclusively on 4 limbs, had a sign of a shift in the axis of the center of gravity to its middle, which, in the light of the discovery of aardonyx, could be interpreted as a reversal of evolution.