Aardonyx (Afrikaans aard, "earth" + Greek , "nail, claw") is a genus of basal sauropodomorph dinosaur. It is known from the type species Aardonyx celestae found from the Early 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.
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. Australian 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. 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".
According to Matthew Bonnan, a co-author of 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."
Description
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. 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.
Classification
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).
Cladogram showing the position of Aardonyx within Sauropodomorphaafter Yates et al., 2010:
The following cladogram shows the position of Aardonyx within Massopoda, according to Oliver W. M. Rauhut and colleagues, 2020:
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.