Abrictosaurus (; "wakeful lizard") is a genus of heterodontosaurid dinosaur that lived during the Early Jurassic in what is now in parts of southern Africa such as Lesotho and South Africa. It was a bipedal herbivore or omnivore and was one of the most basal heterodontosaurids. It was approximately long and weighed less than .
This dinosaur is known from the fossil remains of only two individuals, found in the Upper Elliot Formation of Qacha's Nek District in Lesotho and Cape Province in South Africa. The Upper Elliot is thought to date from the Hettangian and Sinemurian stages of the Early Jurassic Period, approximately 200 to 190 million years ago.
Description
Heterodontosaurids like
Abrictosaurus were small, early
, named for their markedly
heterodont dentition. They are best known for the large,
canine tooth-like tusks (often called
) in both upper and lower jaws. There were no teeth in the front of the jaws, where a hard
rhamphotheca was used to crop vegetation. There were three
teeth, with the first two small and conical and the third enlarged to form the upper caniniform, counterpart to the even larger lower caniniform, which was the first
dentary tooth. In the upper jaw, a large gap (or diastema) accommodated the lower caniniform tooth and separated the premaxillary teeth from the wider chewing teeth of the
maxilla. Similar teeth lined the remainder of the lower jaw.
Abrictosaurus is usually considered the most basal member of the family Heterodontosauridae.
History and naming
Both specimens of
Abrictosaurus are housed in the collection of University College London. The
holotype specimen was discovered in Lesotho and consists of a partial
skull and skeleton (UCL B54).
Paleontologist Richard Thulborn, who first described the specimen in 1974, considered it a new
species of
Lycorhinus and named it
L. consors, using the
Latin word
consors which means 'companion' or 'spouse'. As UCL B54 lacked the caniniforms which had been found in the
type species,
Lycorhinus angustidens, Thulborn believed it to be female.
Neither the skull nor the skeletons of
Abrictosaurus have been fully described in the literature. A tooth from the latest
Triassic of
Switzerland has been assigned to
Abrictosaurus sp., but this has not been supported, as the specimen does not have unique characteristics of
Abrictosaurus, heterodontosaurids, or ornithischians in general.
In 1975,
James Hopson redescribed a fragmentary heterodontosaur skull (UCL A100) found in
South Africa that Thulborn had previously assigned to
Lycorhinus angustidens.
After showing that UCL A100 could not belong to
L. angustidens but was instead more similar to UCL B54, Hopson erected a new genus to contain both specimens. The generic name
Abrictosaurus (from the
Ancient Greek αβρικτος (
abriktos) meaning 'wakeful' and σαυρος (
sauros) meaning 'lizard') refers to Hopson's disagreement with Thulborn's
hypothesis that heterodontosaurids underwent periods of
aestivation (
hibernation during hot and/or dry seasons). The specific name was retained, creating the new binomial
Abrictosaurus consors.
Despite Hopson's renaming, Thulborn continued to consider
Lycorhinus angustidens,
Heterodontosaurus tucki, and
Abrictosaurus consors to be three species of the genus
Lycorhinus. Most paleontologists maintain all three genera separately, although there is no precise definition of a species or genus in paleontology.
Sexual dimorphism
The hypothesis of sexual dimorphism in heterodontosaurids has long centered on
Abrictosaurus. Tusks are a sexually dimorphic trait in many modern mammals, including
musk deer,
walrus,
and many
Suidae, with tusks being present primarily in males. The lack of tusks in UCL B54 led to suggestions that it was female; perhaps even a female of another species.
The discovery of caniniforms in UCL A100 showed that
A. consors also has this 'male' characteristic, suggesting that it is at least a valid species in its own right. However, UCL B54 may actually be a juvenile, based on its short face and unfused
sacrum (hip)
. Therefore, the lack of tusks could be a juvenile trait instead of a secondary sexual characteristic, weakening the case for sexual dimorphism.
