Temporal range: Early–Middle Jurassic, 199–155Ma
The Fabrosauridae was originally proposed by Galton (1972), as a group of basal ornithischian dinosaurs from the Early to Middle Jurassic. However, recent studies indicate the Fabrosaurs are not a natural grouping of dinosaurs, and instead consist of unrelated genera.
The proposed "Fabrosaurs" descended from a Lesothosaurus-like animal. Proposed fabrosaurids were 1–2 meters long, and were lightly built and bipedal. Their skulls were triangular and had very large eye sockets. They were herbivorous and would have used agility to escape predators.
Fabrosaurids are known from southern Africa in the Upper Triassic and Lower Jurassic, as well as in China from the Middle and Upper Jurassic periods. Fabrosaurids bear a semblance to many other members of Ornithischia, and have attracted much interest in phylogenetic studies. However, most genera that were previously referred to as Fabrosauridae are only known from fragmented and partial remains, with most based on no more than isolated teeth, and their place in Fabrosauridae is questionable. For example, in 1990, specimens of Agilisaurus found in the Middle Jurassic of Zigong, Sichuan, China, provided further evidence that led to paleontologists resolving systematic problems of the group Fabrosauridae.
Galton (1972), originally proposed the creation of Fabrosauridae based on the finding of cheek teeth. The recent discovery of a Middle Jurassic primitive ornithischian, Agilisaurus louderbacki, found in the Xiashaximiao Formation of Zigong, Sichuan Basin, China, has led to paleontologists learning more about the phylogeny of fabrosaurids.
Fabrosaurids have many characters that are unique to their physiology, particularly unique tooth morphology and structure, and short forelimb lengths compared to hindlimbs. Unlike many other primitive Ornithischians, such as heterodontosaurids and hypsilophodontids, the teeth of fabrosaurids are very thinly and uniformly enameled. Compared to Agilisaurus, Fabrosaurus is much more primitive in that it has six premaxillary teeth, and a stout prepubis. Agilisaurus differs from Fabrosaurus in that it appears to be larger and more well developed in structure.
The Upper Jurassic ornithischian dinosaur, Gongubusaurus, from China, is thought to be more closely related to fabrosaurids than hypsilophodontids. The type species, G. shiyii, is from the Shangshaximiao Formation in the Rongxian County in Sichuan Basin, has been classified from only a few scattered teeth.
Fabrosaurids were relatively small ornithischian dinosaurs, averaging about 1–2 meters in body length, with a triangular skull that had large circular orbits on the sides. This suggests that fabrosaurids had relatively huge eyes that faced laterally. The forelimbs of fabrosaurids are considerably shorter than their hindlimbs. A small forelimb such as those present in Fabrosauridae would not have been useful for locomotion, and it is evident that fabrosaurids were bipedal dinosaurs. The entire skeleton was lightly built, with a largely fenestrated skull and a very stout neck and trunk. The tail is nearly half of the dinosaurs' overall length. The long tail presumably acted as a counterbalance and as a compensating mechanism for shifts in the creature's center of gravity. The hindlimbs of fabrosaurids show that the tibia is considerably longer than the femur, a feature that suggests that fabrosaurids were adapted for bipedality, and were fast runners.
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