Lesothosaurus is a monospecific genus of ornithischian dinosaur that lived during the Early Jurassic in what is now South Africa and Lesotho. It was named by paleontologist Peter Galton in 1978, the name meaning "lizard from Lesotho". The genus has only one valid species, Lesothosaurus diagnosticus. Lesothosaurus is one of the most completely-known early ornithischians, based on numerous skull and postcranial fossils from the Upper Elliot Formation. It had a simpler tooth and jaw anatomy than later ornithischians, and may have been omnivorous in some parts of the year.
Discovery and naming
left|thumb|Illustration of the [[Fabrosaurus holotype dentary.]]
Fossils referrable to Lesothosaurus may have been known from as early as 1959, when a right dentary (lower jawbone) fragment bearing three teeth was collected by French geologist Jean Fabre from the Red Beds of the Upper Elliot Formation near Mapheteng in Lesotho, Southern Africa, dating to the Early Jurassic (199(?)-190 million years ago). A joint expedition between the NHMUK, London University College, Yale University, and the South African Museum collected many additional specimens of Lesothosaurus from the same site in 1967–68. This included very well preserved cranial material, some of the best known, that was described in the 1991. The generic name Lesothosaurus is derived from the Kingdom of Lesotho, where the fossils were discovered, and the Latin root sauros meaning "lizard", a root commonly used in dinosaur names. Redescription of the syntypes came in 2015 and 2017, including the integration of CT technology. Studies of the tooth wear have shown much less abrasion on the teeth than would be expected of a plant-eater feeding mainly on tough, arid-climate plants, and concluded that Lesothosaurus was probably an opportunistic omnivore, feeding primarily on small animals during seasons when softer plants were not available.
Skull and dentition
left|thumb|Skull diagram
The skull and teeth of Lesothosaurus are more generalized than the heavily specialized and unusual anatomies of the contemporary scelidosaurid and heterodontosaurid ornithischians, which exhibit traits like osteoderms and extreme heterodonty. The cranium is widest across the postorbitals in dorsal view. It tapers anteriorly to the premaxillae, which creates a short, strongly pointed muzzle. The skull is widest at the midshafts of the quadrates in occipital view.
The mandible (lower jaw) of Lesothosaurus has a nearly straight ventral margin and bears only a slight upturn at its anterior tip. The mandible is made up mostly of the dentary, which is 50% of its length in lateral view. Characteristically of Ornithischians, there is a small beak-like bone at the tip of the dentary known as the predentary. The predentary is shaped like an arrowhead in ventral view, with one elongated central keel with smaller lateral processes jutting off the sides. The oral margin is smooth and straight in lateral view, with an uncurved anterior tip. Two foramina are preserved on and near the lateral processes, suggesting that this element was well-supplied with blood and nerves. There is a well-developed coronoid eminence, but it does not expand dorsally into the tall, distinct processes (projection of bone) like in advanced Ornithischians like Triceratops and Zalmoxes. The jaw joint is slightly depressed relative to the alveolar bar that takes up most of the mandible. The mandible preserves an anteroposteriorly elongated fenestra between the dentary, angular, and surrangular, similar to the ones in the skull, that would make the mandible lighter. The mandible differentiates from those of other Ornithischians greatly in that an inturned, 'spout-like' mandibular symphysis is preserved. The sacral vertebrae series had 5 vertebrae with sacral ribs and a sacrodorsal with a short sacral rib. The sacrals notably had large transverse processes that were muscle attachments to the sacral ribs. The number of caudal (tail) vertebrae is unknown, but the proximal caudals are well preserved. The centra of these caudals became more cylindrical as they became located distally, in contrast to the spool-shaped dorsal centra.
The Butler et al., 2005 analysis placed Lesothosaurus at the base of Neornithischia:
Basal neornithischians like Lesothosaurus are known from several time periods and regions, with Nanosaurus fossils coming from the Upper Jurassic Morrison Formation (155–148 mya) of the western United States, Hexinlusaurus from the Shaximiao Formation, dating to the Middle Jurassic (170–168 mya), in southern China, and Hypsilophodon from the Early Cretaceous (130–125 mya) of England. However, the phylogenetic status of basal neornithischians is constantly in a flux and some analyses have recovered these taxa as basal ornithopods or in other groups. The Upper Elliot Formation is characterised by animals that appear to be more lightly built than those of the Lower Elliot Formation, which may have been an adaptation to the drier climate at this time in southern Africa. Both formations are famous for their abundant vertebrate fossils, including temnospondyl amphibians, turtles, lepidosaurs, aetosaurs, crocodylomorphs, and non-mammal cynodonts.
Other dinosaurs from these formations include the heterodontosaurid Heterodontosaurus, the basal sauropodomorph Massospondylus, and the theropod Megapnosaurus. The Upper Elliot Formation shows the largest known heterodontosaurid diversity of any rock unit; besides Heterodontosaurus, it contained Lycorhinus, Abrictosaurus, and Pegomastax. Yet another member of the family, Geranosaurus, is known from the Clarens Formation. The high heterodontosaurid diversity have led researchers to conclude that different species might have fed on separate food sources in order to avoid competition (niche partitioning).
References
Bibliography
- P. M. Galton. 1978. Fabrosauridae, the basal family of ornithischian dinosaurs (Reptilia: Ornithischia). Paläontologische Zeitschrift 52(1/2):138-159
- Butler, R.J., 2005. "The 'fabrosaurid' ornithischian dinosaurs of the Upper Elliot Formation (Lower Jurassic) of South Africa and Lesotho." Zoological Journal of the Linnean Society 145: 175–218.