Lesothosaurus is a monospecific genus of ornithischian dinosaur that lived during the Early Jurassic in what is now South Africa and Lesotho.

Lesothosaurus is one of the most completely-known early ornithischians, based on numerous skull and postcranial fossils from the Upper Elliot Formation.

[4] 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(?

[5] Ginsburg placed it in the family Scelidosauridae and diagnosed it based on its unusual tooth morphology when compared to the only other contemporary ornithischian Heterodontosaurus.

[7] The holotype was all that was known until expeditions by the London University College to the same site in Lesotho from 1963 to 1964 recovered scores of fossils from Lesothosaurus, including a partial skeleton including a skull and another isolated partial skull (NHMUK PV RU B17 & NHMUK PV RU B23).

[11] The specific name diagnosticus is derived from the Greek root diagnostikos meaning “distinguished” in reference to Lesothosaurus being a distinct member of Fabrosauridae.

[11] This “large fabrosaurid” was finally named in 2005, dubbed Stormbergia dangershoeki, on the basis of the partial postcranial skeleton SAM-PK-K1105.

[7] Fossils from Elliot Formation sites in South Africa outside of Lesotho in Jamestown were described in the 2000s, including a nearly complete skeleton of an adult preserved in articulation.

[3] A study published in 2017 by Baron, Norman & Barrett demonstrated that the differences between Stormbergia and Lesothosaurus are most likely related to the animal's growth.

[13][3] Like all ornithischians, the tips of Lesothosaurus upper and lower jaws were horny, forming a beaklike structure.

[15] 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 supratemporal fenestrae are anteroposteriorly longer than mediolaterally wide, with a sub-ovary to sub-triangular outline in dorsal view.

The craniomandibular joint (where the skull contacts the jaw bone) is depressed relative to the margin of the maxillary alveoli.

The alveolar foramina are on the medial wall of the maxilla and dentary, one per tooth position, and represent the area where the neurovascular supply accesses the dental lamina.

[2][18] The mandible (lower jaw) of Lesothosaurus has a nearly straight ventral margin and bears only a slight upturn at its anterior tip.

The predentary is shaped like an arrowhead in ventral view, with one elongated central keel with smaller lateral processes jutting off the sides.

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.

There are 6 premaxillary teeth on the left and right sides of the premaxilla which are preceded by a small edentulous (tooth-lacking) section that shows signs of preserving a large rhampthotheca (beak) made of keratin.

The lingual surface of the more medial premaxillary teeth have a vertical furrow and an adjacent sharp ride that extends towards the crown's mesial edge.

The posterior process of the premaxilla lacks alvelovi, creating a small diastema (gap) in between the premaxillary and maxillary tooth rows.

Ossified tendons are preserved attached to the neural spines of anterior dorsals, suggesting they were arranged longitudinally as in Heterodontosaurus, Scelidosaurus, & Hypsilophodon.

The humerus was elongate and straight in anterior and lateral views, with expanded proximal and distal ends linked by a long, slender shaft.

[7][18][3] Peter Galton considered Lesothosaurus to be a basal ornithopod in the family Fabrosauridae, which included several other ornithischians such as Nanosaurus (from the Late Jurassic of North America), Echinodon (from the Lower Cretaceous of England), and Fabrosaurus (which Galton considered distinct from Lesothosaurus but only included the holotype).

However, a 1991 redescription by Paul Sereno suggested that Lesothosaurus and many other "fabrosaurids" were actually basal members of Ornithischia, one of the two main orders of Dinosauria (the other being Saurischia).

[1] This opinion has been supported by later cladistic studies of basal Ornithischia, which have also found it as the basalmost member of Neornithischia (a group that includes pachycephalosaurs, ceratopsians, and ornithopods) and related to Agilisaurus, Hexinulsaurus, and Nanosaurus.

[22] The Butler et al., 2005 analysis placed Lesothosaurus at the base of Neornithischia: Euparkeria Marasuchus Herrerasaurus Pisanosaurus Heterodontosauridae Eocursor Thyreophora Lesothosaurus Agilisaurus Hexinlusaurus Othnielosaurus Cerapoda 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,[23] Hexinlusaurus from the Shaximiao Formation, dating to the Middle Jurassic (170-168 mya), in southern China,[24] and Hypsilophodon from the Early Cretaceous (130-125 mya) of England.

Both formations are famous for their abundant vertebrate fossils, including temnospondyl amphibians, turtles, lepidosaurs, aetosaurs, crocodylomorphs, and non-mammal cynodonts.

[28][29] 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.

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).