Lesothosaurus
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
Fossils referrable to Lesothosaurus may have been known from as early as 1959, when a right dentary 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. The dentary was described as the holotype of a new genus and species, Fabrosaurus australis, by paleontologist Leonard Ginsburg in 1964. 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. Due to its fragmentary nature, Fabrosaurus is now seen as a nomen dubium, though the holotype is likely from an individual of Lesothosaurus. 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. These specimens were described in the 1970s as belonging to Fabrosaurus by geologist Richard A. Thulborn. 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.British paleontologist Peter Galton named Lesothosaurus diagnosticus in 1978, with NHMUK PV RU B17 and NHMUK PV RU B23 as the syntypes. 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. The specific name diagnosticus is derived from the Greek root diagnostikos meaning "distinguished" in reference to Lesothosaurus being a distinct member of Fabrosauridae. In the material referred to Lesothosaurus, Galton stated that some of it was instead from a "large fabrosaurid". This "large fabrosaurid" was finally named in 2005, dubbed Stormbergia dangershoeki, on the basis of the partial postcranial skeleton SAM-PK-K1105. This species almost certainly represents the adult form of Lesothosaurus. Stormbergia was named for the Stormberg Series of rocks in southern Africa, which includes the Elliot Formation, and the location in South Africa at which the type specimen was found. The type specimen consists of a partial postcranial skeleton, with two additional referred specimens assigned to the species. 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. 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. The authors argued that Stormbergia is a junior subjective synonym of Lesothosaurus and should be regarded as invalid. Several other skull and postcranial specimens have been discovered since, including the description of two partial skulls in 2002, which preserved signs of individual variation. Redescription of the syntypes came in 2015 and 2017, including the integration of CT technology.
Description
Due to the great quantity and quality of specimens known from Lesothosaurus, information about its anatomy is known in detail. Lesothosaurus was a lightly built, bipedal animal that varied between 1 to 2 meters long. It was one of the earliest ornithischians. Its long slender legs, small arms with hands that would not have been able to grasp properly, and slender tail all suggest that it was a fast runner. Like all ornithischians, the tips of Lesothosaurus upper and lower jaws were horny, forming a beaklike structure. Behind the beak were leaf-shaped teeth that lined the jaws. The teeth of the premaxillae are more slender and curved than the maxillary teeth. Analysis of its teeth has shown that Lesothosaurus sliced up its food with its beak and was not able to chew its food. 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.The small skull of Lesothosaurus was narrow and pointed, with large eye sockets. It had large cavities for the eye and jaw muscles. The tip of the snout likely ended in a small beak, based on a blade-like predentary bone and a roughly-texture front end of the cranium. Its teeth were pointed with grooved edges. The skull was mounted on a short but flexible neck.
A bonebed of Lesothosaurus described in 2016 includes material from three large individuals. This association suggest that this early ornithischian dinosaur may have lived in groups.
Skull and dentition
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 best preserved skull is NHMUK PV RU B23, though it is missing some elements. The skull was unlike the triangular skull of Heterodontosaurus in that the caudal half was boxy while the anterior half was tapered and elongated. The height apex of the cranium is just behind the orbit in lateral view and the skull roof are gently rounded in lateral view. The snout is smoothly tapered to the premaxilla. The skull lacks a break in slope along the snout anterior to the orbit, as in the contemporary Heterodontosaurus. The orbits are round and large relative to the skull size. The skulls bears a relatively small, sub-triangular antorbital fenestra with an apex pointing dorsally and a length that is circa 13% of basal skull length. The supratemporal fenestrae are anteroposteriorly longer than mediolaterally wide, with a sub-ovary to sub-triangular outline in dorsal view. In contrast, the infratemporal fenestrae are sub-rectangular in lateral view and extend for most of the skull's height. The infratemporal fenestra is oblong with an oblique axis, while the supratemporal fenestra is oval in outline. The anterior naris had several small, sub-ovate openings along its length. The craniomandibular joint is depressed relative to the margin of the maxillary alveoli. There are 15-16 tooth positions in the maxilla, in contrary to the 11 preserved in PisanosaurusThe mandible 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 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.
Lesothosaurus has two types of teeth preserved: long, curved, sharp premaxillary teeth at the front of the mouth; wide, short, robust maxillary and dentary teeth. There are 6 premaxillary teeth on the left and right sides of the premaxilla which are preceded by a small edentulous section that shows signs of preserving a large rhampthotheca made of keratin. A neck and slight swelling divides the base of the tooth crown from the long, subcylindrical roots. 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 last two teeth crowns in the series acquire distal and medial denticles. The posterior process of the premaxilla lacks alvelovi, creating a small diastema in between the premaxillary and maxillary tooth rows. The maxillary and dentary teeth are low, triangular, and "leaf-shaped" with a distinct neck and cingulum. The denticles are coarse on the medial and distal tooth borders, with sporadically developed high-angled marginal tooth wearing. This suggests rapid tooth replacement in these teeth.