Nanosaurus ("small or dwarf lizard") is the name given to a genus of neornithischian dinosaur that lived about 155 to 148 million years ago, during the Late Jurassic-age. Its fossils are known from the Morrison Formation of the south-western United States. The type and only species, Nanosaurus agilis, was described and named by Othniel Charles Marsh in 1877. The taxon has a complicated taxonomic history, largely the work of Marsh and Peter M. Galton, involving the genera Laosaurus, Hallopus, Drinker, Othnielia, and Othnielosaurus, the latter three now being considered to be synonyms of Nanosaurus. It had historically been classified as a hypsilophodont or fabrosaur, types of generalized small bipedal herbivore, but more recent research has abandoned these groupings as paraphyletic and Nanosaurus is today considered a basal member of Neornithischia.
Nanosaurus is known from material from all parts of the body, including two good skeletons, although the skull is still poorly known.[1] It was a small animal, with specimens previously assigned to Drinker and Othnielosaurus measuring 2–2.2 metres (6.6–7.2 ft) long and weighing 20–30 kilograms (44–66 lb).[2]
It was a bipedal dinosaur with short forelimbs and long hindlimbs with large processes for muscle attachments.[3] The hands were short and broad with short fingers. The head was small. It had small leaf-shaped cheek teeth (triangular and with small ridges and denticles lining the front and back edges), and premaxillary teeth with less ornamentation.[4]
Like several other neornithischian dinosaurs, such as Hypsilophodon, Thescelosaurus, and Talenkauen, Nanosaurus had thin plates lying along the ribs. Called intercostal plates, these structures were cartilaginous in origin.[5]
Nanosaurus has had a long and complicated taxonomic history. In 1877, Marsh named two species of Nanosaurus in separate publications, based on partial remains from the Morrison Formation of Garden Park, Colorado. One paper described N. agilis, based on YPM 1913, with remains including impressions of a dentary, and postcranial bits including an ilium, thigh bones, shin bones, and a fibula.[6] The other paper named N. rex, a second species which Marsh based on YPM 1915 (also called 1925 in Galton, 2007), a complete thigh bone.[4][7] He regarded both species as small ("fox-sized") animals.[7] A third species, N. victor, was named, which he soon recognized to be something completely different, and is now known as the small, bipedal crocodylomorph Hallopus.[6][8]
The next year, he named the new genus Laosaurus on material collected by Samuel Wendell Williston from Como Bluff, Wyoming. Two species were named: the type species L. celer, based on parts of eleven vertebrae (YPM 1875);[9] and the "smaller" L. gracilis, originally based on a back vertebra's centrum, a caudal centrum, and part of an ulna (review by Peter Galton in 1983 finds the specimen to now consist of thirteen back and eight caudal centra, and portions of both hindlimbs).[9][10] A third species, L. consors, was established by Marsh in 1894 for YPM 1882, which consists of most of one articulated skeleton and part of at least one other individual.[11] The skull was only partially preserved, and the fact that the vertebrae were represented only by centra suggests a partially grown individual. Galton (1983) notes that much of the current mounted skeleton was restored in plaster, or had paint applied.[10]