| Nanosaurus | |
|---|---|
 | |
| Mounted skeleton, on temporary exhibit in theNational Museum of Denmark | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Genasauria |
| Clade: | †Neornithischia |
| Family: | †Nanosauridae Marsh, 1877 |
| Genus: | †Nanosaurus Marsh,1877 |
| Species: | †N. agilis |
| Binomial name | |
| †Nanosaurus agilis Marsh, 1877 | |
| Synonyms | |
Genus synonymy
Species synonymy
| |
Nanosaurus ("small or dwarf lizard") is an extinctgenus ofneornithischiandinosaur that lived about 155 to 148 million years ago, during theLate Jurassic in North America. Itsfossils are known from theMorrison Formation of the south-westernUnited States. Thetype and onlyspecies,Nanosaurus agilis, wasdescribed and named byOthniel Charles Marsh in 1877. Thetaxon has a complicated taxonomic history, largely the work of Marsh andPeter M. Galton, involving the generaLaosaurus,Hallopus,Drinker,Othnielia, andOthnielosaurus, the latter three now being considered to be synonyms ofNanosaurus. It had historically been classified as ahypsilophodont orfabrosaur, types of generalized small bipedal herbivore, but more recent research has abandoned these groupings asparaphyletic andNanosaurus is today considered abasal member of Neornithischia.
Nanosaurus has had a long and complicated taxonomic history. In 1877, Marsh named two species ofNanosaurus in separate publications, based on partial remains from the Morrison Formation ofGarden Park,Colorado. One paper describedN. agilis, based onYPM 1913, with remains including impressions of adentary, andpostcranial bits including anilium,thigh bones,shin bones, and afibula.[1] The other paper namedN. rex, a second species which Marsh based on YPM 1915 (also called 1925 in Galton, 2007), a complete thigh bone.[2][3] He regarded both species as small ("fox-sized") animals.[3] A third species,N. victor, was named, which he soon recognized to be something completely different, and is now known as the small, bipedalcrocodylomorphHallopus.[1][4]
The next year, he named the new genusLaosaurus on material collected bySamuel Wendell Williston fromComo Bluff,Wyoming. Two species were named: thetype speciesL. celer, based on parts of elevenvertebrae (YPM 1875);[5] and the "smaller"L. gracilis, originally based on a back vertebra'scentrum, acaudal centrum, and part of anulna (review byPeter Galton in 1983 finds the specimen to now consist of thirteen back and eight caudal centra, and portions of both hindlimbs).[5][6] 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.[7] 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.[6]
These animals attracted little professional attention until the 1970s and 1980s, when Peter Galton reviewed many of the "hypsilophodonts" in a series of papers. In 1973, he andJim Jensen described a partial skeleton (BYU ESM 163 as of Galton, 2007) missing the head, hands, and tail asNanosaurus?rex, which had been damaged by other collectors prior to description.[8] By 1977, he had concluded thatNanosaurus agilis was quite different fromN. rex and the new skeleton, and coinedOthnielia for the latter species. The paper (primarily concerning the transcontinental nature ofDryosaurus) consideredLaosaurus consors andL. gracilis synonyms ofO. rex without elaboration, and consideredL. celer an invalidnomen nudum.[9]
In 1990,Robert Bakker, Peter Galton, James Siegwarth, and James Filla described remains of a dinosaur they namedDrinker nisti. The name is somewhat ironic;Drinker, named for renownedpalaeontologistEdward Drinker Cope whose infamous "bone wars" with rivalOthniel Charles Marsh produced many dinosaur fossils which are world-famous today, was described as a probable close relative ofOthnielia, named for Marsh. The species name refers to the National Institute of Standards and Technology (NIST). Discovered by Siegwarth and Filla in upperMorrison Formation beds atComo Bluff,Wyoming, it wasbased on a partialsubadult skeleton (listed as CPS 106 originally, then as Tate 4001 by Bakker 1996[10]) including partial jaws, vertebrae, and partial limbs. Several other specimens found in the same area were assigned to it, mostly consisting of vertebral and hindlimb remains, and teeth.[11] The holotype specimen's current location is unknown; according to Carpenter and Galton (2018), the previous two institutions reported to have had it did not ever curate the specimen, and the collection it was originally said to be in never existed at all.[10]
Several decades later, in his 2007 study of the teeth of Morrison ornithischians, Galton concluded that the holotype femur ofOthnielia rex is not diagnostic, and reassigned the BYU skeleton toLaosaurus consors, which is based on better material. As the genusLaosaurus is also based on nondiagnostic material, he gave the speciesL. consors its own genus,Othnielosaurus. As a result, in practical terms, what had been thought of asOthnielia is now known asOthnielosaurus consors. RegardingNanosaurus agilis, Galton considered it a potentially valid basalornithopod, and noted similarities toheterodontosaurids in the thigh bone. He tentatively assigned to it some teeth that had been referred toDrinker.[2]
Another decade later, in 2018, Galton, alongsideKenneth Carpenter, described a new ornithischian specimen. They found it very similar to the fragmentary holotype ofNanosaurus, but more clear in its anatomical features. Their new specimen was also found to display extreme similarity with the specimens ofOthnielosaurus andDrinker. Due to the new data, they concluded that all three species, alongsideOthnielia, represented the same animal, united under the nameNanosaurus agilis. This painted a new picture of a singular, very common small dinosaur known from a large amount of material.[10] This conclusion has been recognized by papers since, some of which incorporating the new, all-encompassing taxon into theirphylogenetic analyses.[12][13][14]
Nanosaurus is known from material from all parts of the body, including two good skeletons, although the skull is still poorly known.[15] It was a small animal, with specimens previously assigned toDrinker andOthnielosaurus measuring 2–2.2 metres (6.6–7.2 ft) long and weighing 20–30 kilograms (44–66 lb).[16]
It was a bipedal dinosaur with short forelimbs and long hindlimbs with largeprocesses formuscle attachments.[17] 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), andpremaxillary teeth with less ornamentation.[2]
Like several other neornithischian dinosaurs, such asHypsilophodon,Thescelosaurus, andTalenkauen,Nanosaurus had thin plates lying along the ribs. Called intercostal plates, these structures werecartilaginous in origin.[18]
Thecladogram below results from analysis by Herne et al., 2019.[12]
Nanosaurus was one of the smaller members of the diverseMorrison Formation dinosaurfauna, diminutive in comparison to the giantsauropods.[19] The Morrison Formation is interpreted as asemiarid environment with distinctwet anddry seasons, and flatfloodplains.[20] Vegetation varied fromriver-lininggallery forests ofconifers,tree ferns, andferns, to fernsavannas with rare trees.[21] It has been a rich fossil hunting ground, holding fossils ofgreen algae,fungi,mosses,horsetails, ferns,cycads,ginkgoes, and several families ofconifers. Other fossils discovered includebivalves,snails,ray-finned fishes,frogs,salamanders,turtles,sphenodonts,lizards, terrestrial and aquaticcrocodylomorphs, several species ofpterosaur, numerous dinosaur species, and earlymammals such asdocodonts,multituberculates,symmetrodonts, andtriconodonts. Such dinosaurs as the theropodsCeratosaurus,Allosaurus,Ornitholestes, andTorvosaurus, the sauropodsApatosaurus,Brachiosaurus,Camarasaurus, andDiplodocus, and theornithischiansCamptosaurus,Dryosaurus, andStegosaurus are known from the Morrison.[22]Nanosaurus is present in stratigraphic zones 2-5.[23]
Typically,Nanosaurus has been interpreted like other hypsilophodonts as a small,swiftherbivore,[24] although Bakker (1986) interpretedNanosaurus as anomnivore.[25]
| |||||||||||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||||||||||
