Xiaosaurus is a genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of small herbivorous

Herbivore

Herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...

 dinosaur

Dinosaur

Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...

 from the middle Jurassic

Jurassic

The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

, approximately 169 to 163 mya. Xiaosaurus lived in what is now the Sichuan Basin

Sichuan basin

The Sichuan Basin is a lowland region in southwestern China. Despite its historical name, it is not only synonymous to Sichuan province, comprising its central and eastern portions as well as part of Chongqing Municipality...

 of China

China

Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

.

In 1979 and 1980, two specimens were discovered of a small herbivorous dinosaur during excavations near Dashanpu in Sichuan

Sichuan

' , known formerly in the West by its postal map spellings of Szechwan or Szechuan is a province in Southwest China with its capital in Chengdu...

. In 1983 Dong Zhiming

Dong Zhiming

Dong Zhiming , from the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, is one of China's leading paleontologists. He began working at the IVPP in 1962, learning from Yang Zhongjian who was director at the time...

 and Tang Zilu named the fossils under the type species

Type species

In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...

 Xiaosaurus dashanpensis. The generic name is derived from Chinese

Chinese language

The Chinese language is a language or language family consisting of varieties which are mutually intelligible to varying degrees. Originally the indigenous languages spoken by the Han Chinese in China, it forms one of the branches of Sino-Tibetan family of languages...

 xiáo, 膮, "dawn", a reference to the age of the fossil. The specific name refers to Danshanpu.

The holotype

Holotype

A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...

, IVPP V6730A, was found in the lower Xiashaximiao Formation of which the age is uncertain: both the Bajocian

Bajocian

In the geologic timescale, the Bajocian is an age or stage in the Middle Jurassic. It lasted from approximately 171.6 Ma to around 167.7 Ma . The Bajocian age succeeds the Aalenian age and precedes the Bathonian age....

 and the Bathonian

Bathonian

In the geologic timescale the Bathonian is an age or stage of the Middle Jurassic. It lasted from approximately 167.7 Ma to around 164.7 Ma...

–Callovian

Callovian

In the geologic timescale, the Callovian is an age or stage in the Middle Jurassic, lasting between 164.7 ± 4.0 Ma and 161.2 ± 4.0 Ma. It is the last stage of the Middle Jurassic, following the Bathonian and preceding the Oxfordian....

 have been proposed. It consists of a partial skeleton including a jaw fragment with a single tooth, two cervical vertebrae, four caudal vertebrae, a humerus

Humerus

The humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....

, a partial left femur

Femur

The femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...

 and a complete right hindlimb. The paratype

Paratype

Paratype is a technical term used in the scientific naming of species and other taxa of organisms. The exact meaning of the term paratype when it is used in zoology is not the same as the meaning when it is used in botany...

 IVPP V6730B is a second partial skeleton including a right femur, a dorsal vertebra, two sacral vertebrae, a phalanx

Phalanx bones

In anatomy, phalanx bones are those that form the fingers and toes. In primates such as humans and monkeys, the thumb and big toe have two phalanges, while the other fingers and toes consist of three. Phalanges are classified as long bones.The phalanges do not have individual names...

, a rib and two teeth.

In 1992 Peng Guangzhao renamed Agilisaurus

Agilisaurus

Agilisaurus is a genus of ornithischian dinosaur from the Middle Jurassic Period of what is now eastern Asia. The name is derived from the Latin agilis meaning 'agile' and the Greek sauros meaning 'lizard', and refers to the agility suggested by its lightweight skeleton and long legs...

 multidens He & Cai 1983 (now Hexinlusaurus

Hexinlusaurus

Hexinlusaurus is a genus of basal ornithischian dinosaur from the Middle Jurassic of China. The holotype , consists of an almost complete, articulated skull and some postcranial material, collected from a terrestrial sandstone within the Lower Shaximiao Formation at the famous dinosaur-bearing...

) into a second species of Xiaosaurus: Xiaosaurus multidens, but this has not been accepted.

Xiaosaurus was a small bipedal animal with an estimated length of one metre. The femur is 11 centimetres (4.3 in) long.

The remains are too fragmentary to easily classify the genus. The describers assigned it both to the Fabrosauridae

Fabrosauridae

The Fabrosauridae was originally proposed by Galton , as a group of basal ornithischian dinosaurs from the Early to Middle Jurassic. However, recent studies indicate the Fabrosaurs are not a natural grouping of dinosaurs, and instead consist of unrelated genera.The proposed "Fabrosaurs" descended...

 and the Hypsilophodontidae, considering it an evolution

Evolution

Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

ary link between Lesothosaurus

Lesothosaurus

Lesothosaurus is an herbivorous ornithischian dinosaur. It was named by paleontologist Peter Galton in 1978, the name meaning "lizard from Lesotho". The genus is monotypic, having only one valid species, Lesothosaurus diagnosticus, within the genus....

and Hypsilophodon

Hypsilophodon

Hypsilophodon is an ornithopod dinosaur genus from the Early Cretaceous period of Europe. It was a small bipedal animal with an herbivorous or possibly omnivorous diet...

. Xiaosaurus has sometimes been considered a nomen dubium

Nomen dubium

In zoological nomenclature, a nomen dubium is a scientific name that is of unknown or doubtful application...

and an ornithischia

Ornithischia

Ornithischia or Predentata is an extinct order of beaked, herbivorous dinosaurs. The name ornithischia is derived from the Greek ornitheos meaning 'of a bird' and ischion meaning 'hip joint'...

n of uncertain affinities

Incertae sedis

, is a term used to define a taxonomic group where its broader relationships are unknown or undefined. Uncertainty at specific taxonomic levels is attributed by , , and similar terms.-Examples:*The fossil plant Paradinandra suecica could not be assigned to any...

, possibly a basal cerapod

Cerapoda

Neornithischia is a clade of the dinosaur order Ornithischia. They are the sister group of the Thyreophora within the clade Genasauria. Neornithischians are united by having a thicker layer of asymmetrical enamel on the inside of their lower teeth...

 or marginocephalia

Marginocephalia

Marginocephalia is a clade of ornithischian dinosaurs that includes the thick-skulled pachycephalosaurids, and horned ceratopsians. They were all herbivores, walking on two or four legs, and are characterized by a bony ridge or frill the back of the skull...

n. However, Paul Barrett

Paul Barrett

Paul Franklyn "Legs" Barrett is the UK's best known agent and manager of 1950s style Rock and Roll artistes, an author and previously a singer and film actor...

 et al. in 2005 concluded it to be provisionally valid, as it possessed a single unique derived trait or autapomorphy

Autapomorphy

In cladistics, an autapomorphy is a distinctive anatomical feature, known as a derived trait, that is unique to a given terminal group. That is, it is found only in one member of a clade, but not found in any others or outgroup taxa, not even those most closely related to the group...

: a mediolaterally (seen from the front) straight humerus.