''Xiaosaurus'' ("dawn lizard", ), is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of small

herbivorous A herbivore is an animal anatomically and physiologically evolved to feed on plants, especially upon vascular tissues such as foliage, fruits or seeds, as the main component of its diet. These more broadly also encompass animals that eat n ...

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...

from the middle

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

, approximately 170.3 to 163.5 mya. ''Xiaosaurus'' lived in what is now the

Sichuan Basin The Sichuan Basin (), formerly transliterated as the Szechwan Basin, sometimes called the Red Basin, is a lowland region in southwestern China. It is surrounded by mountains on all sides and is drained by the upper Yangtze River and its tributar ...

China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

Discovery and naming

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

Dashanpu The Shaximiao Formation () is a Middle Jurassic, Middle to Late Jurassic aged geological formation in Sichuan, China, most notable for the wealth of dinosaurs fossils that have been excavated from its strata. The Shaximiao Formation is exposed in ...

Sichuan Sichuan is a province in Southwestern China, occupying the Sichuan Basin and Tibetan Plateau—between the Jinsha River to the west, the Daba Mountains to the north, and the Yunnan–Guizhou Plateau to the south. Its capital city is Cheng ...

. In 1983

Dong Zhiming Dong Zhiming (Chinese language, Chinese: 董枝明, Pinyin: ''Dǒng Zhimíng''; January 1937 – 20 October 2024) was a Chinese vertebrate paleontologist formerly employed at the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) ...

and Tang Zilu named the fossils under the

type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...

''Xiaosaurus dashanpensis''. The generic name is derived from Chinese ''xiáo'', 曉, "dawn", a reference to the age of the fossil. The

specific name Specific name may refer to: * in Database management systems, a system-assigned name that is unique within a particular database In taxonomy, either of these two meanings, each with its own set of rules: * Specific name (botany), the two-part (bino ...

refers to Danshanpu. The

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

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

Bajocian In the geologic timescale, the Bajocian is an age and stage in the Middle Jurassic. It lasted from approximately 170.9 ±0.8 Ma to around 168.2 ±1.2 Ma (million years ago). The Bajocian Age succeeds the Aalenian Age and precedes the Bathonia ...

and the

Bathonian In the geologic timescale the Bathonian is an age (geology), age and stage (stratigraphy), stage of the Middle Jurassic. It lasted from approximately 168.2 ±1.2 annum, Ma to around 165.3 ±1.1 Ma (million years ago). The Bathonian Age succeeds ...

–

Callovian In the geologic timescale, the Callovian is an age and stage in the Middle Jurassic, lasting between 165.3 ± 1.1 Ma (million years ago) and 161.5 ± 1.0 Ma. It is the last stage of the Middle Jurassic, following the Bathonian and preceding the ...

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

vertebra Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spina ...

e, four caudal vertebrae, a

humerus The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...

, a partial left

femur The femur (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the knee. In many quadrupeds, four-legged animals the femur is the upper bone of the hindleg. The Femo ...

and a complete right hindlimb. The

paratype In zoology and botany, a paratype is a specimen of an organism that helps define what the scientific name of a species and other taxon actually represents, but it is not the holotype (and in botany is also neither an isotype (biology), isotype ...

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

phalanx The phalanx (: phalanxes or phalanges) was a rectangular mass military formation, usually composed entirely of heavy infantry armed with spears, pikes, sarissas, or similar polearms tightly packed together. The term is particularly used t ...

, a rib and two teeth. In 1992 Peng Guangzhao renamed '' Agilisaurus multidens'' He & Cai 1983 (now '' Hexinlusaurus'') into a second species of ''Xiaosaurus'': ''Xiaosaurus multidens'', but this has not been accepted.

Description

''Xiaosaurus'' was a small bipedal animal with an estimated length of . The femur is long.

Classification

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

Hypsilophodontidae Hypsilophodontidae (or Hypsilophodontia) is a traditionally used family (biology), family of ornithopod dinosaurs, generally considered invalid today. It historically included many small bodied bipedal neornithischian taxa from around the world, ...

, considering it an

evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

ary link between ''

Lesothosaurus ''Lesothosaurus'' is a Monotypic taxon, 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 L ...

'' and ''

Hypsilophodon ''Hypsilophodon'' (; meaning "high-crested tooth") is a neornithischian dinosaur genus from the Early Cretaceous period of England. It has traditionally been considered an early member of the group Ornithopoda, but recent research has put this ...

''. ''Xiaosaurus'' has sometimes been considered a ''

nomen dubium In binomial nomenclature, a ''nomen dubium'' (Latin for "doubtful name", plural ''nomina dubia'') is a scientific name that is of unknown or doubtful application. Zoology In case of a ''nomen dubium,'' it may be impossible to determine whether a ...

'' and an

ornithischia Ornithischia () is an extinct clade of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds. The name ''Ornithischia'', or "bird-hipped", reflects this similarity and is derived from the Greek ...

n of uncertain affinities, possibly a basal cerapod or marginocephalian. However,

Paul Barrett Paul Franklyn "Legs" Barrett (14 December 1940 – 20 January 2019) was a UK agent and manager of 1950s style rock and roll artists, an author and previously a singer, songwriter and film actor. Barrett was the discoverer, mentor and first man ...

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

autapomorphy In phylogenetics, an autapomorphy is a distinctive feature, known as a Synapomorphy, derived trait, that is unique to a given taxon. That is, it is found only in one taxon, but not found in any others or Outgroup (cladistics), outgroup taxa, not ...

: a mediolaterally (seen from the front) straight humerus.Barrett, P.M., Butler, R.J., and Knoll, F. 2005. Small-bodied ornithischian dinosaurs from the Middle Jurassic of Sichuan, China. ''Journal of Vertebrate Paleontology'' 25(4):823-834

Notes

References

DinoDictionary.com's entry for ''Xiaosaurus''
{{Taxonbar, from=Q1203400 Neornithischia Dinosaur genera Bathonian dinosaurs Shaximiao Formation Dinosaurs of China Fossil taxa described in 1983 Taxa named by Dong Zhiming