''Xixianykus'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

alvarezsaurid Alvarezsauridae is a family of small, long-legged dinosaurs. Although originally thought to represent the earliest known flightless birds, they are now thought to be an early diverging branch of maniraptoran theropods. Alvarezsaurids were highly ...

theropod Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally c ...

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

from the late

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

period of China.Xu, X., Wang, D.Y., Sullivan, C., Hone, D.W.E., Han, F.L., Yan, R.H. and Du, F.M. (2010). "A basal parvicursorine (Theropoda: Alvarezsauridae) from the Upper Cretaceous of China.". ''Zootaxa'', 2413: 1-19.

Discovery and naming

The fossil,

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of sever ...

XMDFEC V0011, was found in the

Majiacun Formation The Majiacun Formation is a Santonian to Coniacian geologic Formation (geology), formation in China. * ''Zhanghenglong, Zhanghenglong yangchengensis''Xing et al., 2014 * Baryonychinae, Baryonychinae indet.? (possibly an indeterminate theropod due ...

Henan Henan (; or ; ; alternatively Honan) is a landlocked province of China, in the central part of the country. Henan is often referred to as Zhongyuan or Zhongzhou (), which literally means "central plain" or "midland", although the name is al ...

Province, China. The fossil consists of a partial skeleton, without the skull. Parts of the hind legs, pelvis and spine has also been recovered. ''Xixianykus'' is among the oldest of the derived alvarezsauroids, the parvicursorines, dated to the

Santonian The Santonian is an age in the geologic timescale or a chronostratigraphic stage. It is a subdivision of the Late Cretaceous Epoch or Upper Cretaceous Series. It spans the time between 86.3 ± 0.7 mya (million years ago) and 83.6 ± 0.7 mya. ...

– Coniacian, as opposed to other parvicursorines, which are either

Campanian The Campanian is the fifth of six ages of the Late Cretaceous Epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campani ...

Maastrichtian The Maastrichtian () is, in the ICS geologic timescale, the latest age (uppermost stage) of the Late Cretaceous Epoch or Upper Cretaceous Series, the Cretaceous Period or System, and of the Mesozoic Era or Erathem. It spanned the interval ...

.Hone, D. (2010)
''Xixianykus zhangi'' - A New Alvarezsaur
Dave Hone's Archosaur Musings, April 23, 2011. The type species is ''Xixianykus zhangi'' described in 2010 by Xu Xing. The genus name refers to

Xixia County Xixia County (; postal: Sisia) is a county in the southwest of Henan province, China, bordering Shaanxi province to the west. It is under the administration of the prefecture-level city of Nanyang, and has an area of and a population of 420,000 a ...

and to the Greek word for claw ("''onyx''"). The latter is a common element used in names for

Alvarezsaurids Alvarezsauridae is a family of small, long-legged dinosaurs. Although originally thought to represent the earliest known flightless birds, they are now thought to be an early diverging branch of maniraptoran theropods. Alvarezsaurids were highly ...

. The species name is in honor of Zhang Xinglao.

Description

''Xixianykus'' was a small animal, about long and high. It is one of the smallest-known dinosaurs (not counting

avians Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...

). It appears to have many adaptations towards a

cursorial A cursorial organism is one that is adapted specifically to run. An animal can be considered cursorial if it has the ability to run fast (e.g. cheetah) or if it can keep a constant speed for a long distance (high endurance). "Cursorial" is often us ...

(running) lifestyle. It was about 50 centimetres long but had legs and a short

femur The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates wit ...

combined with a long

tibia The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it connects ...

and

metatarsus The metatarsal bones, or metatarsus, are a group of five long bones in the foot, located between the tarsal bones of the hind- and mid-foot and the phalanges of the toes. Lacking individual names, the metatarsal bones are numbered from the me ...

, which are good indicators of it being a fast runner. It was probably covered in feathers.

References

External links

Agile 'roadrunner' dinosaur fossil discovered in China
Alvarezsaurids Late Cretaceous dinosaurs of Asia Fossil taxa described in 2010 Taxa named by Xu Xing {{Theropod-stub