''Alvarezsaurus'' (; "Alvarez's lizard") 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 ...

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 Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', ...

, living in

Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, th ...

approximately 86 - 83 million years ago. It was a small dinosaur, measuring long and weighing approximately . It was found in the

Bajo de la Carpa Formation The Bajo de la Carpa Formation is a geologic formation of the Neuquén Basin that crops out in northern Patagonia, in the provinces of Río Negro and Neuquén, Argentina. It is the oldest of two formations belonging to the Río Colorado Subgr ...

and was named by

paleontologist Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...

José Bonaparte José Fernando Bonaparte (14 June 1928 – 18 February 2020) was an Argentine paleontologist who discovered a plethora of South American dinosaurs and mentored a new generation of Argentine paleontologists . One of the best-known Argentine paleo ...

in 1991 after the historian Don Gregorio Alvarez. The

type species In 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 species that contains the biological type specime ...

is ''A. calvoi''.

Description

''Alvarezsaurus'' was a

biped Bipedalism is a form of terrestrial locomotion where an organism moves by means of its two rear limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from Latin ''bis'' 'double' ...

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 ...

. Like other lightweight theropods, it had a long tail, and its leg structure suggests that it was a fast runner. The most proximal elements of ''Alvarezsaurus'' caudal vertebrae exhibited ventrally sharp centra and the transerve processes of these vertebrae were sub-triangular and laterodistally directed, features seen in other alvarezsaurids like ''Shuvuuia''. Spinal processes were entirely absent or poorly developed, and each caudal vertebra supported short prezygapophyses. The scapula was visibly curved and proportionally smaller than those of other alvarezsaurids, and unlike its relatives ''Alvarezsaurus'' did not have a fused astragalus and calcaneum. It was unique in that its metatarsal III was its longest, followed by an unusually long metatarsal IV. It may have been insectivorous.

Classification

''Alvarezsaurus'' is considered basal to better-known members of its family, such as ''

Mononykus ''Mononykus'' ( , sometimes ; meaning "one claw") is a genus of alvarezsaurid dinosaur that lived during the Late Cretaceous in what is now Asia on the Nemegt Formation, about 70 million years ago. ''Mononykus'' was a very small theropod, estim ...

'' and '' Shuvuuia''. It has been alternately classified with both non-

avian Avian may refer to: *Birds or Aves, winged animals *Avian (given name) (russian: Авиа́н, link=no), a male forename Aviation *Avro Avian, a series of light aircraft made by Avro in the 1920s and 1930s *Avian Limited, a hang glider manufacture ...

dinosaurs and early

bird 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 ...

s, but a move of the alvarezsaurids to be recognized as more closely related to neornithine birds proved controversial despite being supported by earlier studies. It was once believed that the

Patagonia Patagonia () refers to a geographical region that encompasses the southern end of South America, governed by Argentina and Chile. The region comprises the southern section of the Andes Mountains with lakes, fjords, temperate rainforests, and g ...

n alvarezsaur taxa were the most basal of their family, but the discovery of a more basal member, ''

Haplocheirus ''Haplocheirus'' is a genus of theropod dinosaur. Upon its description, it was considered the oldest alvarezsauroid, predating all other members by about 63 million years. This has subsequently been questioned. A 2019 study considered ''Haploc ...

'', disproved that when its fossils were discovered in China.

References

External links

Natural History Museum, London
{{Taxonbar, from=Q134182 Alvarezsaurids Late Cretaceous dinosaurs of South America Fossil taxa described in 1991 Taxa named by José Bonaparte Bajo de la Carpa Formation