''Avisaurus'' (meaning "bird 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 ...

enantiornithine The Enantiornithes, also known as enantiornithines or enantiornitheans in literature, are a group of extinct avialans ("birds" in the broad sense), the most abundant and diverse group known from the Mesozoic era. Almost all retained teeth and c ...

avialan Avialae ("bird wings") is a clade containing the only living dinosaurs, the birds, and their closest relatives. It is usually defined as all theropod dinosaurs more closely related to birds (Aves) than to deinonychosaurs, though alternative defi ...

from the

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent 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 ''cre ...

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

Discovery

''Avisaurus archibaldi'' was discovered in the Late Cretaceous

Hell Creek Formation The Hell Creek Formation is an intensively studied division of mostly Upper Cretaceous and some lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana. The Formation (stratigraphy), formation s ...

of North America (

Maastrichtian The Maastrichtian ( ) is, in the International Commission on Stratigraphy (ICS) geologic timescale, the latest age (geology), age (uppermost stage (stratigraphy), stage) of the Late Cretaceous epoch (geology), Epoch or Upper Cretaceous series (s ...

, from c.70.6-66

million years ago Million years ago, abbreviated as Mya, Myr (megayear) or Ma (megaannum), is a unit of time equal to (i.e. years), or approximately 31.6 teraseconds. Usage Myr is in common use in fields such as Earth science and cosmology. Myr is also used w ...

), making it one of the last enantiornithids. It was collected in 1975 in the UCMP locality V73097, in Garfield County, Montana, USA. The holotype is represented by a single fossil of a

tarsometatarsus The tarsometatarsus is a bone that is only found in the lower leg of birds and some non-avian dinosaurs. It is formed from the fusion of several bird bones found in other types of animals, and homologous to the mammalian tarsus (ankle bones) a ...

in the collection of the University of California Museum of Paleontology. It has the catalog number UCMP 117600. The species name honors J. David Archibald, its discoverer, from

The University of California, Berkeley The University of California, Berkeley (UC Berkeley, Berkeley, Cal, or California), is a public land-grant research university in Berkeley, California, United States. Founded in 1868 and named after the Anglo-Irish philosopher George Berkeley ...

. It was initially described as the left tarsometatarsus of a non-avian

theropod Theropoda (; from ancient Greek , (''therion'') "wild beast"; , (''pous, podos'') "foot"">wiktionary:ποδός"> (''pous, podos'') "foot" is one of the three major groups (clades) of dinosaurs, alongside Ornithischia and Sauropodom ...

by Brett-Surman and Paul in 1985. It was later redescribed as the right tarsometatarsus of an

bird by Chiappe in 1992.Chiappe, Luis M. (1992) "Enantiornithine (Aves) Tarsometatarsi and the Avian Affinities of the Late Cretaceous Avisauridae" "Journal of Vertebrate Paleontology" September 3, 1992, Volume 12 no. 3 pp. 344-350 ''Avisaurus gloriae'' Varricchio and Chiappe 1995, discovered in the late

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. Campa ...

Upper

Two Medicine Formation The Two Medicine Formation is a geological formation, or rock body, in northwestern Montana and southern Alberta that was deposited between 82.4 Ma and 74.4 Ma, during Campanian (Late Cretaceous) time. It crops out to the east of the Rocky Mountai ...

of Glacier County, Montana, USA, was renamed '' Gettyia'' by Atterholt et al. (2018). In 2024, another species of ''Avisaurus'' from the Hell Creek Formation was named as ''Avisaurus darwini'', with the specific name in honor of

Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English Natural history#Before 1900, naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all speci ...

Description

''Avisaurus'' was a large enantiornithine, with ''A. darwini'' weighing up to and ''A. archibaldi'' weighing up to . The type specimen of ''A. archibaldi'' has a maximum length of , making it one of the largest known tarsometatarsi of an enantiornithine.Varricchio, David J., Chiappe, Luis M. (1995) "A New Enantiornithine Bird From the Upper Cretaceous Two medicine Formation of Montana" " Journal of Vertebrate Paleontology" March 14, 1995, Vol. 15, no. 1, pp. 201 - 204

Classification

This genus belongs to the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Avisauridae, which also contains similar animals from

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...

such as '' Soroavisaurus'', '' Neuquenornis'' and '' Intiornis''. Brett-Surman and Paul (1985) explicitly considered the possibility that ''A. archibaldi'' was an enantiornithine. The authors described and named UCMP 117600 formally, but they looked at other enantiornithine material, including the "

metatarsus The metatarsal bones or metatarsus (: metatarsi) are a group of five long bones in the midfoot, located between the tarsal bones (which form the heel and the ankle) and the phalanges ( toes). Lacking individual names, the metatarsal bones are ...

" PVL 4690 from Argentina. The authors assigned this latter fossil to ''Avisaurus'' sp. From this they concluded that members of the genus ''Avisaurus'' existed in both North and South America in the Late Cretaceous. Moreover, the authors concluded that the length/width ratio and degree of metacarpal fusion of these bones were more like those of non — avian dinosaurs. A terrestrial dinosaur genus in both continents would then support Brett — Surman's theory that there had been a land connection between the two continents.Brett-Surman, Michael K., Paul, Gregory S. (1985) "A new family of bird-like dinosaurs linking Laurasia and Gondwanaland." "Journal of Vertebrate Paleontology" 5(2): 133-138. Further discoveries and further study by Chiappe showed that all of the material belonged to enantiornithine birds, and PVL 4690 was given its own genus '' Soroavisaurus''.

Paleoenvironment and paleobiology

''Avisaurus'' remains fossilized in the

humid Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity depe ...

low-lying swamps, lakes, and river basins of the western shore of the

Western Interior Seaway The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, or the Western Interior Sea) was a large inland sea (geology), inland sea that existed roughly over the present-day Great Plains of ...

. Inwardly curved claws of the ''Avisaurus'' tarsis resemble those of eudromaeosaurs, indicating that it was predatory. It likely preyed on other vertebrates, with similarity to some modern raptors. File:Avisaurus and Brachychampsa by tomozsaurus.jpg, Reconstruction of ''Avisaurus'' and '' Brachychampsa'' File:Reconstruction of an avisaurid (e.g., A. darwini).png, Reconstruction of ''Avisaurus darwini'' with prey

Discovery

Description

Classification

Paleoenvironment and paleobiology

References

Further reading