Avisaurus

Taxobox
name = "Avisaurus"
fossil_range = fossil range|77|65.5 Late Cretaceous
regnum = Animalia
phylum = Chordata
classis = Aves
subclassis = Enantiornithes
superordo = Euenantiornithes
ordo = Enantiornithiformes
familia = Avisauridae
genus = "Avisaurus"
genus_authority = Brett-Surman & Paul, 1985
subdivision_ranks = Species
subdivision = "Avisaurus archibaldi" (type) "Avisaurus gloriae"

"Avisaurus" (meaning "bird lizard") is a genus in a group of Cretaceous birds called Enantiornithes. Its first known remains were leg bones of the type species "A. archibaldi", discovered in the Late Cretaceous Hell Creek Formation of North America (Maastrichtian, c.70.6-65.5 million years ago), making it one of the last and most advanced enantiornitids. Subsequently, the older species "A. gloriae" was described from the Campanian Two Medicine Formation (c. 77 - 71 mya).

"Avisaurus" probably had a wingspan of about 1.2 meters at the largest and hunted smaller birds, mammals, and young non-avian dinosaurs. Palentologists think that enantiornithines had a long growing period, but left the nest soon after hatching (Cambra-Moo "et al." 2006). Consequently, "Avisaurus" individuals likely occupied different ecological niches in different periods of their lifespan rather than immediately starting off as predators of vertebrates.

The species of "Avisaurus" are known from the humid low-lying swamps, lakes, and river basins of the western shore of the Western Interior Seaway, and from the much more arid uplands between that area and the Cordilleran Overthrust Belt which eventually formed the Rocky Mountains. Residence in such different habitat types coupled with the low morphological differentiation further strengthens the case for "Avisaurus" being predators of larger animals, instead of relying more directly on the primary production which strongly varies according to habitat.

This genus belongs to the Avisauridae, which also contain similar animals from South America such as "Neuquenornis" (Chiappe 1993). These were the equivalent to the birds of prey of our time in the Late Cretaceous Americas, which at that time were still separated by a branch of the Tethys Ocean. The avisaurids had probably few species, in contrast with the roughly 300 species of today's Falconiformes.

References

* Brett-Surman, Michael K. & Paul, Gregory S. (1985): A new family of bird-like dinosaurs linking Laurasia and Gondwanaland. "J. Vertebr. Paleontol." 5(2): 133-138.
* Cambra-Moo, Oscar; Delgado Buscalioni, Ángela; Cubo, Jorge; Castanet, Jacques; Loth, Marie-Madeleine; de Margerie, Emmanuel & de Ricqlès, Armand (2006): Histological observations of Enantiornithine bone (Saurischia, Aves) from the Lower Cretaceous of Las Hoyas (Spain). "C. R. Palevol" 5(5): 685–691. DOI|10.1016/j.crpv.2005.12.018 [http://www.academie-sciences.fr/publications/comptes_rendus/pdf/CRPalevol_article.pdf PDF fulltext]
* Chiappe, Luis M. (1993): Enantiornithine (Aves) Tarsometatarsi from the Cretaceous Lecho Formation of Northwestern Argentina. "American Museum Novitates" 3083: 1-27. [English with Spanish abstract] [http://digitallibrary.amnh.org/dspace/bitstream/2246/4981/1/N3083.pdf PDF fulltext]