Apsaravis

Taxobox
name = "Apsaravis"
status = fossil
fossil_range = fossil range|78Late Cretaceous (Campanian)
regnum = Animalia
phylum = Chordata
classis = Aves
subclassis = Ornithurae
familia = Ambiortidae?
genus = "Apsaravis"
genus_authority =
species = "A. ukhaana"
binomial = "Apsaravis ukhaana"
binomial_authority =

"Apsaravis" is a Mesozoic bird genus from the Late Cretaceous. The single known species, "Apsaravis ukhaana", lived about 78 million years ago, in the Campanian age of the Cretaceous period. Its fossilized remains were found in the Camel's Humps sublocality of the Djadokhta Formation, at Ukhaa Tolgod in the Gobi Desert of Mongolia. They were collected in the 1998 field season by the Mongolian Academy of Sciences/American Museum of Natural History Paleontological Expeditions. It was described by Norell and Clarke (2001). Norell Mark A. ,Clarke, Julia A. (2001) "Fossil that fills a critical gap in avian evolution" "Nature" Vol. 409 11 January 2001 pp.181-184,. ]

Its habitat was presumably very arid open landscape much like it is today, perhaps hotter still and with more (but nonetheless intermittent) rain. Permanent freshwater would have been scarce.

Implications

"Apsaravis" is important in avian Paleontology. It has provided evidence that is directly relevant to at least four issues;

auriurae

The Sauriurae is a putative clade of primitive birds that includes "Archaeopteryx", "Confuciusornis", and Enantiornithes. It is thought by Feduccia and Martin to be phylogenetically separate from the Ornithurae and, thus, from modern birds. "Apsaravis" has features of both Sauriurae and Ornithurae. "Apsaravis" has several characters that place it near Aves (sensu Gauthier), including the presence of at least ten sacral vertebrae, a pubis and ischium that are closely appressed, distal pubes that do not touch, an 'obturator flange' on the ischium, loss of the cuppedicus muscle fossa on the ilium, a patellar groove on the distal femur, an anterior sternal keel, completely heterocoelus vertebrae, curved scapular shaft, and several features of the forelimb, ankle, and foot. Apsaravis also retains primitive characters shared withEnantiornithes and more basal theropods, including a narrow intercondylar groove and barrel - shaped condyles of the tibiotarsus, a dorsal fossa of the coracoid, into which the the supracoracoideus nerve foramen opens, and several features of the humerus. This intermediate anatomy is evidence against the validity of the clade "Sauriurae". Clarke,,Julia. A. , Norell, Mark. A. (2002) "The Morphology and Phylogenetic Position of Apsaravis ukhaana from the Late Cretaceous of Mongolia" American Museum Novitates, No. 3387, American Museum of Natural History, New York, NY 10024. ]

Enantiornithine monophyly

(See Enantiornithes)In their cladistic analyses, Clarke and Norell (2002) found that "Apsaravis" had a mixture of primitive and advanced characters (described above in "Sauriurae") that removed most of the supporting characters for the cladeEnantiornithes. 27characters have been used to support enantiornithine monophyly, but "Apsaravis" brings the number down to only 4. The discovery of further basal ornithurine fossils like "Apsaravis" could render the Enantiornithes paraphyletic. This would mean that, rather than a radiation of primitive birds separate from the radiation that led to modern birds, "enantiornithines" would actually be steps along the way to becoming modern birds.

Ecological bottle neck

Prior to the discovery of "Apsaravis", most Ornithurine birds had been found in marine, lacustrine, or littoral sediments. This led Feduccia (1996) Feduccia, Alan. (1996) "The origin and evolution of birds"New Haven, CT:Yale University Press 420pp. ] and Martin (1983) Martin, Larry D. (1983) "The origin and early radiation of birds" pp.291-338"in" "Perspectives on Ornithology" Brush, A.H., Clark, G.A. New York: Cambridge University Press] to deduce that the ancestors of modern birds were restricted to aquatic environments, and that they were all basal members of the Charadriiformes. Because they believed that all such birds had a "shorebird ecology", they described this limited habitat as an "ecological bottleneck", with all other ecological niches being dominated by enantiornithine birds instead. Apsaravis, however, was found in a sand dune environment, and it has no obviously aquatic anatomical adaptations , giving clear evidence that not all early members of Ornithurae were shorebirds. Clarke,,Julia. A. , Norell, Mark. A. (2002) "The Morphology and Phylogenetic Position of Apsaravis ukhaana from the Late Cretaceous of Mongolia" American Museum Novitates, No. 3387, American Museum of Natural History, New York, NY 10024. ]

Automatic extension of the manus

Apsaravis is the most basal bird that possesses an extensor process. This is a bony projection on metacarpal I that develops at the insertion of the m.extensor metacarpi radialis muscle and the propatagial ligaments. This anatomy functions to "automate" extension of the manus during extension of the forelimb in Aves. This is a key function for the flight stroke of modern birds.

Phylogenetic Position

Clarke and Norell (2002) found that "Apsaravis" is the most basal ornithurine bird, but more advanced than Enantiornithes and "Patagopteryx".

Mortimer's (2004) cladistic analysis confirms that "Apsaravis" is in the lineage that led to modern birds, together with the famous "Gansus yumenensis" for example, but not especially close to "Ambiortus". Mortimer found it to be a not-too-distant relative of the common ancestor of all living birds. [Mortimer (2004)]

Footnotes

References

* (2004): The Theropod Database: [http://home.comcast.net/~eoraptor/Phylogeny%20of%20Taxa.html Phylogeny of taxa] . Retrieved 2008-AUG-14.