''Apsaravis'' is a

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

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 ...

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 ...

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 ...

. The single known species, ''Apsaravis ukhaana'', lived about 78 million years ago, in the

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 ...

age of the Cretaceous period. Its

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

ized remains were found in the Camel's Humps sublocality of the

Djadokhta Formation The Djadochta Formation (sometimes transcribed and also known as Djadokhta, Djadokata, or Dzhadokhtskaya) is a highly fossiliferous geological formation situated in Central Asia, Gobi Desert, dating from the Late Cretaceous period, about 75 mill ...

, at Ukhaa Tolgod in the

Gobi Desert The Gobi Desert ( Chinese: 戈壁 (沙漠), Mongolian: Говь (ᠭᠣᠪᠢ)) () is a large desert or brushland region in East Asia, and is the sixth largest desert in the world. Geography The Gobi measures from southwest to northeast a ...

Mongolia Mongolia; Mongolian script: , , ; lit. "Mongol Nation" or "State of Mongolia" () is a landlocked country in East Asia, bordered by Russia to the north and China to the south. It covers an area of , with a population of just 3.3 million ...

. 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 A region is arid when it severely lacks available water, to the extent of hindering or preventing the growth and development of plant and animal life. Regions with arid climates tend to lack vegetation and are called xeric or desertic. Most ...

open landscape much like it is today, perhaps hotter still and with more (but nonetheless intermittent) rain. Permanent

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does incl ...

would have been scarce.

Implications

''Apsaravis'' is important in avian paleontology. It has provided evidence that is directly relevant to at least four issues:

Sauriurae

The

Sauriurae Sauriurae (meaning "lizard tails" in Greek) is a now-deprecated subclass of birds created by Ernst Haeckel in 1866. It was intended to include ''Archaeopteryx'' and distinguish it from all other birds then known, which he grouped in the sister-gr ...

are a putative clade of primitive birds that includes ''

Archaeopteryx ''Archaeopteryx'' (; ), sometimes referred to by its German name, "" ( ''Primeval Bird''), is a genus of bird-like dinosaurs. The name derives from the ancient Greek (''archaīos''), meaning "ancient", and (''ptéryx''), meaning "feather" ...

'', ''

Confuciusornis ''Confuciusornis'' is a genus of basal crow-sized avialan from the Early Cretaceous Period of the Yixian and Jiufotang Formations of China, dating from 125 to 120 million years ago. Like modern birds, ''Confuciusornis'' had a toothless beak, b ...

'', and

Enantiornithes 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 cla ...

. It is thought by Feduccia and Martin to be phylogenetically separate from the

Ornithurae Ornithurae (meaning "bird tails" in Greek) is a natural group which includes the common ancestor of '' Ichthyornis'', '' Hesperornis'', and all modern birds as well as all other descendants of that common ancestor. Classification Ernst Haeckel ...

and, thus, from modern birds. ''Apsaravis'' has features of both Sauriurae and Ornithurae. ''Apsaravis'' has several characters that place it near

Aves Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

(sensu Gauthier), including the presence of at least ten sacral

vertebrae The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristi ...

, a pubis and

ischium The ischium () form ...

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 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 ...

, 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 with Enantiornithes and more basal

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 ...

s, including a narrow intercondylar groove and barrel-shaped

condyle A condyle (;Entry "condyle"
in
tibiotarsus, a dorsal fossa of the

coracoid A coracoid (from Greek κόραξ, ''koraks'', raven) is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is prese ...

, into which the supracoracoideus nerve

foramen In anatomy and osteology, a foramen (;Entry "foramen"
in

opens, and several features of the

humerus The humerus (; ) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of a r ...

. 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.

Enantiornithine monophyly

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 clade

. Twenty-seven characters have been used to support enantiornithine monophyly, but ''Apsaravis'' brings the number down to only four. The discovery of further basal ornithurine fossils like ''Apsaravis'' could render the Enantiornithes

paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...

. 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 Charadriiformes (, from ''Charadrius'', the type genus of family Charadriidae) is a diverse order of small to medium-large birds. It includes about 390 species and has members in all parts of the world. Most charadriiform birds live near water a ...

. 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

were shorebirds.

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

. 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

and '' Patagopteryx''. Subsequent

cladistic Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived char ...

analysis indicates that it and the more advanced '' Palintropus'' – long believed to be a modern bird –, and perhaps ''

Ambiortus ''Ambiortus'' is a prehistoric bird genus. The only known species, ''Ambiortus dementjevi'', lived sometime during the Barremian age between 136.4 and 125 million years agoHoltz, Thomas R. Jr. (2012). ''Dinosaurs: The Most Complete, Up-to-D ...

'' with which the preceding two had occasionally been allied, form a distinct lineage. This has been named "Palintropiformes", but Apsaraviformes was proposed earlier for the ''Apsaravis'' lineage and is thus the

senior synonym The Botanical and Zoological Codes of nomenclature treat the concept of synonymy differently. * In botanical nomenclature, a synonym is a scientific name that applies to a taxon that (now) goes by a different scientific name. For example, Linnae ...

. And if this group is held to include ''Apatornis'' too, it would receive the name Ambiortiformes, which was proposed even earlier.

Footnotes

References

* (2004): The Theropod Database
Phylogeny of taxa
Retrieved 2013-MAR-02. {{Taxonbar, from=Q4782167 Bird genera Fossil taxa described in 2001 Late Cretaceous birds of Asia Prehistoric euornitheans