Apsaravis
Encyclopedia
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 fossil
ized 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).
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.
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 with Enantiornithes 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 supracoracoideus nerve foramen opens, and several features of the humerus. This intermediate anatomy is evidence against the validity of the clade "Sauriurae".
. 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. 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.
. 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.
and Patagopteryx
.
Subsequent ccladistic analysis indicates that it and the more advanced Palintropus
– long believed to be a modern bird –, and perhaps Ambiortus
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. And if this group is held to include Apatornis too, it would receive the name Ambiortiformes, which was proposed even earlier.
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
from the Late Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
. The single known species, Apsaravis ukhaana, lived about 78 million years ago, in the Campanian
Campanian
The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...
age of the Cretaceous period. Its fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
ized remains were found in the Camel's Humps sublocality of the Djadokhta Formation, at Ukhaa Tolgod in the Gobi Desert
Gobi Desert
The Gobi is a large desert region in Asia. It covers parts of northern and northwestern China, and of southern Mongolia. The desert basins of the Gobi are bounded by the Altai Mountains and the grasslands and steppes of Mongolia on the north, by the Hexi Corridor and Tibetan Plateau to the...
of Mongolia
Mongolia
Mongolia is a landlocked country in East and Central Asia. It is bordered by Russia to the north and China to the south, east and west. Although Mongolia does not share a border with Kazakhstan, its western-most point is only from Kazakhstan's eastern tip. Ulan Bator, the capital and largest...
. 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).
Its habitat was presumably very arid
Arid
A region is said to be arid when it is characterized by a severe lack of available water, to the extent of hindering or even preventing the growth and development of plant and animal life...
open landscape much like it is today, perhaps hotter still and with more (but nonetheless intermittent) rain. Permanent freshwater
Freshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...
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 SauriuraeSauriurae
Sauriurae 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-group Ornithurae...
is a putative clade of primitive birds that includes Archaeopteryx
Archaeopteryx
Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...
, Confuciusornis
Confuciusornis
Confuciusornis is a genus of primitive crow-sized birds from the Early Cretaceous Yixian and Jiufotang Formations of China, dating from 125 to 120 million years ago...
, and Enantiornithes
Enantiornithes
Enantiornithes is an extinct group of primitive birds. They were the most abundant and diverse avialans of the Mesozoic. Almost all retained teeth and clawed fingers on each wing, but otherwise looked much like modern birds externally. Over 50 species of Enantiornithines have been named, but some...
. It is thought by Feduccia and Martin to be phylogenetically separate from the Ornithurae
Ornithurae
Ornithurae is the name of a natural group which includes all modern birds as well as their extinct relatives with plough-shaped pygostyles, a bone at the end of the tail which allows the tail feathers to fan and retract....
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 with Enantiornithes 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 supracoracoideus nerve foramen opens, and several features of the humerus. This intermediate anatomy is evidence against the validity of the clade "Sauriurae".
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 EnantiornithesEnantiornithes
Enantiornithes is an extinct group of primitive birds. They were the most abundant and diverse avialans of the Mesozoic. Almost all retained teeth and clawed fingers on each wing, but otherwise looked much like modern birds externally. Over 50 species of Enantiornithines have been named, but some...
. 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. 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) and Martin (1983) to deduce that the ancestors of modern birds were restricted to aquatic environments, and that they were all basal members of the CharadriiformesCharadriiformes
Charadriiformes is a diverse order of small to medium-large birds. It includes about 350 species and has members in all parts of the world. Most Charadriiformes live near water and eat invertebrates or other small animals; however, some are pelagic , some occupy deserts and a few are found in thick...
. 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
Ornithurae
Ornithurae is the name of a natural group which includes all modern birds as well as their extinct relatives with plough-shaped pygostyles, a bone at the end of the tail which allows the tail feathers to fan and retract....
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 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 EnantiornithesEnantiornithes
Enantiornithes is an extinct group of primitive birds. They were the most abundant and diverse avialans of the Mesozoic. Almost all retained teeth and clawed fingers on each wing, but otherwise looked much like modern birds externally. Over 50 species of Enantiornithines have been named, but some...
and Patagopteryx
Patagopteryx
Patagopteryx is an extinct monotypic genus of birds that lived during the Late Cretaceous, around 80 mya, in what is now the Sierra Barrosa in northwestern Patagonia, Argentina. About the size of a chicken, it is the earliest known unequivocal example of secondary flightlessness: its skeleton...
.
Subsequent ccladistic analysis indicates that it and the more advanced Palintropus
Palintropus
Palintropus is a prehistoric bird genus from the Late Cretaceous. A single species has been named based on a proximal coracoid from the Lance Formation of Wyoming, dated to the latest Maastrichtian, 65.5 million years ago...
– long believed to be a modern bird –, and perhaps Ambiortus
Ambiortus
Ambiortus is a prehistoric bird genus. The only known species, Ambiortus dementjevi, lived about 130 million years ago during the Early Cretaceous in today's Mongolia....
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. And if this group is held to include Apatornis too, it would receive the name Ambiortiformes, which was proposed even earlier.