Ambiortus is a prehistoric 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...

. The only known species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

, Ambiortus dementjevi, lived about 130 million years ago during the Early Cretaceous

Early Cretaceous

The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...

 in today's 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...

.

It belongs to 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....

, which include the living birds, their ancestors, and some extinct Mesozoic

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...

 groups like the toothed Hesperornithes

Hesperornithes

Hesperornithes is an extinct and highly specialized clade of Cretaceous toothed birds. Hesperornithine birds, apparently limited to former aquatic habitats in the Northern Hemisphere, include genera such as Hesperornis, Parahesperornis, Baptornis, Enaliornis, and probably Potamornis, all...

 and Ichthyornis

Ichthyornis

Ichthyornis is a genus of toothed seabirds from the Late Cretaceous of North America. Its fossil remains are known from the chalks of Alberta, Alabama, Kansas, New Mexico, Saskatchewan, and Texas, in strata that were laid down in the Western Interior Seaway during the Turonian-Campanian ages,...

. Among these, it appears to be fairly ancestral, similar to the better-known and slightly younger Gansus yumenensis. These taxa suggest that the ancestors of the birds of our time had separated from the 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...

 (the dominant subclass of Mesozoic

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...

 birds), not too long after the times of 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"...

.

The family

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 Ambiortidae is sometimes used for this bird, especially if it is considered a close relative of the much younger Apsaravis

Apsaravis

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...

which is then included therein.

The results of a cladistic analysis published in 2011, on the other hand, indicate that at least Apsaravis and 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...

(which was mistaken as a much more modern bird initially) are very closely related. They might thus represent more advanced forms of the Ambiortus lineage, and in this case the Ambiortidae, or at least the more inclusive Ambiortiformes, might apply to all of them.