Apatornis is a prehistoric terrestrial

Terrestrial animal

Terrestrial animals are animals that live predominantly or entirely on land , as compared with aquatic animals, which live predominantly or entirely in the water , or amphibians, which rely on a combination of aquatic and terrestrial habitats...

 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...

 endemic to North America

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

 during 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...

 living about 83.5 mya. It is known from a single fossil specimen: a synsacrum, the fused series of vertebrae over the hips.

Taxonomy

Apatornis was named by Marsh (1873). Its type is Ichthyornis celer. It was assigned to Ichthyornidae by Marsh (1873); to Odontotormae by Marsh (1880); to Ichthyornithiformes by Sepkoski (2002); to Neornithes by Hope (2002); and to 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....

by Padian (2004).

It lived around the Santonian

Santonian

The Santonian is an age in the geologic timescale or a chronostratigraphic stage. It is a subdivision of the Late Cretaceous epoch or Upper Cretaceous series. It spans the time between 85.8 ± 0.7 mya and 83.5 ± 0.7 mya...

-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 ...

 boundary. Its remains were found in the Smoky Hill Chalk

Smoky Hill Chalk

The Smoky Hill Chalk Member of the Niobrara Chalk formation is a Cretaceous conservation Lagerstätte, or fossil rich geological formation, known primarily for its exceptionally well-preserved marine reptiles. The Smoky Hill Chalk Member is the uppermost of the two structural units of the Niobrara...

 at Twin Butte Creek, USA. It was long allied with 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,...

in error. A single 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...

 is known, Apatornis celer. The supposed "A." retusus is now in 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...

, and "A". marshi is now in Iaceornis

Iaceornis

Iaceornis is a prehistoric marine bird genus endemic to North America during the Late Cretaceous living about 83.5 mya. It is known from a single fossil specimen consisting of a partial skeleton lacking a skull. Since it was first discovered by Othniel Charles Marsh in 1877, the specimen was long...

.

Its relationships are unresolved, mainly due to the paucity of material. While it is sometimes considered a neornithine and specifically to belong to the waterfowl (Anseriformes

Anseriformes

The order Anseriformes contains about 150 living species of birds in three extant families: the Anhimidae , Anseranatidae , and the Anatidae, which includes over 140 species of waterfowl, among them the ducks, geese, and swans.All species in the order are highly adapted for an aquatic existence at...

), this is far from certain. Though waterfowl must have been around by the time it lived, a phylogenetic analysis could not find any particularly close relationship with the ancestors of modern birds, and thus A. celer cannot be classified at present with more certainty than placing it in the Carinatae

Carinatae

This article is about bird taxonomy; for the topic in pottery and glassware design, see Carinate.The Carinatae are, in phylogenetic taxonomy, the last common ancestor of the Neornithes and Ichthyornis , and all its descendants...

 incertae sedis

Incertae sedis

, is a term used to define a taxonomic group where its broader relationships are unknown or undefined. Uncertainty at specific taxonomic levels is attributed by , , and similar terms.-Examples:*The fossil plant Paradinandra suecica could not be assigned to any...

.