''Apatornis'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

ornithuran Ornithurae (meaning "bird tails" in Greek) is a natural group that includes modern birds and their very close relatives such as the ichthyornithines and the hesperornithines. This clade is defined in the ''PhyloCode'' by Juan Benito and collea ...

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...

s endemic to

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

during the late

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

. It currently contains a single

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

, ''Apatornis celer'', which lived around the

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 86.3 ± 0.7 mya ( million years ago) and 83.6 ± 0.7 m ...

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. Campa ...

boundary, dated to about 83.5 million years ago. The remains of this species were found in the Smoky Hill Chalk of the

Niobrara Formation The Niobrara Formation , also called the Niobrara Chalk, is a geologic formation in North America that was deposited between 87 and 82 million years ago during the Coniacian, Santonian, and Campanian stages of the Late Cretaceous. It is com ...

Kansas Kansas ( ) is a landlocked U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Nebraska to the north; Missouri to the east; Oklahoma to the south; and Colorado to the west. Kansas is named a ...

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

. It is known from a single fossil specimen: a ''synsacrum'', the fused series of vertebrae over the hips. While the known fossil remains are very incomplete, enough has been found to reasonably estimate that the body length was between . The type specimen of ''A. celer'', YPM 1451, was reportedly discovered by

Othniel Charles Marsh Othniel Charles Marsh (October 29, 1831 – March 18, 1899) was an American professor of paleontology. A prolific fossil collector, Marsh was one of the preeminent paleontologists of the nineteenth century. Among his legacies are the discovery or ...

in October 1872 at Butte Creek in

Logan County, Kansas Logan County is a county located in the U.S. state of Kansas. Its county seat and largest city is Oakley. As of the 2020 census, the county population was 2,762. The county was named for John Logan, a general during the American Civil ...

. This location is now recognized as falling between Marker Units 15 and 19 of the Smoky Hill Chalk geological formation. An additional, more complete specimen had also been referred to ''Apatornis celer'' by Marsh.O. C. Marsh. 1873. On a new sub-class of fossil birds (Odontornithes). ''American Journal of Science and Arts'' 5(2):161-162 This more complete specimen had historically been the one used almost exclusively to form the basis of what was known about ''Apatornis''. However,

Julia Clarke Julia Allison Clarke is an American paleontologist and evolutionary biologist who studies the evolution of birds and the dinosaurs most closely related to living birds. She is the John A. Wilson Professor in Vertebrate Paleontology in the Jackso ...

noted in 2004 that because the second specimen did not preserve any of the same bones as the first, the two could not be scientifically compared. Clarke therefore reclassified the second specimen as its own genus and species, '' Iaceornis marshi''.

Classification

The traditional genus ''Apatornis'' has been defined as a

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

, specifically as all species more closely related to the type specimen YPM 1451 than to either ''Ichthyornis'' or modern birds.Clarke, J.A. (2004). "Morphology, phylogenetic taxonomy, and systematics of ''Ichthyornis'' and ''Apatornis'' (Avialae: Ornithurae)." ''Bulletin of the American Museum of Natural History'', 1-179. ''Apatornis celer'' was recognized as a distinct species by Marsh (1873). Its type species was originally classified as ''Ichthyornis celer''. ''A. celer'' was long allied with ''

Ichthyornis ''Ichthyornis'' (meaning "fish bird", after its fish-like vertebrae) is an extinct genus of toothy seabird-like ornithuran from the late Cretaceous period of North America. Its fossil remains are known from the chalks of Alberta, Alabama, Kansa ...

'', having been assigned to Ichthyornidae by Marsh (1873), to Odontotormae by Marsh (1880), and to Ichthyornithiformes by Sepkoski (2002). ''Apatornis'' differs from ''Ichthyornis'' primarily in that it had at least one additional rib attached to the hip vertebrae (sacrum), possibly more as the entire synsacrum was not preserved in the only known specimen. ''Apatornis'' also lacked the ossified tendons covering the top of the sacral region in ''Ichthyornis''. In a 2004 paper, Julia Clarke noted that the few unique characteristics preserved in the ''A. celer'' specimen, rather than suggesting a close relationship with ''Ichthyornis'' as originally thought, actually suggest it is more closely related to modern birds, a hypothesis also supported by Hope in 2002. In 2022, Benito and colleagues noted the variability in the sacral anatomy among known ''Ichthyornis'' specimens, and suggested that the validity of ''Apatornis'' should be re-evaluated. Its exact relationships are unresolved, mainly due to the paucity of fossil remains. Though it has sometimes been considered to be closely related to modern waterfowl (

Anseriformes Anseriformes is an order (biology), order of birds also known as waterfowl that comprises about 180 living species of birds in three families: Anhimidae (three species of screamers), Anseranatidae (the magpie goose), and Anatidae, the largest f ...

),Chiappe, L.M., & Dyke, G.J. (2002). "The Mesozoic radiation of birds." ''Annual Review of Ecology and Systematics'', 91-124. most researchers today consider it to be an early member of the clade

Ornithurae Ornithurae (meaning "bird tails" in Greek) is a natural group that includes modern birds and their very close relatives such as the ichthyornithines and the hesperornithines. This clade is defined in the ''PhyloCode'' by Juan Benito and collea ...

References

{{Taxonbar, from=Q4779241 Ornithurae Late Cretaceous dinosaurs of North America Fossil taxa described in 1873 Taxa named by Othniel Charles Marsh