| Apatornis | |
|---|---|
 | |
| Holotype ofA. celer | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | Avialae |
| Clade: | Ornithurae |
| Genus: | †Apatornis Marsh, 1873b |
| Species: | †A. celer |
| Binomial name | |
| †Apatornis celer (Marsh, 1873a) | |
| Synonyms | |
| |
Apatornis is agenus ofornithurandinosaurs endemic toNorth America during the lateCretaceous. It currently contains a singlespecies,Apatornis celer, which lived around theSantonian-Campanian boundary, dated to about 83.5 million years ago. The remains of this species were found in theSmoky Hill Chalk of theNiobrara Formation inKansas,United States. It is known from a single fossil specimen: asynsacrum, 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 7–8 inches (18–20 cm).[1]
The type specimen ofA. celer, YPM 1451, was reportedly discovered byOthniel Charles Marsh in October 1872 at Butte Creek inLogan County, Kansas. 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 toApatornis celer by Marsh.[2] This more complete specimen had historically been the one used almost exclusively to form the basis of what was known aboutApatornis. However,Julia Clarke 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.[3]
The traditional genusApatornis has been defined as aclade, specifically as all species more closely related to the type specimen YPM 1451 than to eitherIchthyornis or modern birds.[3]
Apatornis celer was recognized as a distinct species by Marsh (1873). Its type species was originally classified asIchthyornis celer.A. celer was long allied withIchthyornis, having been assigned toIchthyornidae by Marsh (1873), toOdontotormae by Marsh (1880), and toIchthyornithiformes by Sepkoski (2002).Apatornis differs fromIchthyornis 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 inIchthyornis. In a 2004 paper, Julia Clarke noted that the few unique characteristics preserved in theA. celer specimen, rather than suggesting a close relationship withIchthyornis as originally thought, actually suggest it is more closely related tomodern birds, a hypothesis also supported by Hope in 2002.[3] In 2022, Benito and colleagues noted the variability in the sacral anatomy among knownIchthyornis specimens, and suggested that the validity ofApatornis should be re-evaluated.[4]
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),[5] most researchers today consider it to be an early member of the cladeOrnithurae.[3]
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