''Calciavis'' is an extinct genus of bird from the

Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...

Wyoming Wyoming () is a U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the south ...

. It is a lithornithid, a member of a lineage of flying

palaeognath Palaeognathae (; ) is a infraclass of birds, called paleognaths, within the class Aves of the clade Archosauria. It is one of the two extant infraclasses of birds, the other being Neognathae, both of which form Neornithes. Palaeognathae contain ...

s, distantly related to modern

ratites A ratite () is any of a diverse group of flightless, large, long-necked, and long-legged birds of the infraclass Palaeognathae. Kiwi, the exception, are much smaller and shorter-legged and are the only nocturnal extant ratites. The systematics o ...

. Like many other fossils from the

Green River Formation The Green River Formation is an Eocene geologic formation that records the sedimentation in a group of intermountain lakes in three basins along the present-day Green River in Colorado, Wyoming, and Utah. The sediments are deposited in very fine ...

, it is exceptionally well preserved, bearing soft-tissue impressions of skin and feathers.

Discovery and Description

The original specimen of ''Calciavis'', AMNH 30578, was found in the Fossil Butte Member of the

, dating to around 51 million years ago. It is an exceptionally well preserved specimen, preserving most of the post-cranial skeleton aside from the femur and associated pelvic region, as well as soft-tissue impressions of feathers, skin, foot scales and claw sheaths. Another specimen, AMNH 30560 , was found in the vicinity, composed of a similarly complete most mostly disarticulate skeleton. It is diagnosed by several features of the skull and pelvis as well as having metatarsals IV and II subequal in distal extent. It differs from the contemporary ''

Pseudocrypturus ''Pseudocrypturus'' is a genus of extinct paleognathous bird. One species is known, ''Pseudocrypturus cercanaxius''. It is a relative of such modern birds as ostriches. It lived in the early Eocene. The holotype fossil is in the collection of th ...

'' by a shorter skull - in ''Calciavis'' the skull is shorter than the humerus, while in inverse happens in ''

'' -, as well as a proportionally narrower coracoid shaft and longer

tarsometatarsus The tarsometatarsus is a bone that is only found in the lower leg of birds and some non-avian dinosaurs. It is formed from the fusion of several bones found in other types of animals, and homologous to the mammalian tarsus (ankle bones) and meta ...

, from ''

Lithornis promiscuus ''Lithornis'' is a genus of extinct paleognathous birds. Although ''Lithornis'' was able to fly well, their closest relatives are the extant tinamous (which are poor flyers) and ratites (which are flightless birds). Fossils of ''Lithornis'' a ...

'' in aspects of the

ischium The ischium () form ...

, and from ''

Paracathartes ''Paracathartes'' is a genus of extinct bird from the Wasachtian horizon of lower Eocene Wyoming. One species, ''Paracathartes howardae'' has been described. It is a paleognathous bird, turkey-like in stature and size, that probably resembled ...

'' in a less curved and more gracile scapular blade. Feather imprints show abundant

plumage Plumage ( "feather") is a layer of feathers that covers a bird and the pattern, colour, and arrangement of those feathers. The pattern and colours of plumage differ between species and subspecies and may vary with age classes. Within species, ...

with long primaries and remiges. In AMNH 30578 most of it is damaged due to post-mortem decomposition, with disorganised patches in the pectoral and pelvic region and the left wing traces and impressions being damaged, but the right wing is mostly intact, even showing evidence of

barbule Feathers are epidermal growths that form a distinctive outer covering, or plumage, on both avian (bird) and some non-avian dinosaurs and other archosaurs. They are the most complex integumentary structures found in vertebrates and a premier ...

s; in AMNH AMNH 30560 a wing is similarly well preserved. It is unclear if it had a tail, as the left wing feathers block the caudal region in AMNH 30578, but other lithornithids lack tail feathers.

Phylogeny

''Calciavis'' is recovered as a sister taxa to ''

'', as sister-taxa to all other lithornithids.

Ecology

Studies suggest that there was bare minimum of two species-level lithornithids in the

at a given time, suggesting a high local diversity. ''Calciavis'' indeed co-existed directly at least with ''

'', and though both genera/species are the only currently diagnosable lithornithids several other contemporary species probably belong to separate species. Comparisons with other palaeognaths render the possibility of both birds being different sexes of the same species unlikely, and both ''Calciavis'' specimens belong to adult birds, not juveniles of ''

''. As mentioned above, skull and limb proportions are different among both species, implying that they occupied different ecological niches.

Flight

A recent study on ''Calciavis'' vindicates previous statements of competent flying in lithornithids, indicating that not only they were capable of sustained flapping but also migratory behaviour.

References

{{Taxonbar, from=Q27990781 Lithornithidae Paleogene birds of Europe Eocene genus extinctions Paleogene birds of North America Fossil taxa described in 2016 Paleontology in Wyoming