''Sororavis'' is an extinct genus of bird from the Early

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

London Clay The London Clay Formation is a Sediment#Shores and shallow seas, marine formation (geology), geological formation of Ypresian (early Eocene Epoch, c. 54-50 million years ago) age which outcrop, crops out in the southeast of England. The London C ...

Walton-on-the-Naze Walton-on-the-Naze is a seaside town on the North Sea coast. It is part of the parish of Frinton and Walton, in the Tendring District, Tendring district in Essex, England. The town is located north of Clacton and south of the port of Harwich; ...

, United Kingdom. It contains a single named species, ''Sororavis solitarius''. It is a member of the

Morsoravidae Morsoravidae is a family of extinct birds known from the Early Eocene of Europe and North America, related to the Psittacopasseres, the clade containing parrots and songbirds. They have specialised feeding ecologies, and one morsoravid, '' Pumil ...

Discovery and naming

The holotype of ''Sororavis'', NMS.Z.2021.40.75, was discovered in 1986 by the late Michael Daniels, a fossil collector who also collected other specimens of Early Eocene zygodactyl birds and numerous other birds. The fossil was collected from the Walton Member of the

, of lower

Ypresian In the geologic timescale the Ypresian is the oldest age (geology), age or lowest stage (stratigraphy), stratigraphic stage of the Eocene. It spans the time between , is preceded by the Thanetian Age (part of the Paleocene) and is followed by th ...

age (between 54.6 and 55 million years old), near the town of

. It consists of the tip of an upper

beak The beak, bill, or rostrum is an external anatomical structure found mostly in birds, but also in turtles, non-avian dinosaurs and a few mammals. A beak is used for pecking, grasping, and holding (in probing for food, eating, manipulating and ...

, fragments of the

mandible In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...

, the

coracoid A coracoid is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is present as part of the scapula, but this is n ...

s, parts of the

furcula The (Latin for "little fork"; : furculae) or wishbone is a forked bone found in most birds and some species of non-avian dinosaurs, and is either an interclavicle or formed by the fusion of the two clavicles. In birds, its primary function is ...

and

sternum The sternum (: sternums or sterna) or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major bl ...

, portions of the

humeri The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of ...

and an

ulna The ulna or ulnar bone (: ulnae or ulnas) is a long bone in the forearm stretching from the elbow to the wrist. It is on the same side of the forearm as the little finger, running parallel to the Radius (bone), radius, the forearm's other long ...

, portions of the

tibiotarsi The tibiotarsus is the large bone between the femur and the tarsometatarsus in the leg of a bird. It is the fusion of the proximal part of the tarsus with the tibia. A similar structure also occurred in the Mesozoic Heterodontosauridae. These sm ...

and

tarsometatarsi 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 bird bones found in other types of animals, and homologous to the mammalian tarsus (ankle bones) an ...

, and several

phalanges The phalanges (: phalanx ) are digit (anatomy), digital bones in the hands and foot, feet of most vertebrates. In primates, the Thumb, thumbs and Hallux, big toes have two phalanges while the other Digit (anatomy), digits have three phalanges. ...

. Mayr & Kitchener (2023) named the new genus and species ''Sororavis solitarius'' based on these remains, also erecting the new family

including the genus. The generic name ''Sororavis'' is from Latin ''soror'', meaning "sister", and ''avis'', meaning "bird", in reference to the closeness of ''Sororavis'' to ''

Morsoravis ''Morsoravis'' is an extinct genus of neoavian bird from the Early Eocene Fur Formation of Denmark. It contains a single named species, ''Morsoravis sedilis''. Fossils of Morsoravis have also been found in the Green River Formation of Wyoming and ...

''. The names are intended to sound similar. The specific ephitet ''solitarius'' is from Latin ''solitaries'', meaning "single", and is based on the presence of only one specimen of ''Sororavis'' in Michael Daniels' large collection of bird fossils. Sororavis holotype

Description

''Sororavis'' was a bird of similar size to ''

sedilis'', but larger than another related bird, '' Pumiliornis tessellatus''. The beak is similar to that of

, and resembles that of

thrushes The thrushes are a passerine bird family, Turdidae, with a worldwide distribution. The family was once much larger before biologists reclassified the former subfamily Saxicolinae, which includes the chats and European robins, as Old World flycat ...

in shape. Its tip is narrow and pointed. The coracoid is similar in proportions to that of

Psittacopedidae Psittacopedidae is an extinct family of birds related to passerines. It had zygodactyl feet, likely due to common ancestry with parrot Parrots (Psittaciformes), also known as psittacines (), are birds with a strong curved beak, upright stance ...

and extant

songbird A songbird is a bird belonging to the suborder Passeri of the perching birds (Passeriformes). Another name that is sometimes seen as the scientific or vernacular name is Oscines, from Latin ''oscen'', "songbird". The Passeriformes contains 5,00 ...

s. There was a long procoracoid process but no foramen for the supracoracoid nerve. The pneumatic fossa of the humerus lacks pneumatic openings, possibly related to the presence of pleurocoelous vertebra. The tibiotarsus is very long and slender, and the tarsometatarsus is longer and narrower than that found in relatives of ''Sororavis''. It has a flattened shape. The trochlea of the fourth metatarsal is deflected downwards and has a flange that is indicative of a

zygodactyl In biology, dactyly is the arrangement of digits (fingers and toes) on the hands, feet, or sometimes wings of a tetrapod animal. The term is derived from the Greek word () meaning "finger." Sometimes the suffix "-dactylia" is used. The derive ...

foot arrangement.

Classification

''Sororavis'' is a member of the Morsoravidae, a family also including such birds as ''

'' and '' Pumiliornis''. This family has only been formally described from fossils discovered in Europe, but it is also thought to have been present in North America, on the basis of a bird fossil 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 (Colorado River), Green River in Colorado, Wyoming, and Utah. The sedimen ...

of the western United States. The precise position of the Morsoravidae is not clear. Mayr & Kitchener (2023), in an analysis including ''Sororavis'' recovered the morsoravids as the sister taxon to a clade including the songbirds and the

Zygodactylidae Zygodactylidae is a family of extinct birds found in Europe and North America from the Eocene epoch to the Middle Miocene. First named in 1971, based on fragmentary remains of two species from Germany, a more complete description of the birds bec ...

, as per the simplified cladogram:

Paleobiology

''Sororavis'', like other

, has a distinctive morphology of the tarsometatarsus, including facilities for a foot that was at least semi-zygodactyl. The tarsometatarsus is generally similar to that of the

mousebird The mousebirds are birds in the order Coliiformes. They are the sister group to the clade Cavitaves, which includes the Leptosomiformes (the cuckoo roller), Trogoniformes ( trogons), Bucerotiformes ( hornbills and hoopoes), Piciformes ( wo ...

s, and the tibiotarsus is also similar in its dimensions. ''Sororavis'' might have been able to hang and dangle from branches in order to better access food, like fruits or flowers. The related taxon '' Pumiliornis'' has been found with

pollen Pollen is a powdery substance produced by most types of flowers of seed plants for the purpose of sexual reproduction. It consists of pollen grains (highly reduced Gametophyte#Heterospory, microgametophytes), which produce male gametes (sperm ...

fossilised in its gut, indicating that it probably ate

nectar Nectar is a viscous, sugar-rich liquid produced by Plant, plants in glands called nectaries, either within the flowers with which it attracts pollination, pollinating animals, or by extrafloral nectaries, which provide a nutrient source to an ...

. ''Sororavis'' could have been a small, arboreal, and acrobatic bird, living much lke modern mousebirds.

References

{{Taxonbar, from=Q123501478 Eocene life Prehistoric birds of Europe Prehistoric bird genera Neognathae