''Annakacygna'' is a genus of flightless marine swan from the Miocene of Japan. Named in 2022, ''Annakacygna'' displays a series of unique adaptations setting it apart from any other known swan, including a

filter feeding Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...

lifestyle, a highly mobile tail and wings that likely formed a cradle for their hatchlings in a fashion similar to modern

mute swans The mute swan (''Cygnus olor'') is a species of swan and a member of the waterfowl family Anatidae. It is native to much of Eurosiberia, and (as a rare winter visitor) the far north of Africa. It is an introduced species in North America, home ...

. Additionally, it may have used both wings and tail as a form of display. All of these traits combined have led the researchers working on it to dub it "the ultimate bird". Two species are known, ''A. hajimei'', which was approximately the size of a black swan, and ''A. yoshiiensis'' which exceeded the mute swan in both size and weight. The describing authors proposed the vernacular name Annaka short-winged swan for the genus.

History and naming

The

type specimen In biology, a type is a particular specimen (or in some cases a group of specimens) of an organism to which the scientific name of that organism is formally attached. In other words, a type is an example that serves to anchor or centralizes th ...

(GMNH-PV-678) of ''Annakacygna'' is a nearly complete, almost articulate skeleton discovered in the year 2000 by Hajime Nakajima close to the

Usui river The traditional chinese calendar divides a year into 24 solar terms. ''Yǔshuǐ'' / 雨水, ''Usui'', ''Usu'', or ''Vũ thủy'' (in vietnamese lenguage), literally meaning ''rain water'', is the 2nd one of them. It begins when the Sun reach ...

Annaka, Gunma 240px, Edo-period Usui Magistrate’s office in Annaka is a city located in Gunma Prefecture, Japan. , the city had an estimated population of 47,911 in 24,749 households, and a population density of 210 persons per km². The total area of t ...

. The fossils were found encased in a slab of siltstone in sediments of the Haraichi Formation, a

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

formation Formation may refer to: Linguistics * Back-formation, the process of creating a new lexeme by removing or affixes * Word formation, the creation of a new word by adding affixes Mathematics and science * Cave formation or speleothem, a secondar ...

preserving a marine environment. Early research believed it to be a relative of '' Megalodytes'', a flightless bird from the Miocene of western North America. Preparation of the material however showed that ''Annakacygna'' was a distinct taxon and it was described as such by Hiroshige Matsuoka and Yoshikazu Hasegawa in 2022. The second species, only known from a distal

tibiotarsus 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 s ...

(GMNH-PV-1685) unearthed in 1995, was collected from the riverbed of the Kabura River, 11.5 km further southeast than the remains of the type species. This fossil was donated to the

Gunma Museum of Natural History is a museum of the natural sciences in Tomioka, Gunma Prefecture, Japan. History The museum first opened in 1978 under its old Japanese name, after repairs to the former . In 1996, the old museum closed, and its collection was transferred, befor ...

in 2005. The name ''Annakacygna'' is a combination of the town Annaka and the feminine form of the Latin "cygnus", meaning swan. ''A. hajimei'' was named after its discoverer, Hajime Nakajima, while ''A. yoshiiensis'' was named after the town of Yoshii-machi near its type locality.

Description

The skull of the holotype is crushed and missing most of the

rostrum Rostrum may refer to: * Any kind of a platform for a speaker: **dais **pulpit * Rostrum (anatomy), a beak, or anatomical structure resembling a beak, as in the mouthparts of many sucking insects * Rostrum (ship), a form of bow on naval ships * Ros ...

, however based on the preserved

mandible In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

a length of has been estimated for the upper beak. The head of the animal was proportionally large and it is described as "head-heavy" by the authors. A bony shelf extends over the posterior parts of the

orbits In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...

, which meets the

postorbital The ''postorbital'' is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some ...

process and forms a dorsal notch thought to represent a

salt gland The salt gland is an organ for excreting excess salts. It is found in the cartilaginous fishes subclass elasmobranchii (sharks, rays, and skates), seabirds, and some reptiles. Salt glands can be found in the rectum of sharks. Birds and reptiles ...

. The

lacrimal bones The lacrimal bone is a small and fragile bone of the facial skeleton; it is roughly the size of the little fingernail. It is situated at the front part of the medial wall of the orbit. It has two surfaces and four borders. Several bony landmarks o ...

are V-shaped and highly unusual due to the fact that they are not fused together. The origin for the lower jaw depressor is massive and they meet dorsally to form a false sagital crest. This is not seen in any other known bird. The musculature of the lower jaw, specifically the adductor mandibulae externus profundus, leaves a wide and deep impression in the bone indicating its origin. The element itself is quite thin and a single antero-posteriorly oriented

condyle A condyle (;Entry "condyle"
in
mandibular rami that are deeply bowed when viewed from the side. The coronoid process is set far back, with the distance between it and the posterior articular surface only being a fourth of the length of the mandibular rami. The tip of the mandible is poorly preserved, thin and fragile, but based on the preserved elements the lower jaw would have had a somewhat spoon-like shape. Seventeen cervical and

thoracic vertebrae In vertebrates, thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. In humans, there are twelve thoracic vertebrae and they are intermediate in size between the cervical ...

are known and although many are distorted, they are well enough preserved to reveal their position within the skeleton. Subsequently, Matsuoka and Hasegawa managed to determine the presence of twenty-three neck and seven thoracic vertebrae. They are generally similar to those of the extant Black swan (''Cygnus atratus''), however notably wider and heavier than any modern ''Cygnus'' species. Despite the more robust and shorter vertebrae, the neck of ''Annakacygna'' was still long and flexible. The

sternum The sternum 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 blood vessels from injury. Sha ...

is poorly preserved, but shows only weak attachment for the

coracobrachialis muscle The coracobrachialis muscle is the smallest of the three muscles that attach to the coracoid process of the scapula. (The other two muscles are pectoralis minor and the short head of the biceps brachii.) It is situated at the upper and medial part ...

and the

coracoid A coracoid (from Greek κόραξ, ''koraks'', raven) 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 prese ...

resembles that of the flightless sea duck '' Chendytes'' from

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was finally confirmed in ...

North America. The

scapula The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on eith ...

is much stronger than those of extant flying anseriformes with a better developed scapulohumeralis caudalis muscle and initially does not resemble that of a flightless animal. Both humerus are preserved and are similar in length to those of Black swans, but with an unusually straight shaft lacking a pronounced sigmoid curve. Only in lateral or medial view can the remnants of the curvature be seen, expressed in an ʃ-shape. The proximal head is large and well developed, both proportionally and absolutely larger than in the

whooper swan The whooper swan ( /ˈhuːpə(ɹ) swɒn/) (''Cygnus cygnus''), also known as the common swan, pronounced ''hooper swan'', is a large northern hemisphere swan. It is the Eurasian counterpart of the North American trumpeter swan, and the type speci ...

(''Cygnus cygnus''). The distal end is much smaller and the entire bone is thickened. The

ulna The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...

is straight and incredibly short, only 57% the length of the humerus, while in modern swans the ulna and humerus are of almost equal length. The

radius In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...

is highly unusual among birds. It is also shortened and shows a round crosssection towards its proximal end, before widening and flatting as it moves away from the humerus. The articulation with the humerus is reduced. The radiale is proportionally large while the

carpometacarpus The carpometacarpus is a bone found in the hands of birds. It results from the fusion of the carpal and metacarpal bone, and is essentially a single fused bone between the wrist and the knuckles. It is a smallish bone in most birds, generally flat ...

is reduced in length, but retaining strong articular surfaces that indicate that the element was still used. The wing phalanges are barely recognizable. The pelvis is almost completely preserved and would be gently arched if accounting for the preservation of the fossil. Three caudal vertebrae are preserved and much better developed than the caudals in other birds. The caudals contribution to the stiffened synsacrum is short, another highly unusual feature. The tarsometatarsus is narrow with an eliptical crosssection, an adaptation observed in grebes and

loons Loons ( North American English) or divers (British / Irish English) are a group of aquatic birds found in much of North America and northern Eurasia. All living species of loons are members of the genus ''Gavia'', family Gaviidae and order ...

that helps reduce drag while moving through water. The second species, ''A. yoshiiensis'', is only known from the distal end of a tibiotarsus, which stands apart from the type materiel due to its greater size. This bone is 1.3 times larger than the corresponding material in ''A. haijimei'', which lead the authors to believe that it should represent a species distinct from the type.

Paleobiology

Diet

Although the upper beak is not preserved, based on the anatomy of the mandible it was likely long and deep with a forward spreading form superficialy resembling modern shovelers. The unique jaw musculature indicated by the cranial and mandibular bones suggests that ''Annakocygna'' moved its jaws in a somewhat

seesaw A seesaw (also known as a teeter-totter or teeterboard) is a long, narrow board supported by a single pivot point, most commonly located at the midpoint between both ends; as one end goes up, the other goes down. These are most commonly found a ...

-like motion, which is unlike anything observed in extant swans. While the lower jaw moved back, the upper jaw would have retracted at the same time, which has been interpreted to mean that ''Annakocygna'' was a highly derived filter feeder. Subsequently, the beak was likely lined with soft

lamellae Lamella (plural lamellae) means a small plate or flake in Latin, and in English may refer to: Biology * Lamella (mycology), a papery rib beneath a mushroom cap * Lamella (botany) * Lamella (surface anatomy), a plate-like structure in an animal * ...

to support this

planktivorous A planktivore is an aquatic organism that feeds on planktonic food, including zooplankton and phytoplankton. Planktivorous organisms encompass a range of some of the planet's smallest to largest multicellular animals in both the present day and ...

diet.

Parental care

Like in geese and the

coscoroba swan The coscoroba swan (''Coscoroba coscoroba'') is a species of waterfowl in subfamily Anserinae of the family Anatidae.HBW and BirdLife International (2021) Handbook of the Birds of the World and BirdLife International digital checklist of the bi ...

(''Coscoroba coscoroba''), the pelvis is wide and short and does not resemble the long and narrow pelvis of birds using their feet to swim. However, the foot of ''Annakacygna'' show the opposite, resembling those of foot-propelled seabirds. Matsuoka and Hasegawa suggest that the pelvis may have been a specific adaptation to stabilize the bird while at sea. The strangely long and robust caudals meanwhile, together with other adaptations of the pubic area, would have likely given the bird an exceptionally strong and mobile tail. While the wings clearly show that it was flightless, the arm bones while shortened do not show the same kind of reduction observed in other flightless birds and instead rather show a different specialisation. The

shoulder joint The shoulder joint (or glenohumeral joint from Greek ''glene'', eyeball, + -''oid'', 'form of', + Latin ''humerus'', shoulder) is structurally classified as a synovial ball-and-socket joint and functionally as a diarthrosis and multiaxial joint ...

was highly mobile, allowing the wings to move back to a great extent with musculature supporting this movement usually only observed when birds fold their wings. To the contrary, the

wrist joint In human anatomy, the wrist is variously defined as (1) the carpus or carpal bones, the complex of eight bones forming the proximal skeletal segment of the hand; "The wrist contains eight bones, roughly aligned in two rows, known as the carpal ...

was restrictive, allowing the phalanges to only fold to a position of approximately 60° and flex to a position of 135°. Additionally, the ulnimetacarpalis ventralis muscle would have allowed the hands to "hyperpronate", elevating the

primary feathers Flight feathers (''Pennae volatus'') are the long, stiff, asymmetrically shaped, but symmetrically paired pennaceous feathers on the wings or tail of a bird; those on the wings are called remiges (), singular remex (), while those on the tai ...

upward beyond the wrist itself. A similar positioning of the wings is observed in several modern species of swan that carry their young in a "piggyback" style, elevating their wings slighly while juveniles rest on the adults. All three species displaying such behavior are also notably less vocal than other swan species, instead using their wings for communication. Due to this combination of skeletal elements and the corresponding musculature, Matsuoka and Hasegawa propose that ''Annakocygna'' used its highly specialised wings as a cradle for its young in a fashion similar to extant

. The mobile tail could have possibly served as part of this cradle, being raised to protect the back. Additionally, both wings and tail may have been utilized not just to protect their young, but also as displays to attract and communicate with mates.

Locomotion

The feet of ''Annakacygna'' are similar in morphology to that of modern diving birds like

and grebes, showing the same narrowing of the tarsometatarsus, but lacking the shortening of the femur. Although the authors do not believe it to have been a deep diver, it shows none the less a clear adaptation to life in the ocean. Furthermore the

appears similar to a variety of other marine or diving birds including albatrosses, anhingas and sulids. The ankle joints had strong

dorsiflexion Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relativ ...

and its hypothesized that the webbed feet would have been passively folded in life. The thickened bones likely helped the birds lifestyle. Pachyostotic bones are commonly associated with diving, as they make the animal heavier, but in the case of ''Annakacygna'' it is more likely that they helped stabilize the animal at sea while foraging.

Paleoenvironment

Both species of ''Annakacygna'' are found in the middle layers of the Miocene Haraichi Formation, which preserves a marine environment. Other animals found in the formation include the desmostylian ''

Paleoparadoxia ''Paleoparadoxia'' ("ancient paradox") is a genus of large, herbivorous aquatic mammals that inhabited the northern Pacific coastal region during the Miocene epoch (). It ranged from the waters of Japan (Tsuyama and Yanagawa), to Alaska in the n ...

'', sharks, allodesmine seals and a variety of cetaceans like ''

Joumocetus ''Joumocetus'' is a genus of extinct baleen whale in the family Cetotheriidae containing the single species ''Joumocetus shimizui''. The species is known only from a partial skeleton found in Miocene age sediments of Japan. History and classifi ...

'', '' Kentriodon'' and '' Norisdelphis''.

References

{{Taxonbar, from=Q111804523 Birds described in 2022 Fossil taxa described in 2022 Extinct flightless birds Prehistoric bird genera Anseriformes Swans