''Arktocara'' is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...

river dolphin River dolphins are a polyphyletic group of fully aquatic mammals that reside exclusively in freshwater or brackish water. They are an informal grouping of dolphins, which itself is a paraphyletic group within the infraorder Cetacea. Extant r ...

from the

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but ...

epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided by ...

of Alaska, containing one

species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...

, ''A. yakataga''. Having been discovered in 25-million-year-old

strata In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as e ...

near the

60th parallel north The 60th parallel north is a circle of latitude that is 60 degrees north of Earth's equator. It crosses Europe, Asia, the Pacific Ocean, North America, and the Atlantic Ocean. Although it lies approximately twice as far away from the Equator a ...

, it is perhaps the oldest-known

crown A crown is a traditional form of head adornment, or hat, worn by monarchs as a symbol of their power and dignity. A crown is often, by extension, a symbol of the monarch's government or items endorsed by it. The word itself is used, partic ...

toothed whale The toothed whales (also called odontocetes, systematic name Odontoceti) are a parvorder of cetaceans that includes dolphins, porpoises, and all other whales possessing teeth, such as the beaked whales and sperm whales. Seventy-three species ...

and the northmost

discovered. It was a member of the now-extinct

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Allodelphinidae Allodelphinidae is a family of primitive platanistoid river dolphins found in marine deposits in the eastern North Pacific region, Alaska, and Japan. Description Kimura and Barnes (2016, pp. 3–4) diagnose the family as follows: System ...

, along with the

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial ...

Allodelphis ''Allodelphis'' is an extinct genus of whale belonging to Allodelphinidae found in marine deposits of the eastern North Pacific. Description ''Allodelphis'' has a very elongated rostrum and a polydont, heterodont dentition. It differs from ...

'', '' Goedertius'', '' Ninjadelphis'', and '' Zarhinocetus''. It measured approximately , comparable to its closest living relative, the

South Asian river dolphin South Asian river dolphins are toothed whales in the genus ''Platanista'', which inhabit fresh water habitats in the northern Indian subcontinent. They were historically considered to be one species (''P. gangetica'') with the Ganges river dol ...

, which measures . However, the animal probably had an elongated beak and neck, so it may have been longer. The animal is known only from a partially preserved skull. Its

ecology Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overl ...

may have been similar to the modern-day

Dall's porpoise Dall's porpoise (''Phocoenoides dalli'') is a species of porpoise endemic to the North Pacific. It is the largest of porpoises and the only member of the genus ''Phocoenoides''. The species is named after American naturalist W. H. Dall. Taxo ...

, and it may have competed with contemporaneous delphinoids. Its remains were found in the

Poul Creek Formation The Poul Creek Formation is a geologic formation in Alaska. It preserves fossils dating back to the Paleogene period. See also * List of fossiliferous stratigraphic units in Alaska * Paleontology in Alaska Paleontology in Alaska refers to pal ...

, which has also yielded several mollusk species.

Taxonomy

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 the ...

, the only specimen, of ''Arktocara yakataga'', an incomplete skull, was collected by

geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, alt ...

Donald J. Miller in 1951 in the

, who was mapping the Yakataga District, from where the species name ''yakataga'' derives from, on behalf of the

United States Geological Survey The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, ...

(USGS). The genus name ''Arktocara'' derives from

Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic p ...

''arktos'' (north) and ''cara'' (face), translating to "the face of the north". "Yakataga" translates to "canoe road" in the

Tlingit The Tlingit ( or ; also spelled Tlinkit) are indigenous peoples of the Pacific Northwest Coast of North America. Their language is the Tlingit language (natively , pronounced ),

language, which is apparently a reference to the

reef A reef is a ridge or shoal of rock, coral or similar relatively stable material, lying beneath the surface of a natural body of water. Many reefs result from natural, abiotic component, abiotic processes—deposition (geology), deposition of ...

s which form a canoe passage to a village. ''Arktocara'' was later described 65 years after the collection of the skull (labelled as specimen USNM 214830) by Smithsonian

paleontologist Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of foss ...

s Alexandra Boersma and Nicholas Pyenson in 2016, being recognized as a new species. ''Arktocara'' was a

of the

superfamily SUPERFAMILY is a database and search platform of structural and functional annotation for all proteins and genomes. It classifies amino acid sequences into known structural domains, especially into SCOP superfamilies. Domains are functional, str ...

Platanistoidea, and the extinct

. The sole surviving member of Platanistoidea is the

(''Platanista gangetica'') which inhabits the

tropics The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred ...

, making it its closest living relative. Allodelphinidae also included the

'', '' Goedertius'', '' Ninjadelphis'', and '' Zarhinocetus''. Its discovery further reinforces the theory that the Platanistoidea river dolphins originated in the oceans. Of the characteristics it shared with other platanistoids was a cylindrical projection of the

periotic bone The periotic bone is the single bone that surrounds the inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In ...

in the ear, deeply grooved

rostral Rostral may refer to: Anatomy * Rostral (anatomical term), situated toward the oral or nasal region * Rostral bone, in ceratopsian dinosaurs * Rostral organ, of certain fish * Rostral scale, in snakes and scaled reptiles Other uses * Rostral col ...

sutures on the beak, the widening of the

cranium The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, ...

, a depression on the roof of the

orbit 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 a ...

in the eye socket, a smaller exposed

palatine bone In anatomy, the palatine bones () are two irregular bones of the facial skeleton in many animal species, located above the uvula in the throat. Together with the maxillae, they comprise the hard palate. (''Palate'' is derived from the Latin ...

in the roof of the mouth, an elongated and convex spine in the

tympanic bulla The tympanic part of the temporal bone is a curved plate of bone lying below the squamous part of the temporal bone, in front of the mastoid process, and surrounding the external part of the ear canal. It originates as a separate bone (tympanic ...

of the ear, single rooted back-teeth, and more than 25 teeth.

Description

''Arktocara'' was around in length, based on the distance from cheekbone to cheekbone, and it may have had a similar size to the modern-day South Asian river dolphin, which is in length and in weight. Based on other allodelphinids, it probably had a long beak; and it also had a long neck, as identified by the elongated, unfused

cervical vertebrae In tetrapods, cervical vertebrae (singular: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In ...

in the neck. This is unusual in other cetaceans which generally have a short neck with short and fused cervical vertebrae. This elongated neck may have made it longer than the estimated size. Unlike other platanistoids, ''Arktocara'' had a

sagittal crest A sagittal crest is a ridge of bone running lengthwise along the midline of the top of the skull (at the sagittal suture) of many mammalian and reptilian skulls, among others. The presence of this ridge of bone indicates that there are exceptiona ...

, and its

frontal sinus The frontal sinuses are one of the four pairs of paranasal sinuses that are situated behind the brow ridges. Sinuses are mucosa-lined airspaces within the bones of the face and skull. Each opens into the anterior part of the corresponding midd ...

es were positioned behind the nasals. The nostrils were oriented vertically, and the

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has ...

formed a rectangular plateau surrounding them. The nostrils were perhaps . The

lacrimal bone 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 of ...

in the

eye socket In anatomy, the orbit is the cavity or socket of the skull in which the eye and its appendages are situated. "Orbit" can refer to the bony socket, or it can also be used to imply the contents. In the adult human, the volume of the orbit is , of ...

wrapped around the outer edge of the

brow ridge The brow ridge, or supraorbital ridge known as superciliary arch in medicine, is a bony ridge located above the eye sockets of all primates. In humans, the eyebrows are located on their lower margin. Structure The brow ridge is a nodule or crest ...

and was fused to the cheek bones. It further differed from other platanistoids in that it had a smaller

process A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic. Things called a process include: Business and management *Business process, activities that produce a specific se ...

of the

squamosal The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestral co ...

, that is, a projection from its skull. The frontonasal suture between the eyes on the brow ridge, which separates the

frontal bone The frontal bone is a bone in the human skull. The bone consists of two portions.'' Gray's Anatomy'' (1918) These are the vertically oriented squamous part, and the horizontally oriented orbital part, making up the bony part of the forehead, pa ...

from the

nasal bone The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face and by their junction, form the bridge of the upper one third of the nose. Ea ...

, was U-shaped. The sutures indicate that the specimen was an adult. It lacked on the upper side of its head a

condyloid fossa Behind either condyle of the lateral parts of occipital bone is a depression, the condyloid fossa (or condylar fossa), which receives the posterior margin of the superior facet of the atlas when the head is bent backward; the floor of this fossa is ...

, a depression between the head and the neck; and had a symmetrical

vertex Vertex, vertices or vertexes may refer to: Science and technology Mathematics and computer science *Vertex (geometry), a point where two or more curves, lines, or edges meet *Vertex (computer graphics), a data structure that describes the position ...

, the upper part of the head. Also contrary to other platanistoids, ''Arktocara'' lacked the thin plate which extended from the

hard palate The hard palate is a thin horizontal bony plate made up of two bones of the facial skeleton, located in the roof of the mouth. The bones are the palatine process of the maxilla and the horizontal plate of palatine bone. The hard palate spans th ...

in the roof of the mouth and attached to the

greater wing of sphenoid bone The greater wing of the sphenoid bone, or alisphenoid, is a bony process of the sphenoid bone; there is one on each side, extending from the side of the body of the sphenoid and curving upward, laterally, and backward. Structure The greater win ...

in the ear. It lacked a maxillary crest, a bony projection from the

maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The ...

ry and

palatine A palatine or palatinus (in Latin; plural ''palatini''; cf. derivative spellings below) is a high-level official attached to imperial or royal courts in Europe since Roman times.

bones in the roof of the mouth that runs along the

nasal septum The nasal septum () separates the left and right airways of the nasal cavity, dividing the two nostrils. It is depressed by the depressor septi nasi muscle. Structure The fleshy external end of the nasal septum is called the columella or col ...

which separates the two nostrils. It did not have a large tympanosquamosal recess, which in cetaceans receives an air-filled sinus originating from the

middle ear The middle ear is the portion of the ear medial to the eardrum, and distal to the oval window of the cochlea (of the inner ear). The mammalian middle ear contains three ossicles, which transfer the vibrations of the eardrum into waves in ...

. The groove for the

mandibular branch The marginal mandibular branch of the facial nerve passes forward beneath the platysma and depressor anguli oris, supplying the muscles of the lower lip and chin, and communicating with the mental branch of the inferior alveolar nerve. Muscles inn ...

nerve wrapped around the sides of the

pterygoid fossa The pterygoid fossa is an anatomical term for the fossa formed by the divergence of the lateral pterygoid plate and the medial pterygoid plate of the sphenoid bone. Structure The lateral and medial pterygoid plates (of the pterygoid process o ...

, which is located behind the eyes laterally on the sphenoid bone.

Paleobiology

Despite being labelled as a river dolphin, ''Arktocara'', much like other ancient platanistoids, was likely a coastal, ocean-going cetacean. Its lifestyle and diet is speculated to have been similar to the modern-day

(''Phocoenoides dalli''), which is an opportunistic

twilight Twilight is light produced by sunlight scattering in the upper atmosphere, when the Sun is below the horizon, which illuminates the lower atmosphere and the Earth's surface. The word twilight can also refer to the periods of time when this ...

-to-

surface A surface, as the term is most generally used, is the outermost or uppermost layer of a physical object or space. It is the portion or region of the object that can first be perceived by an observer using the senses of sight and touch, and is t ...

predator of soft-bodied fish and

squid True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fitting ...

. Paleontologists Ewan Fordyce and Christian de Muizon speculated in 2001 that, since the modern-day platanistoid, the South Asian river dolphin, inhabits the rivers, ancient platanistoids, such as ''Arktocara'', faced some

competition Competition is a rivalry where two or more parties strive for a common goal which cannot be shared: where one's gain is the other's loss (an example of which is a zero-sum game). Competition can arise between entities such as organisms, ind ...

from other dolphins of the superfamily

Delphinoidea Delphinoidea is the largest group of toothed whales with 66 genera in 6 families. The largest living member of the superfamily is the killer whale, which can reach 6 tons, while the smallest is the vaquita. Taxonomy Based on McGowen ''et al''. ...

, and were eventually outcompeted in marine environments. However, this theory has not been properly explored yet.

Paleoecology

Found near the

, ''Arktocara'' inhabited the subarctic waters of Alaska, and is the northernmost river dolphin known. ''Arktocara'' lived somewhere between 29 and 24 million years ago (mya), from the

Rupelian The Rupelian is, in the geologic timescale, the older of two ages or the lower of two stages of the Oligocene Epoch/ Series. It spans the time between . It is preceded by the Priabonian Stage (part of the Eocene) and is followed by the Chattian ...

Chattian The Chattian is, in the geologic timescale, the younger of two ages or upper of two stages of the Oligocene Epoch/ Series. It spans the time between . The Chattian is preceded by the Rupelian and is followed by the Aquitanian (the lowest sta ...

ages of the

, making it the oldest-known

(Odontoceti), besides possibly '' Waipatia hectori'' from 25.2 mya. This time period is thought to have been characterized by the

diversification Diversification may refer to: Biology and agriculture * Genetic divergence, emergence of subpopulations that have accumulated independent genetic changes * Agricultural diversification involves the re-allocation of some of a farm's resources to ...

of cetaceans. Many mollusks were found in the Poul Creek Formation that were contemporaneous with ''Arktocara'', such as the

nautilus The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species ...

species '' Aturia alaskensis'' and '' A. angustata''; gastropod species such as '' Scaphander alaskensis'' and '' Turritella hamiltonensis''; and

pelecypod Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, biva ...

s such as '' Venericardia yakatagensis'' and '' Cyclocardia yakatagensis''.

References

External links

* {{Good article Extinct animals of the United States Fossil taxa described in 2016 Neogene mammals of North America Oligocene cetaceans Prehistoric cetacean genera Prehistoric toothed whales Oligocene genus first appearances Oligocene genus extinctions River dolphins Taxobox binomials not recognized by IUCN