''Arktocara'' is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of river dolphin from the

Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...

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 b ...

of Alaska, containing one

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

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

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

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 as ...

, it is perhaps the oldest-known crown toothed whale and the northmost river dolphin discovered. It was a member of the now-extinct

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Allodelphinidae, along with the

genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...

'' Allodelphis'', '' 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 the waterways of the Indian subcontinent. They were historically considered to be one species (''P. gangetica'') with the Ganges river dolphin and the Indu ...

, 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 natural science of the relationships among living organisms and their Natural environment, environment. Ecology considers organisms at the individual, population, community (ecology), community, ecosystem, and biosphere lev ...

may have been similar to the modern-day Dall's porpoise, and it may have competed with contemporaneous delphinoids. Its remains were found in the Poul Creek Formation, which has also yielded several mollusk species.

Taxonomy

The

type specimen In biology, a type is a particular wikt:en:specimen, specimen (or in some cases a group of specimens) of an organism to which the scientific name of that organism is formally associated. In other words, a type is an example that serves to ancho ...

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

geologist A geologist is a scientist who studies the structure, composition, and History of Earth, history of Earth. Geologists incorporate techniques from physics, chemistry, biology, mathematics, and geography to perform research in the Field research, ...

Donald J. Miller in 1951 in the Poul Creek Formation, 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), founded as the Geological Survey, is an agency of the U.S. Department of the Interior whose work spans the disciplines of biology, geography, geology, and hydrology. The agency was founded on Mar ...

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

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

''arktos'' (north) and ''cara'' (face), translating to "the face of the north". "Yakataga" translates to "canoe road" in the Tlingit 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 (non-living) processes such as deposition (geol ...

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 as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure geolo ...

s Alexandra Boersma and Nicholas Pyenson in 2016, being recognized as a new species. ''Arktocara'' was a river dolphin of the superfamily Platanistoidea, and the extinct

Allodelphinidae. The sole surviving member of Platanistoidea is the

(''Platanista gangetica'') which inhabits the

tropics The tropics are the regions of Earth surrounding the equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's ax ...

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

'' Allodelphis'', '' 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 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 The rostral scale, or rostral, in snakes and other sca ...

sutures on the beak, the widening of the

cranium The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...

, a depression on the roof of the

orbit In celestial mechanics, an orbit (also known as orbital revolution) 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 ...

in the eye socket, a smaller exposed

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

in the roof of the mouth, an elongated and convex spine in the tympanic bulla 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 (: 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 saurop ...

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 excepti ...

, and its frontal sinuses 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 mammals h ...

formed a rectangular plateau surrounding them. The nostrils were perhaps . The lacrimal bone in the eye socket wrapped around the outer edge of the brow ridge 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 s ...

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

frontal bone In the human skull, the frontal bone or sincipital bone is an unpaired bone which consists of two portions.'' Gray's Anatomy'' (1918) These are the vertically oriented squamous part, and the horizontally oriented orbital part, making up the bo ...

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. Eac ...

, was U-shaped. The sutures indicate that the specimen was an adult. It lacked on the upper side of its head a condyloid fossa, a depression between the head and the neck; and had a symmetrical vertex, 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 ...

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, positioned in the skull behind each eye. There is one on each side, extending from the side of the body of the sphenoid and curving upward, laterally ...

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

maxilla In vertebrates, the maxilla (: maxillae ) 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 two maxil ...

ry and

palatine A palatine or palatinus (Latin; : ''palatini''; cf. derivative spellings below) is a high-level official attached to imperial or royal courts in Europe since Roman Empire, 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 Human nose, nasal cavity, dividing the two nostrils. It is Depression (kinesiology), depressed by the depressor septi nasi muscle. Structure The fleshy external end of the nasal s ...

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 (malleus, incus, and stapes), which transfer the vibrations ...

. The groove for the mandibular branch nerve wrapped around the sides of the pterygoid fossa, which is located behind the eyes laterally on the

sphenoid bone The sphenoid bone is an unpaired bone of the neurocranium. It is situated in the middle of the skull towards the front, in front of the basilar part of occipital bone, basilar part of the occipital bone. The sphenoid bone is one of the seven bon ...

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 Dall's porpoise (''Phocoenoides dalli''), which is an opportunistic

twilight Twilight is daylight illumination produced by diffuse sky radiation when the Sun is below the horizon as sunlight from the upper atmosphere is scattered in a way that illuminates both the Earth's lower atmosphere and also the Earth's surf ...

-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 ...

predator of soft-bodied fish and

squid A squid (: squid) is a mollusc with an elongated soft body, large eyes, eight cephalopod limb, arms, and two tentacles in the orders Myopsida, Oegopsida, and Bathyteuthida (though many other molluscs within the broader Neocoleoidea are also ...

. 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, indi ...

from other dolphins of the superfamily Delphinoidea, and were eventually outcompeted in marine environments. However, this theory has not been properly explored yet.

Paleoecology

Found near the

, ''Arktocara'' inhabited the

subarctic The subarctic zone is a region in the Northern Hemisphere immediately south of the true Arctic, north of hemiboreal regions and covering much of Alaska, Canada, Iceland, the north of Fennoscandia, Northwestern Russia, Siberia, and the Cair ...

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 age (geology), ages or the lower of two stage (stratigraphy), stages of the Oligocene epoch (geology), Epoch/series (stratigraphy), Series. It spans the time between . It is preceded b ...

to Chattian ages of the

, making it the oldest-known crown toothed whale (Odontoceti), besides possibly '' Waipatia hectori'' from 25.2 mya. This time period is thought to have been characterized by the diversification of cetaceans. Many mollusks were found in the Poul Creek Formation that were contemporaneous with ''Arktocara'', such as the

nautilus A nautilus (; ) is any of the various species within the cephalopod family Nautilidae. This is the sole extant family of the superfamily Nautilaceae and the suborder Nautilina. It comprises nine living species in two genera, the type genus, ty ...

species '' Aturia alaskensis'' and '' A. angustata'';

gastropod Gastropods (), commonly known as slugs and snails, belong to a large Taxonomy (biology), taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and fro ...

species such as '' Scaphander alaskensis'' and '' Turritella hamiltonensis''; and pelecypods such as '' Venericardia yakatagensis'' and '' Cyclocardia yakatagensis''.

References

External links