Australodelphis
Encyclopedia
Australodelphis is an extinct Pliocene
Pliocene
The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...

 genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of dolphin with a single described species, Australodelphis mirus. The genus is known from fossils found in the Sørsdal Formation, Vestfold Hills
Vestfold Hills
Vestfold Hills is an area of rounded rock coastal hills, in extent, located at the north side of Sorsdal Glacier on Ingrid Christensen Coast in Antarctica. The hills are subdivided by three west-trending peninsulas bounded by narrow fjords...

, East Antarctica
East Antarctica
East Antarctica, also called Greater Antarctica, constitutes the majority of the Antarctic continent, lying on the Indian Ocean side of the Transantarctic Mountains...

. The genus has been described as an example of convergent evolution
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...

 with whales.

Name history

The name Australodelphis is derived from the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 Australis meaning southern and Delphis meaning dolphin. The species name mirus is Latin for strange or wonderful, and was chosen to reflect the unexpected morphology
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....

 of the type specimen. While not described until 2002, the type specimen
Biological type
In biology, a type is one particular specimen of an organism to which the scientific name of that organism is formally attached...

 of A. mirus was collected between 1985 and 1986, and a further four specimens were found between 1986 and 1994. Prior to the description of Australodelphis in 2002, the genus was mentioned briefly in several publications between 1988 and 1993. The holotype
Holotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...

 skull
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...

 was figured in 1988 by R. E. Fordyce and Australodelphis mirus first appeared as a nomen nudum
Nomen nudum
The phrase nomen nudum is a Latin term, meaning "naked name", used in taxonomy...

name in E. H. Colbert's 1991 "Mesozoic and Cainozoic tetrapod fossils from Antarctica". A second species of Australodelphis was noted in R. E. Fordyce and G. Quilty in their 1993 publication on the stratigraphic context of the Marine Plain sediments, but this second species has yet to be formally described.

Type locality

The type locality of the genus marks Australodelphis as the first Pliocene higher vertebrate to be named from Antarctica, and the first cetacea
Cetacea
The order Cetacea includes the marine mammals commonly known as whales, dolphins, and porpoises. Cetus is Latin and is used in biological names to mean "whale"; its original meaning, "large sea animal", was more general. It comes from Ancient Greek , meaning "whale" or "any huge fish or sea...

n to be named from sediments dating after the final breakup of Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...

. All known specimens of Australodelphis were recovered from sediments of the Sørsdal Formation which outcrops at Marine Plain approximately 8 km south of Davis Station
Davis Station
Davis Station is a permanent base in Antarctica managed by the Australian Antarctic Division . It is the busiest Australian scientific research station...

 in the Vestfold Hills of East Antarctica. The fossils are found in massive to poorly bedded muddy siltstone
Siltstone
Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...

, dated at 4.5 to 4.1 million years old, placing the sediments in the Early Pliocene
Pliocene
The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...

. The cetaceans of the Sørsdal Formation are found in association with the extinct diatom
Diatom
Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...

 Fragilariopsis barronii and the scallop
Scallop
A scallop is a marine bivalve mollusk of the family Pectinidae. Scallops are a cosmopolitan family, found in all of the world's oceans. Many scallops are highly prized as a food source...

 Chlamys tuftensis. Poor sorting and fine grained sediments, combined with cetacean bones and diatom depositions, indicate the area was a sheltered shallow glacio-marine inner shelf.

Specimens

The holotype, CPC 25730, was reconstructed from hundreds of frost
Frost
Frost is the solid deposition of water vapor from saturated air. It is formed when solid surfaces are cooled to below the dew point of the adjacent air as well as below the freezing point of water. Frost crystals' size differ depending on time and water vapour available. Frost is also usually...

-shattered pieces using a combination of polyvinyl acetate
Polyvinyl acetate
Polyvinyl acetate, PVA, PVAc, poly, is a rubbery synthetic polymer with the formula n. It belongs to the polyvinyl esters family with the general formula -[RCOOCHCH2]-...

 and epoxy resin. This prevented the use of acetic acid
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...

 requiring the use of mechanical methods for specimen preparation. CPC 25730 consists of incomplete right and left mandibles and an incomplete skull missing the basicranium. The other four specimens consist of: one partial rostrum
Rostrum (anatomy)
The term rostrum is used for a number of unrelated structures in different groups of animals:*In crustaceans, the rostrum is the forward extension of the carapace in front of the eyes....

; a partial skull consisting of the right side; the rear section of a skull including basicranium; and a partial skull consisting of the narial
Nose
Anatomically, a nose is a protuberance in vertebrates that houses the nostrils, or nares, which admit and expel air for respiration in conjunction with the mouth. Behind the nose are the olfactory mucosa and the sinuses. Behind the nasal cavity, air next passes through the pharynx, shared with the...

 region and a partial endocranial cast. A number of features of Australodelphis cranium indicate its position in the Delphinidae family. These feature include asymmetry in the premaxilla
Premaxilla
The incisive bone is the portion of the maxilla adjacent to the incisors. It is a pair of small cranial bones at the very tip of the jaws of many animals, usually bearing teeth, but not always. They are connected to the maxilla and the nasals....

 ends, an inflated pars cochlearis, and a tympanoperiotic which is not sutured with the squamosal
Squamosal
The squamosal is a bone of the head of higher vertebrates. It is the principal component of the cheek region in the skull, lying below the temporal series and otic notch and bounded anteriorly by postorbital. Posteriorly, the squamosal articulates with the posterior elements of the palatal complex,...

. The genus also shows a number of similarities to the modern genus Mesoplodon, a member of the Ziphiidae
Beaked whale
Beaked whales are 21 species of toothed whales, members of the family Ziphiidae, are notable for their elongated beaks. Beaked whales are one of the world's most extreme divers. They can dive for long periods—20 to 30 minutes is common, and 85 minute dives have been recorded—and to...

 family of beaked whales, possessing an elongated toothless rostrum with wide maxillary flanges and laterally compressed tympanic bulla.

Taxonomic placement

The overall suture patterns of the skull are closest to Delphinidae while the topography of the rostrum and upper side of the skull is very similar to Ziphiidae, making placement of the genus difficult. It is noted by O. Lambert that Australodelphis has rostrum features similar to those of the Ziphiidae family of whales. However, the first true members of Delphinidae appeared in the late Oligocene
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...

, thus the last common ancestor of both Delphinidae and Ziphiidae is estimated to have lived 30 million years ago. The evidence, both morphological and temporal, is that Australodelphis is a member of the Delphinidae family. In their discussion of the species Archaeoziphius microglenoideus, the describing authors O. Lambert and S. Louwye note the distinct similarities between ziphiids but reaffirm the placement of Australodelphis. The Messapicetus sp. specimen CMM-V-3138, found in the St. Marys Formation of Calvert Cliffs, Maryland
Calvert Cliffs State Park
Calvert Cliffs State Park is a state park in Calvert County, Maryland, USA, situated on the Chesapeake Bay. On the 1612 John Smith map, the site is called "Rickard's Cliffes"...

, is noted to be similar enough in structure to possibly be derived from Australodelphis. However there are enough differences to make this possibility remote. Australodelphis facial structure indicates a probable feeding style similar to the ziphiid whales, consisting of a rapid opening of the mouth to produce suction for capturing soft bodied prey. This is supported by the toothless rostrum which would make catching prey with a pincer movement difficult, and the small size of the temporal muscles. The structure of the nasal area indicates Australodelphis was likely to have enlarged nasiofacial muscles similar to Mesoplodon and indicate a possible ability to generate high-frequency sounds used in echolocation
Animal echolocation
Echolocation, also called biosonar, is the biological sonar used by several kinds of animals.Echolocating animals emit calls out to the environment and listen to the echoes of those calls that return from various objects near them. They use these echoes to locate and identify the objects...

.
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