''Australodelphis mirus'' 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 ...

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58dolphin A dolphin is an aquatic mammal in the cetacean clade Odontoceti (toothed whale). Dolphins belong to the families Delphinidae (the oceanic dolphins), Platanistidae (the Indian river dolphins), Iniidae (the New World river dolphins), Pontopori ...

.The Paleobiology Database Australodelphis page
/ref> ''A. mirus'' is known from fossils found in the Sørsdal Formation, Mule Peninsula,

Vestfold Hills The Vestfold Hills are rounded, rocky, coastal hills, in extent, on the north side of Sorsdal Glacier on the Ingrid Christensen Coast of Princess Elizabeth Land, Antarctica. The hills are subdivided by three west-trending peninsulas bounded by n ...

East Antarctica East Antarctica, also called Greater Antarctica, constitutes the majority (two-thirds) of the Antarctic continent, lying primarily in the Eastern Hemisphere south of the Indian Ocean, and separated from West Antarctica by the Transantarctic ...

. The genus has been described as an example of

convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...

with beaked whales.

Name history

The generic name ''Australodelphis'' is derived from the

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

''australis'' meaning southern and ''delphis'' meaning dolphin, in reference to its discovery in Antarctica. The species name ''mirus'' is Latin for strange or wonderful, and was chosen to reflect the unexpected

morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology), study of the shapes or forms of artifacts *Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...

of the type specimen. While not described until 2002, 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 ...

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 A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

skull 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 ...

was figured in 1988 by R. E. Fordyce and ''Australodelphis mirus'' first appeared as a ''

nomen nudum In Taxonomy (biology), taxonomy, a ''nomen nudum'' ('naked name'; plural ''nomina nuda'') is a designation which looks exactly like a scientific name of an organism, and may have originally been intended to be one, but it has not been published ...

'' in E. H. Colbert's 1991 "Mesozoic and Cainozoic tetrapod fossils from Antarctica". A second species of ''Australodelphis'' was noted by R. E. Fordyce and P. 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 (; , ) is an infraorder of aquatic mammals belonging to the order Artiodactyla that includes whales, dolphins and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively c ...

n to be named from sediments dating after the final breakup of

Gondwana Gondwana ( ; ) was a large landmass, sometimes referred to as a supercontinent. The remnants of Gondwana make up around two-thirds of today's continental area, including South America, Africa, Antarctica, Australia (continent), Australia, Zea ...

. All known specimens of ''Australodelphis'' were recovered from sediments of the Sørsdal Formation which outcrops at Marine Plain about 8 km south of

Davis Station Davis Station, commonly called Davis, is one of three permanent bases and research outposts in Antarctica managed by the Australian Antarctic Division (AAD). Davis is situated on the coast of Cooperation Sea in Princess Elizabeth Land, Ingrid ...

in the Vestfold Hills of East Antarctica. The fossils are found in massive to poorly bedded muddy

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...

, dated at 4.5 to 4.1 million years old, placing the sediments in the Early

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58 The cetaceans of the Sørsdal Formation are found in association with the extinct

diatom A diatom (Neo-Latin ''diatoma'') is any member of a large group comprising several Genus, genera of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of Earth's B ...

'' Fragilariopsis barronii'' and the

scallop Scallop () is a common name that encompasses various species of marine bivalve molluscs in the taxonomic family Pectinidae, the scallops. However, the common name "scallop" is also sometimes applied to species in other closely related famili ...

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

Specimens

The holotype, CPC 25730, was reconstructed from hundreds of

frost Frost is a thin layer of ice on a solid surface, which forms from water vapor that deposits onto a freezing surface. Frost forms when the air contains more water vapor than it can normally hold at a specific temperature. The process is simila ...

-shattered pieces using a combination of

polyvinyl acetate Polyvinyl acetate (PVA, PVAc, poly(ethenyl ethanoate)), commonly known as wood glue (a term that may also refer to other types of glues), PVA glue, white glue, carpenter's glue, school glue, or Elmer's Glue in the US, is a widely available adh ...

and

epoxy resin Epoxy is the family of basic components or cured end products of epoxy resins. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups. The epoxide functional group is also co ...

. This prevented the use of

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main compone ...

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 may refer to: * Any kind of a platform for a speaker: **dais **pulpit ** podium * 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 ...

, a partial skull consisting of the right side, the rear section of a skull including basicranium, and a partial skull consisting of the narial region and a partial endocranial cast. A number of features of the ''Australodelphis'' cranium indicate its position in the family Delphinidae. These feature include asymmetry in 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 ...

ends, an inflated pars cochlearis, and a tympanoperiotic which is not sutured with 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 ancestra ...

. The genus also shows a number of similarities to the modern genus ''

Mesoplodon Mesoplodont whales are 16 species of toothed whale in the genus ''Mesoplodon'', making it the largest genus in the cetacean order. Two species were described as recently as 1991 (pygmy beaked whale) and 2002 (Perrin's beaked whale), and marine b ...

'', a member of the

beaked whale Beaked whales (systematic name Ziphiidae) are a Family (biology), family of cetaceans noted as being one of the least-known groups of mammals because of their deep-sea habitat, reclusive behavior and apparent low abundance. Only three or four of ...

family, Ziphiidae, 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 Oceanic dolphins or Delphinidae are a widely distributed family of dolphins that live in the sea. Close to forty extant species are recognised. They include several big species whose common names contain "whale" rather than "dolphin", such as the ...

, while the topography of the rostrum and upper side of the skull is very similar to

Ziphiidae Beaked whales (systematic name Ziphiidae) are a family of cetaceans noted as being one of the least-known groups of mammals because of their deep-sea habitat, reclusive behavior and apparent low abundance. Only three or four of the 24 existing s ...

, making placement of the genus difficult. O. Lambert noted that ''Australodelphis'' has rostrum features similar to those of the family Ziphiidae. However, the first true members of Delphinidae appeared in the late

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 ...

, 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 family Delphinidae. 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, 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.

References

{{Taxonbar, from=Q1964584 Oceanic dolphins Prehistoric toothed whales Pliocene cetaceans Extinct animals of Antarctica Monotypic prehistoric cetacean genera Fossil taxa described in 2002