extinct
Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
Pliocene
The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58dolphin
A dolphin is an aquatic mammal within the infraorder Cetacea. Dolphin species belong to the families Delphinidae (the oceanic dolphins), Platanistidae (the Indian river dolphins), Iniidae (the New World river dolphins), Pontoporiidae (the b ...
East Antarctica
East Antarctica, also called Greater Antarctica, constitutes the majority (two-thirds) of the Antarctic continent, lying on the Indian Ocean side of the continent, separated from West Antarctica by the Transantarctic Mountains. It lies almos ...
. 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 com ...
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 a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
''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 of the type specimen. While not described until 2002, 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 ...
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 is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of seve ...
skull
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, th ...
was figured in 1988 by R. E. Fordyce and ''Australodelphis mirus'' first appeared as a ''
nomen nudum
In 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 with an adequate desc ...
'' 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 that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel th ...
n to be named from sediments dating after the final breakup of
Gondwana
Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final sta ...
. 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
The 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, ...
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.Blatt ''et al.'' 1980, ...
, dated at 4.5 to 4.1 million years old, placing the sediments in the Early
Pliocene
The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58 The cetaceans of the Sørsdal Formation are found in association with the extinct
diatom
A diatom ( Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising se ...
scallop
Scallop () is a common name that encompasses various species of marine bivalve mollusks in the taxonomic family Pectinidae, the scallops. However, the common name "scallop" is also sometimes applied to species in other closely related families ...
'' 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 in an above-freezing atmosphere coming in contact with a solid surface whose temperature is below freezing, and resulting in a phase change from water vapor (a g ...
-shattered pieces using a combination of
polyvinyl acetate
Polyvinyl acetate (PVA, PVAc, poly(ethenyl ethanoate)), commonly known as wood glue, PVA glue, white glue, carpenter's glue, school glue, or Elmer's glue in the US, is a widely available adhesive used for porous materials like wood, paper, and ...
and
epoxy resin
Epoxy is the family of basic components or Curing (chemistry), 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 ...
. 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 componen ...
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 base of skull, also known as the cranial base or the cranial floor, is the most inferior area of the skull. It is composed of the endocranium and the lower parts of the calvaria.
Structure
Structures found at the base of the skull are for ...
. The other four specimens consist of one partial rostrum, 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 mammal has ...
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 ...
. 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 ...
'', a member of the
beaked whale
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 and apparent low abundance. Only three or four of the 24 species are reasonably well- ...
family, Ziphiidae, possessing an elongated toothless rostrum with wide maxillary flanges and laterally compressed
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 ...
.
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, 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 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 t ...
, 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
''Messapicetus'' is an extinct genus of beaked whale from the Late Miocene. It currently holds two species, ''M. longirostris'' from the Tortonian of Italy and ''M. gregarius'' from the Pisco Formation of Peru. However, a third unnamed species is ...
'' 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.