''Mammalodon'' 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 nom ...

of archaic baleen whale belonging to the family

Mammalodontidae Mammalodontidae is a family of extinct whales known from the Oligocene of Australia and New Zealand. There are currently two genera in this family: ''Janjucetus'' and ''Mammalodon''. After a new cladistic analysis by Fitzgerald (2010), ''Janjuce ...

Taxonomy

The fossils of ''Mammalodon'' were found to be around 25.7–23.9 million years old, dating to the

Late Oligocene 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 stage ...

. 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 sever ...

for ''M. colliveri'', NMV P199986 though formerly MUGD 1874, is an incomplete skull of an adult individual collected in 1932 by George Baxter Pritchard, Alan Frostick, and Frederick Stanley Colliver—to which owes the

species name In taxonomy, binomial nomenclature ("two-term naming system"), also called nomenclature ("two-name naming system") or binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, bot ...

—in

Jan Juc, Victoria Jan Juc is a suburb of Torquay, Victoria Torquay is a seaside resort in Victoria, Australia, which faces Bass Strait, 21 km south of Geelong and is the gateway to the Great Ocean Road. It is bordered on the west by Spring Creek and i ...

in Australia; specimen NMV P17535, consisting of a lower left molar, probably also belongs to NMV P199986. NMV P173220 consists of a left

periotic bone The periotic bone is the single bone that surrounds the inner ear of mammals. It is formed from the fusion of the prootic, epiotic, and opisthotic bones. References External links * http://www.encyclopedia.com/doc/1O8-perioticbone.html ...

. NMV P199587 is a partial skeleton, consisting of part of the head, spine, and arm; specimen NMV P198871, an

ulna The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...

, very likely also belongs to NMV P199587. The second species, ''M. hakataramea'', was discovered in the Kokoamu Greensand of New Zealand. ''Mammalodon'' fossils have been found in Australia and New Zealand ''Mammalodon'' was, at first, considered to be a member of

Archaeoceti Archaeoceti ("ancient whales"), or Zeuglodontes in older literature, is a paraphyletic group of primitive cetaceans that lived from the Early Eocene to the late Oligocene (). Representing the earliest cetacean radiation, they include the initial ...

, an ancient group of whales, which was evidenced by its apparent ancient features, such as the variety of differently shaped teeth in its jaw ( heterodonty) that modern whales lack. ''Mammalodon'' was first considered to be a baleen whale in a 1982 study despite having no baleen; instead, they cited other similarities such as loosely sutured bones in the snout, a broad and flat roof of the mouth, and an unjointed mandibular symphysis between the two halves of the jawbone. It belongs to the

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 ...

, along with ''

Janjucetus ''Janjucetus'' is an extinct genus of cetacean, and a basal baleen whale (Mysticeti), from the Late Oligocene around 25 million years ago (mya) off southeast Australia, containing one species ''J. hunderi''. Unlike modern mysticetes, it pos ...

''. These whales and '' Llanocetus'' may form a clade of toothed baleen whales of the Southern Hemisphere, a

sister clade In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and t ...

to Aetiocetidae and more modern baleen-bearing baleen whales. The name ''Mammalodon'' is said to be derived from

English English usually refers to: * English language * English people English may also refer to: Peoples, culture, and language * ''English'', an adjective for something of, from, or related to England ** English national ide ...

''mammal'' and

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 ...

''odontos'' tooth, meaning "mammal tooth", as its molar teeth are similar to those found in terrestrial carnivores. The Ancient Greek for "tooth" is, however, (ὀδούς).

Description

''Mammalodon'', with a length of , was smaller and more primitive than modern baleen whales. Unlike other baleen whales, ''Mammalodon'' had a blunt and rounded snout. The left

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. T ...

—upper jaw—of specimen NMV P199986 preserved four premolars and three

molars The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone to ...

, and the space between the teeth (

diastema A diastema (plural diastemata, from Greek διάστημα, space) is a space or gap between two teeth. Many species of mammals have diastemata as a normal feature, most commonly between the incisors and molars. More colloquially, the condition ...

) increased towards back into the mouth. The molars decreased in size back into the mouth, like in archaeocetes, and the bottom jaw had two more molars than the upper jaw. The specimen's lower jaw indicates it had 24 lower teeth in all, all tightly spaced together. The upper teeth all looked the same (monodonty), whereas the bottom teeth varied in shape (polydonty) which is an ancient characteristic of whales. There were three lower

incisor Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, wher ...

teeth, and one upper incisor with possibly two or three

vestigial Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on co ...

incisors. The teeth were likely never replaced, and the whale had the same set of teeth throughout its life. The single upper incisor was markedly smaller than the other teeth, and smaller than the upper incisors of ''Janjucetus''. The cheek teeth—molars and premolars—were all double-rooted, and the lower molars were serrated and triangular. In the holotype of ''M. colliveri'', only the second vertebra of the neck—the

axis An axis (plural ''axes'') is an imaginary line around which an object rotates or is symmetrical. Axis may also refer to: Mathematics * Axis of rotation: see rotation around a fixed axis * Axis (mathematics), a designator for a Cartesian-coordinat ...

—is preserved. Unlike in modern baleen whales, but similar to archaeocetes and the ancient toothed baleen whale '' Aetiocetus'', the breastbone is composed of several pieces. The top-most breastbone, the manubrium, is T-shaped and wider than is long like archaeocetes, but plate-like and compressed like modern baleen whales. Unlike in modern whales though similar to archaeocetes, the thyrohyoid bone of the

hyoid apparatus The hyoid apparatus is the collective term used in veterinary anatomy for the bones which suspend the tongue and larynx. It consists of pairs of stylohyoid, thyrohyoid, epihyoid and ceratohyoid bones, and a single basihyoid bone. The hyoid appara ...

used to hold up the tongue is large and tubular as opposed to plate-like. It probably had a fused mandibular symphysis linking the two halves of the jaw together, unlike in later and modern baleen whales.

Palaeobiology

As with the closely related genus ''

'', ''Mammalodon'' lacked baleen, instead possessing well-developed teeth. As such, it was not able to filter-feed in the same manner as extant baleen whales, making its diet and ecological niche a mystery. As the teeth are widely spaced, they may have developed a method of filter-feeding unlike that of other whales. It may have been a bottom filter feeder, its blunt snout helping to suck up organisms from the sea floor.

References

{{Taxonbar, from=Q2428914 Oligocene cetaceans Baleen whales Prehistoric cetacean genera Extinct mammals of Australia Fossil taxa described in 1939