''Mammalodon'' is an extinct

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 archaic

baleen whale Baleen whales (), also known as whalebone whales, are marine mammals of the order (biology), parvorder Mysticeti in the infraorder Cetacea (whales, dolphins and porpoises), which use baleen plates (or "whalebone") in their mouths to sieve plankt ...

belonging to the family Mammalodontidae.

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 geologic time scale or geological time scale (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the pro ...

. 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 ...

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 binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, both of which use Latin grammatical forms, altho ...

—in Jan Juc, Victoria 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 birds and mammals. It is formed from the fusion of the prootic, epiotic, and opisthotic bones, and in Cetacea forms a complex with the tympanic bone The tympanic part of the ...

. NMV P199587 is a partial skeleton, consisting of part of the head,

spine Spine or spinal may refer to: Science Biology * Spinal column, also known as the backbone * Dendritic spine, a small membranous protrusion from a neuron's dendrite * Thorns, spines, and prickles, needle-like structures in plants * Spine (zoology), ...

, and arm; specimen NMV P198871, an

ulna The ulna or ulnar bone (: ulnae or ulnas) is a long bone in the forearm stretching from the elbow to the wrist. It is on the same side of the forearm as the little finger, running parallel to the Radius (bone), radius, the forearm's other long ...

, 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 an obsolete paraphyletic group of primitive cetaceans that lived from the Early Eocene to the late Oligocene (). Representing the earliest cetacean radiation, they include th ...

, an ancient group of whales, which was evidenced by its apparent ancient features, such as the variety of differently shaped teeth in its jaw (

heterodont In anatomy, a heterodont (from Greek, meaning 'different teeth') is an animal which possesses more than a single tooth morphology. Human dentition is heterodont and diphyodont as an example. In vertebrates, heterodont pertains to animals wher ...

y) that modern whales lack. ''Mammalodon'' was first considered to be a

in a 1982 study despite having no

baleen Baleen is a filter feeder, filter-feeding system inside the mouths of baleen whales. To use baleen, the whale first opens its mouth underwater to take in water. The whale then pushes the water out, and animals such as krill are filtered by th ...

; 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 In human anatomy, the facial skeleton of the skull the external surface of the mandible is marked in the median line by a faint ridge, indicating the mandibular symphysis (Latin: ''symphysis menti'') or line of junction where the two lateral ha ...

between the two halves of the jawbone. It belongs to the

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 ...

Mammalodontidae, along with '' Janjucetus''. These whales and ''

Llanocetus ''Llanocetus'' ( "Llano's whale" ) is a genus of extinct toothed baleen whales from the Late Eocene of Antarctica. The type species, ''Llanocetus denticrenatus'', reached gigantic proportions, with the juvenile specimen reaching an estimated in ...

'' may form a

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

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 ...

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

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 ...

''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 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 ...

—upper jaw—of specimen NMV P199986 preserved four

premolar The premolars, also called premolar Tooth (human), teeth, or bicuspids, are transitional teeth located between the Canine tooth, canine and Molar (tooth), molar teeth. In humans, there are two premolars per dental terminology#Quadrant, quadrant in ...

s and three

molars The molars or molar teeth are large, flat tooth, teeth at the back of the mouth. They are more developed in mammal, mammals. They are used primarily to comminution, grind food during mastication, chewing. The name ''molar'' derives from Latin, '' ...

, and the space between the teeth (

diastema A diastema (: diastemata, from Greek , '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 may be referred to ...

) 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 Cheek teeth or postcanines comprise the molar and premolar teeth in mammals. Cheek teeth are multicuspidate (having many folds or tubercles). Mammals have multicuspidate molars (three in placentals, four in marsupials, in each jaw quadrant) and ...

—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 neck is the part of the body in many vertebrates that connects the head to the torso. It supports the weight of the head and protects the nerves that transmit sensory and motor information between the brain and the rest of the body. Addition ...

—the

axis An axis (: axes) may refer to: Mathematics *A specific line (often a directed line) that plays an important role in some contexts. In particular: ** Coordinate axis of a coordinate system *** ''x''-axis, ''y''-axis, ''z''-axis, common names ...

—is preserved. Unlike in modern baleen whales, but similar to archaeocetes and the ancient toothed baleen whale '' Aetiocetus'', the

breastbone The sternum (: sternums or sterna) or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major blood ve ...

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 appar ...

used to hold up the tongue is large and tubular as opposed to plate-like. It probably had a fused

linking the two halves of the jaw together, unlike in later and modern baleen whales.

Palaeobiology

As with the closely related genus '' Janjucetus'', ''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