Mammalodontidae is a
family
Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...
of
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 ...
whale
Whales are a widely distributed and diverse group of fully aquatic placental marine mammals. As an informal and colloquial grouping, they correspond to large members of the infraorder Cetacea, i.e. all cetaceans apart from dolphins and ...
s known from the
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 the ...
of
Australia
Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
and
New Zealand
New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...
.
There are currently two genera in this family: ''
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 ...
'' and ''
Mammalodon
''Mammalodon'' is an extinct genus of archaic baleen whale 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 holotype for ...
''. After a new
cladistic
Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups (" clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived char ...
analysis by Fitzgerald (2010), ''Janjucetus'' was transferred into Mammalodontidae, thereby making Janjucetidae a junior synonym of Mammalodontidae.
Analysing the jaw morphology of the toothed mysticetes, found eight mandibular characters unique to the members of Mammalodontidae:
* the
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 halves ...
is short, has a rugose joint surface, but lacks a symphyseal groove. In
archaeocetes
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 ...
the symphysis is long. In modern mysticetes, in contrast, the symphysis is very small, its joint surfaces are smooth, and there is a groove on the interior side of the mandible that accommodates the symphyseal ligament which enables them to open their mouth wide.
* the external
on the mandible are relatively large (smaller or absent in later mysticetes)
* the postcanines sit in a longitudinal groove flanked by a lateral edge (the "alveolar margin")
* the alveolar (upper) margin forms an angle with the ventral (lower) margin (like in archaeocetes)
* the ventral margin is straight in the posterior half of the mandible
* teeth have longitudinal ridges
* posterior postcanines have two roots joined below the crown base
* postcanines are densely packed without long
diastemata
From these mandibular features, Fitzgerald 2006 concluded that in mysticetes enlarge oral cavities adapted for
suction feeding
Aquatic feeding mechanisms face a special difficulty as compared to feeding on land, because the density of water is about the same as that of the prey, so the prey tends to be pushed away when the mouth is closed. This problem was first identifi ...
evolved before mandibular adaptations for bulk
filter feeding
Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...
, like for example kinetic jaw joints.
References
Oligocene cetaceans
Chattian extinctions
Baleen whales
Chattian first appearances
Prehistoric mammal families
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