Bilaterally symmetrical animals
SUPERORDER: Therians
The therians give birth to live young, as opposed to eggs. Their ears are external, and their ankles have adapted to allow for versatile land movement.
There are extinct therians, notably from the infraclass trituberculata, that date as far back as 200 million years ago.
The other two infraclasses are called eutheria and metatheria.
Due to the complexity of fossilised animals that could qualify for these infraclasses, there are a lot of species which scientists find difficult to conclusively classify in each infraclass.
Essentially trituberculates are categorised by the presence of a third cusp on the molar teeth, and are considered to be the ancient relative of marsupials. Trituberculata are considered to be extinct with modern day marsupials belonging in the metatheria infraclass.
Metatheria is the infraclass which modern marsupials belong to. The distinguishing factor being that the latter part of an offspring's development takes place within the marsupium, which is an external pouch on the offspring's mother.
Eutherians are distinguished by their unique development of the bone structure of the lower part of their legs and their jaws.
Humans are members of eutheria.
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Mammals with uniquely developed ankles, feet, jaws and teeth. |
METATHERIA Mammals whose offspring partially develop in an external pouch. |
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TRITUBERCULATA Extinct mammals which have three cusped molar teeth. |
