Marsupials
Marsupialia - marsupials
Sarcopterygii; Tetrapoda; Amniota; Synapsida; Theria; Metatheria; Marsupialia |
The marsupials are the sister taxon to the subclass Eutheria (placental
mammals), together forming the clade Theria. Therians are then the
sister clade to the monotremes, forming the
group mammalia - all living mammals. The cladogram below shows these
relationships:
Diversity and Lower
Taxonomy:
The
marsupials are 335 extant species across seven orders of mammals from
Australasia and the Americas, which originated in the Early Cretaceous of North
America.
Three of these orders are now restricted to the Americas -
including opossums, shrew opossums and the charismatic Monito del Monte - while
the other four are restricted to Australasia - including kangaroos, possums,
marsupial moles, bandicoots, and carnivorous marsupials such as the tasmanian
devil. Below is a list of these orders with their representative groups:
o
Didelphimorphia - American
opossums
o
Paucituberculata - shrew
opossums
o
Microbiotheria - Monito
del Monte
o
Notoryctemorphia - marsupial
moles
o
Dasyuromorphia - Australian
carnivorous marsupials
o
Peramelemorphia - bandicoots
and bilbies
o
Diprotodontia - kangaroos,
wallabies, possums, wombats, and koala
Features:
- After a brief gestation in the
womb with no placenta (except bandicoots), the females of certain species
give birth to extremely immature young, who then develop to maturity
through suckling in the mother's pouch.
- In-turned process in the dentary.
- Three premolars and four molars. (NB. This is the ancestral marsupial dentition,
however, many species have actually modified this character to have a
varying number of molars and premolars)
- Three premolars and four molars. (NB. This is the ancestral marsupial dentition,
however, many species have actually modified this character to have a
varying number of molars and premolars)
- Epipubic bones - two thin rod-like bones that extend anteriorly
from the pubic bones of
the pelvic girdle. These
bones, although often called marsupial bones, are also seen in monotremes, and are probably a ancestral
mammalian character, which was lost in the Eutheria.
Labelled images of an opposum skull:
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