Marsupials have the same highly derived skeletons, jaws and teeth as placentals.

Marsupials differ in giving birth to tiny undeveloped young after a short pregnancy, but then lactating for a longer period than placentals. This is more specialized, not primitive.

Marsupials have been thought of as a single order (Marsupialia) for many years. However, recently it has been suggested that there should be 4 (or more) separate orders in order to represent their diversity properly.

 

The peramelids and diprotodontians have syndactylous hind feet. The second and third toes are reduced in                size and share the same skin membrane. They are used for grooming.

A Red Kangaroo

Macropus rufus

	Give birth to live young
	External ear & a cochlea in the inner ear with at least 2.5 coils
	Tribospheric molars
	Distinctive skull and skeleton features:

	lost sclerotic rings
	lost ribs on cervical vertebrae
	more derived hinge joint between tibia and astragalus in ankle

 

	have an inflected angle to the dentary bone
	jugal reaches back to glenoid
	large vacuities in secondary palate
	usually lack or have a reduced form of the 'bulla' in the auditory region
	possess a greater number of incisors and molars but fewer pre-molars than placentals
	the typical marsupial dental formula is: I5/4 C1/1 P3/3 M4/4
	have close excretory and genital tracts
	were primitively aboreal due to opposable big toe that is now much reduced in later forms

	resembles both Eutherian and Monotremata reproduction in certain ways
	similar oestrus cycle to eutherians; controlled by the same hormones
	both marsupials and monotremes have small offspring with all organs present but many do not function.                    As a result offspring are maintained in a pouch in stable conditions and fed on milk

	Have different gestation and lactation periods
	A long development in eutherians allows a fast growth rate
	A slow growth rate in marsupials - longer protection period in homeostatic environment.
	Slow growth rate may be attributed to lower metabolic rate or chorio-allantoic placenta is more efficient organ      than the mammary gland.
	Marsupial grows slower and reproduce less often
	Eutherians grow faster and reproduce more often
	It takes four times as long for a marsupial to gain the weight equivalent to a eutherian but both reach adult          hood at the same age with eutherians the young are independent much earlier with marsupials wholly dependant on mother throughout this time

Specialisations of marsupials

	length of gestation is independent of body size
	marsupial new-borns have well developed fore-limbs, large lungs, increased cranial development and advanced development of jaws, facial muscle and tongue
	The new-born must make way from vagina to the nipple (generally enclosed within a pouch). It climbs up unaided although the mother licks a path from the vagina to the pouch. The young must have an instinct to climb up.
	Lactation continues even after most are old enough to leave the nipple and the joey continues to climb in and        out of the pouch
	Milk from the mother is at first dilute and protein rich but later it is concentrated and richer in fats
	It is possible to have a new born attached to the nipple, a fertilised blastocyst in the uterus and a young joey at      foot still suckling

 

Geography.........

Different major groups of mammals have predominated on different continents through the tertiary. In Europe, Asia and North America different groups of eutherians were present to in Africa. In Australia marsupials were dominant and are still the most diverse but the bats and rodents, which were late arrivals are now the most numerous.

In South America both marsupials and South American groups of eutherians dominated throughout the tertiary but the latter died out during the Pleiocene when North and South America joined up again and Northern eutherians invaded.

 

Humans influences.............

Most of the large marsupials are now extinct and the largest surviving ones are restricted to Australia. The 'large' Australian marsupials are no where near the size of extinct species. There are no longer marsupials in South America. It is both the climate and humans that are responsible for this.

Humans are responsible for introducing non-native species to Australia such as foxes and cats. Competitors have also been introduced such as cattle and rabbits. 

The consequences have been disastrous, particularly on the small marsupial fauna. They have either become extinct like the pig-footed bandicoot or are now restricted to offshore islands, like many wallabies.

Large marsupials are extinct and small marsupials are becoming increasingly endangered. The future for marsupials is looking uncertain......

 

Further Reading

Carroll, R.L. (1988) Vertebrate Palaeontology and Evolution. Chapter 19: pages 425 - 439

Pough, F.H., Janis, C.M. & Heiser J.B. (2002) Vertebrate Life.  Chapter 20: pages 552 – 581

Tyndale-Biscoe, H. (1973) Life of Marsupials. Edward Arnold Publishers Ltd., London. 254pp

Recommended Websites:

http://www.marsupialcrc.com.au/webhome/FACTS - contains information about the current problem and information about the conservation status of many marsupials.