Australia: The Land Where Time Began

A biography of the Australian continent 

Anatomy Based relationship - Sperm morphology and anatomy

Relationships among the marsupials can be indicated by the fine structure and morphology of their spermatozoa. The Didelphidae and the Caenolestidae, the 2 main families of American Marsupials, the sperm occur as conjoined pairs, the heads of the 2 spermatozoa being pressed closely together, remaining in this state until reaching the vicinity of the egg in the female's oviduct. Such conjugation is unknown in any Australian marsupial, and is not known of in any placental. Unlike other American marsupials, Dromiciops also has sperm that is not conjugated, as is the case with Australian marsupials, adding further support to the notion that there were close links between the Australian and American marsupials.

The shape of the spermatozoon is characteristic in each marsupial family. It distinguishes the 3 families of American marsupials from each other, and in Australia, distinguishes the Macropodidae, Dasyuridae and the Peramelidae. The koalas are closely related to the wombats, which is supported by the hook-shaped sperm heads common to both groups, but different from any other marsupial group. It also supports their difference from the possums. Tarsipes, a very small marsupial, with sperm measuring 360 um in length, has the largest sperm known for any mammal, being 4.5 times as long as human sperm.

The males of all South American and most Australian marsupials have a penis with 2 lobes. There are some variations, in the koala and wombat, it is partially divided, and in the Macropodidae it is a single structure, Tarsiops lacks the glans completely. Penis anatomy can be used to classify some of the dasyurids.

The 16 families of extant marsupials have been divided into 5 groups, based on the anatomical characteristics of teeth, feet, brain, sperm and penis shape. of which 1 is found only in South America, 3 exclusively in Australasia, and one in which species are found in both Australasia and South America.

1. Chris Johnson, Australia's Mammal Extinctions, a 50,000 year history, Cambridge University Press, 2006
2. M.Archer, S.J. Hand & H. Godthelp in Hill, Robert S., (ed.), 1994, History of the Australian Vegetation, Cambridge University Press.
3. Tyndale-Biscoe, Hugh, 2005, Life of Marsupials, CSIRO Publishing.