Australia: The Land Where Time Began

A biography of the Australian continent 

Of the features evolved by kangaroos as they adapted to the increasingly dry Australian climate, on increasingly impoverished soils, there are 3 that contribute much to their success - foregut fermentation, hopping and embryonic diapause.

Kangaroo origins

There are at least 20 genera of macropods, in 4 distinct families, that have been found in deposits dating to the Oligocene, indicating the first ancestral groups arose some time before the Oligocene. The Balbaridae and the Bulungamayines were the 2 most common groups of the earliest known period, both dying out some time between 10 and 5 Ma in the Late Miocene.

It is still uncertain exactly how they are related to rat kangaroos and modern kangaroos. It is believed by some that the rat kangaroos arose from the balbarines and the Bulungamayines gave rise to the main branch that led to the kangaroos and Sthenurines (Cooke & Kear, 1999). Others hold the contrary view (Flannery, 1989). The time of the evolution of the hopping gait by the ancestral kangaroos is crucial to determining which opinion is correct. As most fossils recovered from the time when hopping gate would have been evolving are fragmentary, skull fragments or isolated teeth, this is difficult to determine exactly when the transition to hopping occurred. It is commonly believed that the Late Oligocene to Early Miocene was the time when the hopping gate arose. In Balbaroo fangaroo, from Riversleigh, all 4 limbs were of similar length, and it had a 1st digit on the hindfoot, as occurs in the modern musky rat kangaroo, that is quadrupedal. It is believed probable that this species didn't hop, because of the small size of the attachment sites on the femur of the main muscles used for hopping, the quadratus and adductor magnus. The Gunguroo and the Wanburoo, bulungamayines from the Middle to Late Miocene, had lost the 1st digit of the hindfoot, and the 2nd and 3rd digits were reduced and partly beneath the 4th digit, as occurs in modern kangaroos, so are believed to have probably been bipedal. Another adaptation that occurred in their hind foot was the articulation of the ankle bones that restricted sideways movement, a feature associated with hopping locomotion.

The musky rat kangaroo and the balbarines were the only kangaroos that were not bipedal. Based on this, it is widely believed that if there was not a common bipedal ancestor to all, then bipedalism must have arisen independently more than once. It has been suggested that bipedalism possibly arose independently in early potoroines, arising later in the bulungamayines (Kirsch et al., 1997). Others believe the balbarines were related to Propleopus and Hypsiprymnodon, and so are distinct from other macropods (Cooke & Kear, 1999). If this turns out to be the case, the stem group for all the Macropodidae would be the bulungamayines, either declining or being replaced by the radiation of the Potoroinae, Sthenurinae and the Macropodidae as aridification of the continent progressed, with the associated spread of grasslands (Tyndale-Biscoe, 2005).

Sthenurines from the earliest known fossil deposits were about the size of wallabies, but during the Pliocene and Pleistocene their descendants grew up to 3 m tall, the hindfoot reducing further to a single large 4th toe, as in the case of the foot of the horse, a feature that distinguishes them from true kangaroos. They had a short stocky tail, and forelimbs that appear to have been more flexible than those of kangaroos, and are believed to have probably not been used for locomotion, could have been used to reach above their heads to pull branches down to their mouths, as is believed to have been the case with the extinct giant ground sloth of South America. They had massive skulls and a very short muzzle, in which the halves of the lower jaw were fused. They flourished for about 5 million years, finally going extinct between 10,000 and 5.000 years ago, though it has been suggested that the banded hare wallaby may be a extant member of the group.

In the last 5 million years the Potoroines and the Macropodines both underwent a major radiation, and by 2 million years ago all the major genera were represented. Throughout their short history, the Macropodidae have been extremely dynamic, producing a wide diversity of forms as they radiated and adapted to fill a number of new ecological niches as they became available (Flannery, 1989). Their success was based on the 3 main features they evolved to survive and thrive in a greatly changing environment, digestive, locomotion and reproduction.

Sources & Further reading

Tyndale-Biscoe, Hugh, 2005, Life of Marsupials, CSIRO Publishing.