Australia: The Land Where Time Began

A biography of the Australian continent 

Riversleigh Koalas

Living koalas (Phascolarctos ciereus) are unusual in that their brains don't fill their skulls. In spite of this, the family to which koalas belong, the Phascolarctidae, has been around for at least 25 million years, almost unchanged in form and diversity from their first known appearance in the fossil record. 1 or 2 species are present in most Oligocene-Miocene deposits from central Australia. The teeth and skulls of the 3 known genera, Perikoala, Madakoala, and Litokoala, seem to be similar to that of the living species. Like the modern species, they were all arboreal leaf-eaters.

Among the species of Koala found at Riversleigh there are some species of Litokoala that are better persevered than those from other fossil sites. There is also a primitive new genus, that is more primitive than any other known fossil koala. Another genus of several species had complex, high-crowned molars that indicate that they ate something other than gum leaves.

The koalas from other sites are known from teeth and a partial jawbone. Some of the koalas found at Riversleigh are known from partial skulls. Compared with the present populations of koalas, the Riversleigh koalas, and probably those of other rainforest areas in central Australia, were apparently rare. The reason for the rarity in the past is unknown, but it has been speculated that it may have been because the eucalypts that koalas specialised in eating were also rare in the closed canopy rainforests of Riversleigh, as they are in the present-day rainforests. By the time the climate changed and the rainforest species died out in the Riversleigh area the eucalypts had been pre-adapted to survive on the impoverished, arid soils that existed when the rainforest had gone, by adapting to the low-nutrient soils that underlie many Australian rainforests. If this suggestion is correct, as the eucalypts flourished, so did the koalas that fed on them.

The conclusions from studying the fossil record of koalas, is that they are now the most abundant they have been since they first appeared in the Tertiary. It seems the only thing that could stop them is the destruction of them and their habitat by humans. Based on their dental morphology, the living koalas are little more specialised than they were in the earliest known fossil sites. This suggests that they are not in decline, as long as they are not helped on their way by humans.

Links

1. Australian Koala Foundation
2. Fact Sheet, San Diego Zoo

Sources & Further reading

1. Michael Archer, Suzanne J. Hand & Henk Godthelp, Australia's Lost World: Riversleigh, world heritage Site, Reed New Holland