Australia: The Land Where Time Began

A biography of the Australian continent 

Koalas

Koalas have developed the ability to eat the leaves of gum trees (Eucalypts) which are not only low in nutrition but are packed with toxins. This ability to survive on a very widespread food source that no other mammal can eat frees them from the pressures of competition for food from other animals. Gum trees have adapted to survive drought better than most other trees, so koalas have a constant food source that also provides their water needs in dry spells when ground-dwelling animals have much more difficulty finding enough water, either free or in vegetation, to survive.

Skull fragments from 2 extinct species of koala, Litokoala kutjamarpensis and Nimiokoala greystansi, that lived about 20 million years ago, both from the Riversleigh deposits, have been compared with the skulls of modern koalas. It was found that the teeth, palate and jaws of the extinct species differed significantly from those of the living species, Phascolarctos cinereus, even though the remainder of the skull hadn't changed significantly over time.

The comparison suggests that the extinct koalas probably had a more diverse diet, and if they ate eucalypts they would have been of species with leaves less tough than those they eat today. The facial bones of koalas underwent substantial changes as they evolved towards the living species to allow them to get a more powerful bite as they began eating the though leaves of eucalypts with low nutrient levels and high tannin content. In mammals, the bones of the ear evolved from jaw bones, and the jaw is very close to the ear, so the structure of the jaws has an effect on the hearing of the animal. In mammals, changes in jaw structure can impact on the hearing, The shape of the koala's skull has developed the way it has to allow its jaw structure to change without affecting its hearing.

According to the paper below, the auditory bullae (large bony structure enclosing the middle and inner ear) has maintained a similar size and shape between the extinct and extant species. This indicates that their vocalisations probably haven't changed. They probably used their loud bellows, that can be heard by potential mates and rivals up to a kilometre away, to communicate without leaving their tree then and now. It seems they had probably already developed their sedentary habits 20 million years ago.

Journal of Vertebrate Paleontology 29(4):981992, December 2009

Phascolarctos cinereus, the living koala and P. stirtoni, an extinct species, have both been found in the Naracoorte Caves fossil site in South Australia

 

Links

  1. Prehistoric koalas didn't eat gum, study says
  2. Australian Koala Foundation 
  3. Fact Sheet, San Diego Zoo

Sources & Further reading

 

 

 

 

 

 

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Last updated 08/01/2010 

 

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading