Diprotodon Distribution

Holocene 10,000-0

Probable relationships of koalas and their diversity through time. Each block represents a genus and the number of species is indicated in the block if greater than one. (Taken from Long et al. (2002))

late Pleistocene have been found north of Perth at Koala Cave near Yanchep,25 and south of the city at Mammoth Cave near Margaret River.26 Devil's Lair near Boranup has Phascolarctos fossils from the main excavation,27 with other fossils also known from Labyrinth Cave near Augusta.28 Koala fossil deposits have also been found in the Madura Cave on the southern edge of the Nullarbor Plain.29 There are still plenty of viable eucalypt plantations in south-east Western Australia, so it is difficult to establish why there are no koalas.30 One hypothesis is that the

□ Late Oligocene — early Miocene fossils • Pliocene fossils O Pleistocene fossils

Locations of fossil koalas and the predicted distribution of rainforest during the Tertiary period. (Taken from Martin & Handasyde (1999))

Distribution Koala

□ Late Oligocene — early Miocene fossils • Pliocene fossils O Pleistocene fossils

Extent of forests of the Tertiary period

Extent of forests of the Tertiary period koala became extinct in Western Australia after the arrival of the Aborigines.31

The koala family Phascolarctidae appears to have evolved from a group of marsupials now known as the diprotodonts, literally, 'two front teeth'. These included the enormous ancestor of today's wombat, Diprotodon australis, a creature the size of a modern-day rhinoceros, weighing about two tonnes. Although it is not a close relationship, the koala's nearest relatives are

Marcopodidae Burramyidae

Phalangeridae Pseudocheiridae Tarsipedidae ""^j' Petauridae

■ Acrobatidae

Phascolarctidae y/tj

• Microbiotheriidae

■ Dasyuridae ' Caluromyidae

Didelphidae

Caenolestidae

Relationships of the living families of marsupials (with the exception of the Potoroidae andHypsyprymnodontidae families). (Taken from Kirsch, Lapointe & Springer (1997))

the wombats of the family Vombatidae. The two families went their separate ways in the late Eocene, approximately 42 million years ago.32

We do not know how large the common ancestor of the koala and the wombat was, but it is possible that it was a bur-rower who began climbing trees to access a different food source. The koala's backward-facing pouch is an interesting evolutionary legacy of this upwards movement into the trees. The wombat's pouch opens backward to protect the young from flying dirt raised by a digging adult. The koala joey needs no such protection, but now has what must be a pretty vertiginous view down to the ground.

So, we have established that the koala is a marsupial—in fact, it is a syndactylic diprotodont marsupial. But what does that mean? Within the marsupial family, two sub-families— the bandicoots and a group comprising the koalas, wombats, possums and kangaroos—have the second and third toes of their hind feet joined together within the one sheath of skin, a phenomenon known as syndactyly or 'joined toes'.33 As we saw above, 'diprotodont' refers to the great elongation and forward pointing of the two functional lower incisors found in all these species with the exception of the bandicoots.34

The fossil record confirms that Australian marsupials evolved in parallel with many placental mammals. Though they have different reproductive systems, some species have co-evolved very similar adaptations; for example the wolf-like thylacine,

Koala Hand
The right forepaw (A) and hindpaw (B) of the koala. Notice the two thumbs on the forepaw and the joined second and third toes of the hindpaw. (Taken from Martin & Handasyde (1999))

the striped possums with elongated digits like those of the aye-aye lemur of Madagascar, the marsupial gliders with membranes similar to those of flying squirrels, marsupial moles that swim through the desert sands like Africa's golden mole, and kangaroos that hop like hopping mice and spring hares.

The placental mammal most commonly compared to the koala is the sloth, but although they have similarly inactive lifestyles, the resemblance goes no further. Many of the first Europeans to come across the koala assumed that it was a new species of sloth, but sloths are found only in Central and South America. As can be seen from the following illustration, and as we will discuss in more detail in Chapter 4, the koala has very well-developed, strong teeth that include incisors, canines (on a a

Koala Skull
Ventral (a) and lateral (b) view of a koala skull showing the shape and teeth types. (Taken from Lee & Carrick (1989))

the upper jaw), premolars and molars, all of which are coated with enamel. Sloths do not have any incisors or canines and their grinding teeth are simple cylinders that are not coated with enamel.

Despite the diversity of marsupial species there are none that fly like bats, nor any strictly aquatic species such as seals or whales. Only the South American marsupial called the yapok

ExampLes of the pARAneL evoLution between AustrALiAn MARsuPiALs and placental mammals.

Wolf-like carnivore

Wolf-like carnivore

Thylacine Elongated finger for finding insects

Thylacine Elongated finger for finding insects

Striped possum

Sugar glider

Blind underground desert burrowers

Gliders

Marsupial mole Tree-dwelling leafeater that sleeps a lot

Burrower

Hopper

Burrower

Hopper

Wombat

Eastern grey kangaroo

Koala

Wombat

Eastern grey kangaroo

Wolf

Aye-aye

Flying squirrel

Golden mole

Aye-aye

Flying squirrel

Golden mole

Sloth

Badger

Sloth

Badger

Hopping mouse is semi-aquatic. It has webbed feet and spends much time in the water finding food. Only one species, the marsupial mole, lives entirely underground. The numbat and musky-rat kangaroo are alone in being active solely during daylight hours. The extinction of the marsupial lion means that there are also no large carnivores such as lions or tigers. There are no large herbivores such as elephants and rhinoceroses, although a number of such species did exist until approximately 40 000 years ago,35 nor are there marsupials with extreme brain development such as primates.

The koala has an extraordinary evolutionary history and although its environment has changed radically over the last 30 million years, the modern species seems relatively unchanged when compared to its distant ancestors. In fact, the koala has evolved and adapted so well to its environment that not too long ago it was one of the most widespread and abundant marsupial species on the continent. The koala's more recent history is inextricably linked with that of Australia's Aborigines and, as we shall see in Chapter 2, the koala features in many different Dreamtime stories.

How Many Fingers Kolas Have
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  • Costantino
    How many fingers do koalas have?
    7 years ago

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