Australia: The Land Where Time Began

A biography of the Australian continent 

Riversleigh diprotodontoids

The superfamily Protodontoidea has 2 families, diprotodontids and palorchestids.

Diprotodontids

The diprotodontids were similar in shape to wombats, and ranged from the size of a sheep to that of a large hippopotamus. There were 2 families of diprotodontids, Zygomaturines and Diprotodontines.

Zygomaturines have multi-cusped last upper premolars. Among the Riversleigh diprotodontoids these were the most common and diverse. One of the most common was Neohelos tirarensis, that seems to have ranged far and wide across Australian habitats during the Miocene. One genus, Nimbadon, is known from skulls and articulated skeletons, was about the size of a labrador dog, others reached the size of small cattle. A feature of these animals was that they had very small brains. They are believed to have eaten soft leaves. Gradually the smaller forms died out, either becoming extinct or giving rise to larger forms. This evolutionary pattern is common, being found in the fossil record of many vertebrate groups. Most zygomaturines were very large, and most were species of Zygomaturus by the Pliocene. Z. trilobus was still living when the Aboriginal People arrived in Australia. Hulitherium seems to have survived in the New Guinea highlands as late as the Early Holocene. Z. trilobus fossils have been found in the fossil deposits of the Naracoorte Caves.

Diprotodontines had much simpler premolars than the zygomaturines. They followed the same path as the zygomaturines, diversifying and becoming larger, but diversified less than the zygomaturines. The last of the group, Diprotodon optatum, that survived to the Late Pleistocene, was the largest known marsupial. Before their discovery at Riversleigh, the oldest known protodontine was the very large species Pyramios alcootensis from the Alcoota Local Fauna from the Late Miocene.

A very primitive, small diprotodontine has been found at Riversleigh. It has the simple premolars of diprotodontines and the molars of a zygomaturine, suggesting that it may be ancestral to both groups, or at least closely related to the ancestral form, especially as it is older than the oldest known members of both groups, such as P. alcootensis.

Palorchestids. The last-surviving species of this group, Palorchestes azeal, was about the size of a horse. Its shape differs from any other known animal. It had very powerful forearms and very large claws like those of koalas, a long, thin tongue and a large trunk like an elephant. As it had the high-crowned, complex teeth of like the grazers it could have eaten grass, it is assumed it must have eaten abrasive plant material such as grass, but why it had the other characteristics of a trunk, long thin tongue and powerful arms and claws is unknown. These features don't fit with it being a grass-eater.

The oldest named palorchestid genera,  Ngapakaldia and Pitikantia, were found in the central Australian Oligocene-Miocene sites. A species of  Ngapakaldia has been found in the lower deposits at Riversleigh. This find has allowed the biocorrelation of the Riversleigh strata with those of the central Australian sites. Many small palorchestids have also been found at Riversleigh in the local faunas from Sites B and C.

A tooth from Propalorchestes, previously known from the Bullock Creek Local Fauna in the Northern Territory, has been found at Riversleigh.

Some other diprotodontoids from Riversleigh have a combination of features normally thought to be from either diprotodontids or palorchestids. This blend of features has been interpreted as indicating that the common ancestor of diprotodontids and palorchestids had the premolars of palorchestids, the molars and incisors of primitive zygomaturines, and the middle ear region of the skull like that seen in Ngapakaldia species. The common ancestor would be much older than any of the fossils at either Riversleigh or the central sites containing mammals.

Sources & Further reading

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

Links

1. The Age of the Megafauna
2. Australian Beasts