Australia: The Land Where Time Began

A biography of the Australian continent 

Diprotodontids

At a number of sites in widespread parts of Western Australia, such as the Kimberley, Pilbara , Lake Darlot to the north of Leonora, and at Balladonia on the western edge of the Nullarbor Plain, lower jaws, teeth and bone fragments of Diprotodon optatum the largest known marsupial, once lived in Western Australia. In 1991 a complete skeleton of Diprotodon was found in du Boulay Creek, near the mouth of the Fortescue River, in the Pilbara. This specimen, that is at least 80,000 years old, is about 3 m long and 2 m high at the shoulder, is one of the largest diprotodons found so far.

Diprotodon was one of the last surviving members of the family Diprotodontidae, large herbivorous marsupials that are first found in deposits from about 24 Ma. The earliest known diprotodontids were about the size of sheep, though by the Late Pleistocene they had attained the size of D. obtatum, weighing about 2 tonnes and being the size of a rhinoceros. It was heavily built, having stout limbs and a neck that was moderately long. According to the authors¹ it is believed to have had an appearance similar to that of an oversized, long legged wombat, the nearest living relative of Diprotodon, though they are only distantly related. Diprotodon and wombats share several features of the skull, which reflects the evolution of the 2 forms from a common ancestor about 30 Ma.

It is believed Diprotodon probably moved rather slowly. It had unusual feet with massive wrists and ankles but very small toes on all its feet, and on the hind feet the big toe opposed the others.

In Diprotodon the brain was larger than in its ancestors, though relative to body size it was smaller. It had a very large head, with much of the skull containing an open network of air sinuses that were apparently designed to lower the weight of the skull. The brain was contained within an inner brain case inside the massive skull. The authrors¹ suggest the structure of the skull was important as it allowed the external shape of the skull to remain similar in shape as that of its ancestors to accommodate its huge teeth and muscles required to process large amounts of coarse vegetation. Much of the bone composing the skull was also very thin, apparently to reduce the weight of the skull.

An increase in the relative size of the teeth that was required to process sufficient food to support such a large body mass probably relates to the development of the very large skull, and such a large skull would have withstood the chewing of very coarse vegetation. Relatively larger muscles than those of the smaller ancestral diprotodontids would have supported the larger skull.

Diprotodon had a pair of elongate, lower incisors that pointed forward and a pair of chisel-like incisors in the upper jaw which opposed the lower pair. The 5 cm long molars had a characteristic high, narrow, transverse double-ridged structure. Where opposing molars met there was a vertical slicing and grinding motion that would have been ideal for shredding the tough vegetation that they apparently fed on exclusively, possibly comprised of leaves and shoots of small plants and trees.

The nature of the Diprotodon snout region has been the subject of some debate. They had a vertical bone plate that has not been seen in any living mammal that has caused some problems with the reconstructions of the animal. One suggestion that has been made is that the plate was the support for muscles operating either lips that were very large and mobile, or possibly a trunk, that may have given the animal the appearance of a tapir. To date no evidence of the shape of the snout has been found, though impressions of the skin, hair and footpads have been found. The stripping of leaves from shrubs and trees would have been allowed by the combination of very flexible lips and a long, narrow incisors.

It has been suggested that the attainment of large size by such forms as Diprotodon was a response to poorer food sources that would have resulted from a drying climate. It is known that in the Late Cenozoic, 15-2 Ma, the climate of much of Australia became progressively more arid. According to the authors¹ such a drying would have favoured the spread of Diprotodon as the forests contracted, whatever the conclusion of the debate over the possibility of the increasing aridity over the last 2 My led to the extinction of Diprotodon along with the rest of the megafauna.

In Lake Callabonna, South Australia, many complete skeletons of Diprotodon have been found after their bones were found in 1892, with bones of a total of 360 animals being recovered from a limited area of a few hectares, animals that had been bogged in the sticky clay mud around the margins of the lake. The appearance of the skeleton of Diprotodon is not the only information gained by the large number of animals that died in the mud. It is indicated by the large number of animals in this deposit that Diprotodon was relatively common, and that it was probably a social animal that moved around in small herds. As juvenile Diprotodon have been found just in front of the pelvis in adult animals which indicates that females had a pouch for carrying their young. The vegetation found in the rib cage of some specimens, in the position the stomach would have been, so that the final meal of a number of individuals could be determined. These animals had fed on saltbush, as well as other shrubs, that were probably growing along the lake margins.

Zygomaturus

Sources & Further reading

1. McNamara, K & Murray, P., May 2013, Prehistoric Mammals of Western Australia, Western Australia Museum