Diprotodon

A rare find

In October 1991, a worker from the WA Agriculture Protection Board was surveying a noxious weed at the Du Boulay Creek in Western Australia. Unbeknownst to them, they were about to stumble upon a major paleontological discovery – the most complete Diprotodon optatum skeleton found in WA.  

Aided by an army of local volunteers from Karratha, WA Museum representatives Dr Alex Baynes, George Kendrick and visiting USA palaeontologist Dr Dale Guthrie exposed the skeleton and brought it to Perth.  

Prior to this discovery, Diprotodon optatum, a member of the Diprotodontidae family, was only known through a small number of bone fragments, teeth and lower jaws. These fragments were discovered as far apart as the Pilbara, Lake Darlot north of Leonora, and Balladonia at the western edge of the Nullarbor Plain. However, the discovery of a near-complete skeleton measuring three metres long and standing more than two metres high confirmed its suspected status as a member of the largest marsupial species that ever walked the Earth.  

Since then, this more than 80,000-year-old marsupial heavyweight has been on proud display in several of the WA Museum’s permanent exhibitions, where it has delighted young and old visitors alike, including in its most recent home at the WA Museum Boola Bardip, in the Wildlife gallery. 

Diprotodon optatum 

The name ‘Diprotodon’ means “two forward teeth”, which refers to the incisors of the lower jaw, which point straight forward. Coupled with its heavy build, stout limbs and moderately long neck, the Diprotodon may have resembled an oversized, long-legged wombat. Indeed, wombats are the nearest living relatives of the Diprotodon, sharing several skull characters which have been linked to a common ancestor that lived more than 30 million years ago.  

Diprotodon was one of the last surviving members of a family of large herbivorous marsupials, the Diprotodontidae, which first appeared in the fossil record more than 26 million years ago. Early Diprotodontidae were only about the size of modern sheep, however their later evolutionary relatives, the Late Pleistocene Diprotodon optatum, measured nearly two metres high at the shoulder and 2.5 metres in length, growing to roughly the size of a rhinoceros.  

Weighing approximately two tonnes, Diprotodon was likely a slow-moving animal. A peculiar feature of its skeleton was the structure of its feet; at the end of its stout limbs and massive wrists and ankles were absurdly small toes. The body’s enormous weight would likely have been borne mainly by the bones of the wrist and ankle.  

Approximately 15 million to two million years ago, in the Late Cenozoic era, aridity increased across the Australian continent which favoured the spread of Diprotodon as the forest areas contracted. However, this aridity may have also contributed to the eventual extinction of Diprotodon and other large mammals over the last two million years.  

Following the discovery of fossils belonging to 360 individual Diprotodon skeletons in Lake Callabonna in South Australia, within a limited area of only a few hectares, scientists have suggested that Diprotodon was a relatively common animal and was probably a social creature that moved around in small herds. Additionally, the discovery of remains of juveniles found just in front of the pelvis of adult specimens suggests that the female Diprotodon possessed a pouch, much like modern marsupials.  

From these remains, vegetation was also found in the rib cage of some specimens, likely in a position where the stomach may have been. Analysis of this material revealed that the last meal of some Diprotodons consisted of saltbush and other shrubs, which likely would have been growing around the edges of the lake. 

Despite its large head, Diprotodon’s brain was smaller relative to its whole-body size than its ancestors. Instead of a large brain case, the head was filled with an open network of thin bones and air sinuses that acted to lessen the weight of the large skull. This skull structure was necessary to accommodate the huge teeth and muscles needed to process large amounts of coarse vegetation and withstand stresses from excessive chewing.  

Alongside its two lower, elongate incisors, Diprotodon possessed a pair of chisel-like incisors in its upper jaw which opposed its lower pair, for shearing and cutting through coarse vegetation. Its molars reached up to five centimetres in length and were characterised by their high and narrow transverse double-ridged structure, meaning that opposing molars with a vertical slicing and grinding motion to shred tough vegetation including leaves and shoots of small plants and trees. 

There is some debate about the nature of the snout region of Diprotodon. An unusual vertical plate of bone, not found in any living mammal, has posed some problems when attempts have been made to reconstruct the animal. It has been suggested that this plate supported muscles which operated either very large, mobile lips, or even a trunk, perhaps giving the animal a tapir­-like appearance. While impressions of the skin, hair and footpads are known, no evidence of the nature of the nose or mouth has been found. However, the combination of very flexible lips and long, narrow lower incisors would have allowed leaves to be easily stripped from shrubs and trees.