Diprotodon Optatum

''Diprotodon'' (Ancient Greek: "two protruding front teeth") is an extinct genus of marsupial from the Pleistocene of Australia containing one species, ''D. optatum''. The earliest finds date to 1.77 million to 780,000 years ago but most specimens are dated to after 110,000 years ago. Its remains were first unearthed in 1830 in Wellington Caves, New South Wales, and contemporaneous paleontologists guessed they belonged to rhinos, elephants, hippos or dugongs. ''Diprotodon'' was formally described by English naturalist Richard Owen in 1838, and was the first named Australian fossil mammal, and led Owen to become the foremost authority of his time on other marsupials and Australian megafauna, which were enigmatic to European science.

''Diprotodon'' is the largest-known marsupial to have ever lived; it greatly exceeds the size of its closest living relatives wombats and koalas. It is a member of the extinct family Diprotodontidae, which includes other large quadrupedal herbivores. It grew to at the shoulders, over from head to tail, and likely weighed several tonnes, possibly as much as . Females were much smaller than males. ''Diprotodon'' supported itself on elephant-like legs to travel long distances, and inhabited most of Australia. The digits were weak; most of the weight was probably borne on the wrists and ankles. The hindpaws angled inward at 130°. Its jaws may have produced a strong bite force of at the long and ever-growing incisor teeth, and over at the last molar. Such powerful jaws would have allowed it to eat vegetation in bulk, crunching and grinding plant materials such as twigs, buds and leaves of woody plants with its bilophodont teeth.

It is the only marsupial and metatherian that is known to have made seasonal migrations. Large herds, usually of females, seem to have marched through a wide range of habitats to find food and water, walking at around . ''Diprotodon'' may have formed polygynous societies, possibly using its powerful incisors to fight for mates or fend off predators, such as the largest-known marsupial carnivore ''Thylacoleo carnifex''. Being a marsupial, the mother may have raised her joey in a pouch on her belly, probably with one of these facing backwards, as in wombats.

''Diprotodon'' went extinct about 40,000 years ago as part of the Late Pleistocene megafauna extinctions, along with every other Australian mammal over ; the extinction was possibly caused by extreme drought conditions and predation pressure from the first Aboriginal Australians, who likely co-existed with ''Diprotodon'' and other megafauna in Australia for several thousand years prior to its extinction. There is little direct evidence of interactions between Aboriginal Australians and ''Diprotodon''—or most other Australian megafauna. ''Diprotodon'' has been conjectured by some authors to have been the origin of some aboriginal mythological figures—most notably the bunyip—and aboriginal rock artworks, but these ideas are unconfirmable.

Research history

In 1830, farmer George Ranken found a diverse fossil assemblage while exploring Wellington Caves, New South Wales, Australia. This was the first major site of extinct Australian megafauna. Remains of ''Diprotodon'' were excavated when Ranken later returned as part of a formal expedition that was headed by explorer Major Thomas Mitchell.

At the time these massive fossils were discovered, it was generally thought they were remains of rhinos, elephants, hippos, or dugongs. The fossils were not formally described until Mitchell took them in 1837 to his former colleague English naturalist Richard Owen while in England publishing his journal. In 1838, while studying a piece of a right mandible with an incisor, Owen compared the tooth to those of wombats and hippos; he wrote to Mitchell designating it as a new genus ''Diprotodon''. Mitchell published the correspondence in his journal. Owen formally described ''Diprotodon'' in Volume 2 without mentioning a species; in Volume 1, however, he listed the name ''Diprotodon optatum'', making that the type species. ''Diprotodon'' means "two protruding front teeth" in Ancient Greek and ''optatum'' is Latin for "desire" or "wish". It was the first-ever Australian fossil mammal to be described. In 1844, Owen replaced the name ''D. optatum'' with "''D. australis''". Owen only once used the name ''optatum'' and the acceptance of its apparent replacement "''australis''" has historically varied widely but ''optatum'' is now standard.

In 1843, Mitchell was sent more ''Diprotodon'' fossils from the recently settled Darling Downs and relayed them to Owen. With these, Owen surmised that ''Diprotodon'' was an elephant related to or synonymous with ''Mastodon'' or ''Deinotherium'', pointing to the incisors which he interpreted as tusks, the flattening (anteroposterior compression) of the femur similar to the condition in elephants and rhinos, and the raised ridges of the molar characteristic of elephant teeth. Later that year, he formally synonymised ''Diprotodon'' with ''Deinotherium'' as ''Dinotherium Australe'', which he recanted in 1844 after German naturalist Ludwig Leichhardt pointed out that the incisors clearly belong to a marsupial. Owen still classified the molars from Wellington as ''Mastodon australis'' and continued to describe ''Diprotodon'' as likely elephantine. In 1847, a nearly complete skull and skeleton was recovered from the Darling Downs, the latter confirming this elephantine characterisation. The massive skeleton attracted a large audience while on public display in Sydney. Leichhardt believed the animal was aquatic, and in 1844 he said it might still be alive in an undiscovered tropical area nearer the interior. But, as the European land exploration of Australia progressed, he became certain it was extinct. Owen later become the foremost authority of Australian palaeontology of his time, mostly working with marsupials.

Huge assemblages of mostly-complete ''Diprotodon'' fossils have been unearthed in dry lakes and riverbeds; the largest assemblage came from Lake Callabonna, South Australia. Fossils were first noticed here by an aboriginal stockman working on a sheep property to the east. The owners, the Ragless brothers, notified the South Australian Museum, which hired Australian geologist Henry Hurst, who reported an enormous wealth of fossil material and was paid £250 in 1893 to excavate the site. Hurst found up to 360 ''Diprotodon'' individuals over a few acres; excavation was restarted in the 1970s and more were uncovered. American palaeontologist Richard H. Tedford said multiple herds of these animals had at different times become stuck in mud while crossing bodies of water while water levels were low during dry seasons.

In addition to ''D. optatum'', several other species were erected in the 19th century, often from single specimens, on the basis of subtle anatomical variations. Among the variations was size difference: adult ''Diprotodon'' specimens have two distinct size ranges. In their 1975 review of Australian fossil mammals, Australian palaeontologists J. A. Mahoney and William David Lindsay Ride did not ascribe this to sexual dimorphism because males and females of modern wombat and koala species—its closest living relatives—are skeletally indistinguishable, so they assumed the same would have been true for extinct relatives, including ''Diprotodon''.
These other species are:
*''D. annextans'' was erected in 1861 by Irish palaeontologist Frederick McCoy based on some teeth and a partial mandible found near Colac, Victoria; the name may be a typo of ''annectens'', which means linking or joining, because he characterised the species as combining traits from ''Diprotodon'' and ''Nototherium'';
*''D. minor'' was erected in 1862 by Thomas Huxley based on a partial palate; in 1991, Australian palaeontologist Peter Murray suggested classifying large specimens as ''D. optatum'' and smaller ones as "''D. minor''";
*''D. longiceps'' was erected in 1865 by McCoy as a replacement for "''D. annextans''";
*''D. bennettii'' was erected in 1873 by German naturalist Gerard Krefft based on a nearly complete mandible collected by naturalists George Bennet and Georgina King near Gowrie, New South Wales; and
*''D. loderi'' was erected in 1873 by Krefft based on a partial palate collected by Andrew Loder near Murrurundi, New South Wales.
In 2008, Australian palaeontologist Gilbert Price opted to recognise only one species ''D. optatum'' based most-notably on a lack of dental differences among these supposed species, and said it was likely ''Diprotodon'' was indeed sexually dimorphic, with the male probably being the larger form.

Classification

Phylogeny

''Diprotodon'' is a marsupial in the order Diprotodontia, suborder Vombatiformes (wombats and koalas), and infraorder Vombatomorphia (wombats and allies). It is unclear how different groups of vombatiformes are related to each other because the most-completely known members—living or extinct—are exceptionally derived (highly specialised forms that are quite different from their last common ancestor).

In 1872, American mammalogist Theodore Gill erected the superfamily Diprotodontoidea and family Diprotodontidae to house ''Diprotodon''. New species were later added to both groups; by the 1960s, the first diprotodontoids dating to before the Pliocene

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