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.

Later that year, he formally synonymised Diprotodon with Deinotherium as Dinotherium Australe,[9] which he recanted in 1844 after German naturalist Ludwig Leichhardt pointed out that the incisors clearly belong to a marsupial.

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,[c] so they assumed the same would have been true for extinct relatives, including Diprotodon.

Because of this, in 1967, American palaeontologist Ruben A. Stirton subdivided Diprotodontoidea into one family, Diprotodontidae, with four subfamilies; Diprotodontinae (containing Diprotodon among others), Nototheriinae, Zygomaturinae, and Palorchestinae.

[28] Below is the Diprotodontoidea family tree according to Australian palaeontologists Karen H. Black and Brian Mackness, 1999 (top),[29] and Vombatiformes family tree according to Beck et al. 2020 (bottom):[24] Palorchestidae Ngapakaldia Pitikantia Bematherium Pyramios Meniscolophus Euowenia Euryzygoma Diprotodon Silvabestius Alkwertatherium Nimbadon Plaisiodon Kolopsoides Neohelos Kolopsis Maokopia Hulitherium Zygomaturus Thylacoleonidae Phascolarctidae (koalas) Vombatidae (wombats) Ngapakaldia Nimbadon Neohelos Palorchestidae Kolopsis Diprotodon Euryzygoma Zygomaturus Diprotodontidae is the most diverse family in Vombatomorphia; it was better adapted to the spreading dry, open landscapes over the last tens of millions of years than other groups in the infraorder, living or extinct.

[e][33] The oldest vombatomorph (and vombatiform) is Mukupirna, which was identified in 2020 from Oligocene deposits of the South Australian Namba Formation dating to 26–25 million years ago.

[24] Both diprotodontines and zygomaturines were both apparently quite diverse over the Late Oligocene to Early Miocene, roughly 23 million years ago, though the familial and subfamilial classifications of diprotodontoids from this period is debated.

Diprotodontidae also began a gigantism trend, along with several other marsupials, probably in response to the lower-quality plant foods available in a drying climate, requiring them to consume much more.

[34] In 1977, Archer said Diprotodon directly evolved from the smaller Euryzygoma,[27] which has been discovered in Pliocene deposits of eastern Australia predating 2.5 million years ago.

The expanded sinuses increase the surface area available for the temporalis muscle to attach, which is important for biting and chewing, to compensate for a deflated braincase as a result of a proportionally smaller brain.

[40] As in kangaroos and wombats, there is a gap between the jointing of the palate (roof of the mouth) and the maxilla (upper jaw) behind the last molar, which is filled by the medial pterygoid plate.

[57] The general proportions of the scapula (shoulder blade) align more closely with more-basal vertebrates such as monotremes, birds, reptiles, and fish rather than marsupials and placental mammals.

He calculated the food requirement of Diprotodon was 50–60% smaller than expected for Australia's landmass, which he believed was a result of a generally lower metabolism in marsupials compared to placentals—up to 20% lower—and sparser nutritious vegetation than other continents.

They made a regression between the minimum circumference of the femora and humeri of 18 quadrupedal marsupials and 32 placentals against body mass, and then inputted 17 Diprotodon long bones into their predictive model.

[67] Like modern megaherbivores, most evidently the African elephant, Pleistocene Australian megafauna likely had a profound effect on the vegetation, limiting the spread of forest cover and woody plants.

[74] The fossilised, incompletely digested gut contents of one 53,000-year-old individual from Lake Callabonna show its last meal consisted of young leaves, stalks, and twigs.

Kangaroos that predominantly graze have specialised molars to resist the abrasiveness of grass but such adaptations are not exhibited in Diprotodon, which may have had a mixed diet similar to that of a browsing wallaby.

Similarly to many large ungulates (hoofed mammals), the jaws of Diprotodon were better suited for crushing rather than grinding, which would have permitted it to process vegetation in bulk.

[42] In 2016, Australian biologists Alana Sharpe and Thomas Rich estimated the maximum-possible bite force of Diprotodon using finite element analysis.

This individual appears to have been following the Condamine River and, while apparently keeping to the Darling Downs during the three years this tooth had been growing, it would have been annually making a 200 km (120 mi) northwest-southeast round trip.

Like koalas and wombats, the pouch may have faced backwards so the joey could crawl down across its mother's abdomen to enter and attach itself to a teat until it could see—perhaps 260 days—and thermoregulate.

[80][88] In 2005, American geologist Gifford Miller noticed fire abruptly becomes more common about 45,000 years ago; he ascribed this increase to aboriginal fire-stick farmers, who would have regularly started controlled burns to clear highly productive forests and grasslands.

[87][91][92] The frequency of fire could have also increased as a consequence of megafaunal extinction because total plant consumption rapidly fell, leading to faster fuel buildup.

They determined it was inconsistent with bite marks from scavenging Thylacoleo or mice, and concluded it was incised by humans with flint as a counting system or a random doodle.

[103] This specimen became one of the most-cited pieces of evidence humans and megafauna directly interacted until a 2020 re-analysis by Australian palaeoanthropologist Michelle Langley identified the engraver as most-likely a tiger quoll.

[102] In 2016, Australian archaeologist Giles Hamm and colleagues unearthed a partial right radius belonging to a young Diprotodon in the Warratyi rock shelter.

Aboriginal Australians also attempted to fit the finds into their own religious ideas, quickly associating Diprotodon with the bunyip, a large, carnivorous, lake monster.

[3] In 1892, Canadian geologist Henry Yorke Lyell Brown reported Aboriginal Australians identified Diprotodon fossils from Lake Eyre as those of the Rainbow Serpent, which he thought was a giant, bottom-dwelling fish.

[105] If Pleistocene megafauna are the basis of some aboriginal mythology, it is unclear if the stories were based on the creatures when they were alive or their fossils being discovered long after their extinction.

In 1907, Australian anthropologist Herbert Basedow found footprint petroglyphs in Yunta Springs and Wilkindinna, South Australia, which he believed were those of Diprotodon.