Diprotodon

Diprotodon Temporal range: Pleistocene,
Diprotodon skeleton cast, MNHN, Paris
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Infraclass:	Marsupialia
Order:	Diprotodontia
Family:	†Diprotodontidae
Genus:	†Diprotodon
Type species
Diprotodon optatum Owen, 1838
Synonyms
D. australis Owen, 1844 D. annextans McCoy, 1861 D. bennettii Krefft, 1873 D. loderi Krefft, 1873 D. longiceps McCoy, 1865 D. minor Huxley, 1862

Diprotodon is an extinct genus of large marsupials native to Australia from the Pleistocene epoch. It is considered one of Australia's core species of "megafauna", which ranged throughout the continent during the Pleistocene. The genus is currently considered monotypic, containing only Diprotodon optatum, the largest known marsupial to have ever existed. The word diprotodon is constructed from the Ancient Greek words for 'two forward teeth'. Diprotodon existed from about 1.6 million years ago until extinction some 44,000 years ago.

Diprotodon species fossils have been found in sites across mainland Australia, including complete skulls, skeletons, and foot impressions. The largest specimens were hippopotamus-sized: about 3 m (9.8 ft) from nose to tail, standing 2 m (6.6 ft) tall at the shoulder and weighing about 2,790 kg (6,150 lb). Diprotodons may have been depicted on Aboriginal rock art images in Quinkan traditional country (Queensland, Australia). Diprotodon became extinct sometime after 44,000 years ago, after the initial settlement of the continent; the role of human and climatic factors in its extinction are uncertain and contested.

Diprotodon is a member of the extinct family Diprotodontidae. The closest surviving relatives of Diprotodon are the wombats and the koala, and so Diprotodon have sometimes been referred to as "giant wombats" in the popular press.

Research history

Early reconstruction of Diprotodon by Alice B. Woodward, 1912

In 1830, bushman George Ranken found a diverse fossil assemblage while exploring Wellington Caves, New South Wales. This was the first major site of extinct Australian megafauna. They were excavated when Ranken returned later in a formal expedition headed by Major Thomas Mitchell.

At the time these massive fossils were discovered, there were no serious scientists in Australia. It was generally guessed that the fossil assemblage represents rhinos, elephants, hippos, or dugongs, but they were not formally described until Mitchell, while in England publishing his journal, brought them to his former colleague Sir Richard Owen in 1837. In 1838, while studying a piece of a right mandible with an incisor, Owen compared the tooth to that of a wombat or a hippo, and designated it as a new genus in a letter to Mitchell, as Diprotodon. Mitchell published the correspondence in his journal. Owen formally described Diprotodon in volume 2 without mentioning a species, but in volume 1, he listed the name Diprotodon optatum, making that the type species. Diprotodon means "two protruding front teeth". It was the first ever Australian fossil mammal described. In 1844, Owen replaced the name D. optatum with "D. australis". Owen only used optatum the one time, and the acceptance of its apparent replacement "australis" has historically varied widely.

In 1843, Mitchell was sent more Diprotodon fossils from the recently settled Darling Downs, and relayed them to Owen. Owen believed Diprotodon was an elephant related to or synonymous with Mastodon or Deinotherium, having interpreted the incisors as tusks, as well as comparing the flattening (anteroposterior compression) of the femur to the condition in elephants and rhinos, and the raised ridges of the molar to the grinding surfaces 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, nonetheless, still classified the molars from Wellington as "M. australis". He also continued to describe Diprotodon as likely elephantine. In 1847, a nearly complete skull and skeleton was recovered from the Darling Downs, which confirmed this characterisation. The massive skeleton, while on public display in Sydney, attracted quite an audience. Leichhardt believed the animal was aquatic and suggested in 1844 that it might still be alive in an undiscovered tropical area towards the interior, but as the European land exploration of Australia progressed, he became certain it was extinct. Owen would go on to be the foremost authority of Australian palaeontology of his time, mostly working with marsupials.

Several other species were erected through the 19th century because adult Diprotodon specimens come in two distinct size ranges. In their 1975 review of Australian fossil mammals, Australian palaeontologists J. A. Mahoney and William David Lindsay Ride did not ascribed this to sexual dimorphism because males and females among 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", erected in 1861 by Irish palaeontologist Frederick McCoy based on some teeth and a partial mandible near Colac, Victoria; the name may be a typo of annectens which means linking or joining, because the species combines traits from Diprotodon and Nototherium;
• "D. minor", 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", erected in 1865 by McCoy as a replacement for "D. annextans";
• "D. bennettii", 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", 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 suggested that it was likely that Diprotodon was indeed sexually dimorphic, with the male probably being the larger form.

Classification

Phylogeny

Diprotodon's closest living relatives are wombats (left) and koalas (right)

Diprotodon is a marsupial in the order Diprotodontia, suborder Vombatiformes (wombats and koalas), infraorder Vombatomorphia (wombats and allies). It is unclear how different groups of vombatiformes are related to each other as the most completely known members, living or extinct, are exceptionally derived (highly specialised forms which 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 added to both groups in subsequent years. By the 1960s, the first diprotodontoids dating to before the Pliocene were being discovered, better elucidating how they are related to each other. 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. In 1977, Australian palaeontologist Michael Archer synonymised Nototheriinae with Diprotodontinae, and in 1978, Archer and Australian palaeontologist Alan Bartholomai elevated Palorchestinae to family level as Palorchestidae, leaving Diprotodontoidea with two families (Diprotodontidae and Palorchestidae), and Diprotodontidae with two subfamilies (Diprotodontinae and Zygomaturinae).

Diprotodontoidea family tree according to Australian palaeontologists Karen H. Black and Brian Mackness, 1999 (left), and Vombatiformes family tree according to Beck et al. 2020 (right):

Diprotodontoidea	Palorchestidae Diprotodontidae Diprotodontinae Ngapakaldia Pitikantia Bematherium Pyramios Meniscolophus Euowenia Euryzygoma Diprotodon Zygomaturinae Silvabestius Alkwertatherium Nimbadon Plaisiodon Kolopsoides Neohelos Kolopsis Maokopia Hulitherium Zygomaturus

Vombatiformes	Thylacoleonidae Phascolarctidae (koalas) Vombatomorphia Vombatidae (wombats) Diprotodontoidea Ngapakaldia Nimbadon Neohelos Palorchestidae Diprotodontidae Kolopsis Diprotodon Euryzygoma Zygomaturus

Evolution

Diprotodon may have evolved from Euryzygoma (skull above).

Diprotodontidae is the most diverse family in Vombatomorphia, better adapted to the spreading dry and open landscapes over the last tens of millions of years than other groups in the infraorder (living or extinct). Similarly, Diprotodon has been found in every Australian state, making it the most widespread Australian megafauna in the fossil record. The oldest vombatomorph (and vombatiform) is Mukupirna, identified in 2020 from Oligocene deposits of the South Australian Namba Formation dating to 26–25 million years ago. The group probably evolved much earlier, since Mukupirna was already differentiated as a closer relative to wombats than other vombatiformes; and attained a massive size of roughly 150 kg (330 lb) whereas the last common ancestor of vombatiformes was probably a small 1–5.5 kg (2.2–12.1 lb) creature.

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 time period is debated). Compared to zygomaturines, diprotodontines were rare during the Miocene, the only identified genus being Pyramios. By the Late Miocene, diprotodontians became the most common marsupial order in fossil sites, a dominance which indures to present day; at this point, the most prolific diprotodontians were diprotodontids and kangaroos. Diprotodontidae also began a gigantism trend, along with several other marsupials, probably a response to the lower quality plant foods available in a drying climate, requiring them to consume much more. Gigantism appears to evolved six time independently among the vombatiform lineages. Diprotodontine diversity returned in the Pliocene, and Diprotodontidae reached peak diversity with seven genera, coinciding with the spread of open forests. In 1977, Archer suggested that Diprotodon directly evolved from the smaller Euryzygoma, which has been discovered in Pliocene deposits of eastern Australia predating 2.5 million years ago.

In general, there is poor resolution on the ages of Australian fossil sites, and while the geochronology of Diprotodon is one of best for Australian megafauna, it is still quite incomplete and the majority of remains sit undated. The earliest (indirectly dated) Diprotodon fossils were reported by Price and Australian palaeontologist Katarzyna Piper from the Nelson Bay Formation at Nelson Bay, New South Wales, which dates to 1.77 million to 780,000 years ago during the Early Pleistocene. The remains are notably smaller than Late Pleistocene Diprotodon by 8–17%, but they are otherwise indistinguishable. The oldest directly dated fossils come from the Boney Bite site at Floraville, New South Wales. They were deposited approximately 340,000 years old during the Middle Pleistocene based on U-series dating and luminescence dating of quartz and orthoclase. Floraville is the only identified Middle Pleistocene site in tropical northern Australia. Beyond these, almost all dated material comes from Marine Isotope Stage 5 (MIS5) or younger (after 110,000 years ago) during the Late Pleistocene.

Description

Skull

Diprotodon skull reconstructions showing the cranial bones (left) and the frontal sinuses (right)

Diprotodon had a long and narrow skull. Like other marsupials, the top of the skull of Diprotodon was flat or depressed over the small braincase, as well as the sinuses of the frontal bone. Like many other giant vombatiformes, the frontal sinuses are extensive; in a specimen from Bacchus Marsh, they take up 2,675 cc (163.2 cu in)—roughly 25% of skull volume—whereas the brain only occupies 477 cc (29.1 cu in)—only 4% skull volume. Marsupials tend to have smaller brain to body mass ratios than placental mammals, becoming more disparate the bigger the animal, which could be a response to conserve energy, as the brain is a calorically expensive organ, or is proportional to the maternal metabolic rate, which is much less in marsupials since they do not have to gestate for as long. The expanded sinuses increase the surface area available for the temporalis muscle to attach (important for biting and chewing) to compensate for a deflated braincase as a result of a proportionally smaller brain. They may have also helped dissipate stresses produced by biting more efficiently across the skull.

The occipital bone, the back of the skull, slopes forward at a 45° angle, as opposed to most modern marsupials where it is vertical. The base of the occipital is heavily thickened. The occipital condyles (a pair of bones which connect the skull with the vertebral column) are semi-circle shaped, with the bottom half being narrower than the top. The inner border (which forms the foramen magnum, where the spinal cord feeds through) is thin and well-defined. The top margin of the foramen magnum is somewhat flattened rather than arched. The foramen expands backwards towards the inlet, especially vertically, and is more reminiscent of a short neural canal than a foramen magnum.

There is a sagittal crest extending across the midline of the skull from the supraoccipital (the top of the occipital bone) to the interorbital region (between the eyes on the top of the head). The orbit (eye socket) is small and oval-shaped vertically. The nasal bones slightly curve upwards until near their endpoint, where they begin to curve down, giving the bones somewhat of an S-shaped profile. Like many marsupials, most of the nasal septum is made of bone rather than cartilage. The nose would have been quite mobile. The height of the skull from the peak of the occipital bone to the end of the nasals is strikingly almost uniform, but the end of the nasals is the tallest point. The zygomatic arch (cheek bone) is strong and deep like in kangaroos, but unlike koalas or wombats, and also extends all the way from the supraoccipital.

Jaws

Diprotodon skull at the Natural History Museum, London

Like kangaroos and wombats, there is a gap in 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. This would have been the insertion for the medial pterygoid muscle relevant to closing the jaw. Like other grazers, the masseter muscle (also responsible for closing the jaw) seems to have been the dominant jaw muscle, and a probable large temporal muscle compared to the lateral pterygoid muscle may indicate, unlike wombats, a limited range of jaw motion side-to-side (i.e., it would have been better at crushing rather than grinding food). The insertion of the masseter is placed forwards, in front of the orbits, which could have allowed better control over the incisors. Overall, Diprotodon chewing strategy appears to align more with kangaroos than wombats, with a powerful vertical crunch followed by a transverse grinding motion.

Like other marsupials, the ramus of the mandible (the portion that goes up to connect with the skull) angles inward. The condyloid process (which connects the jaw to the skull) is similar to that of a koala. The ramus is straight and extends almost vertically, thickening as it approaches the body of the mandible (where the teeth are). The depth of the body of the mandible increases somewhat from the last to the first molar. The mandibular symphysis (which fuses the two halves of the mandible) is strong, beginning at the frontmost end of the third molar; this would prevent either half of the mandible from moving independently of the other, which kangaroos can do to better control their incisors.

Teeth

Diprotodon molars

The dental formula of Diprotodon is 3.0.1.41.0.1.4, with, in each half of either jaw, three incisors in the upper and one in the lower jaw; no canines; and one premolar and four molars in both jaws. There is a long diastema (gap) separating the incisors from the molars.

The incisors are chisel-like (scalpriform). The first incisors in both jaws grew continuously throughout the animal's life, like wombats and rodents, but the other two upper incisors did not. This condition is not seen in any living marsupial. The cross-section of the upper incisors is circular. In one old male specimen, the first upper incisor measures 28 cm (11 in) of which 220 mm (8.5 in) is within the tooth socket; the second is 100 mm (4 in) where 25 mm (1 in) is in the socket; and the exposed part of the third is 66 mm (2.6 in). The first incisor is convex and curves outwards, but the other two are concave. The lower incisor has a faint upward curve but is otherwise straight, and it has an oval cross-section. In the same old male specimen, it measures 250 mm (10 in) of which 2⁄3 is inside the socket.

The premolars and molars are bilophodont, and each feature two distinct lophs (ridges). The premolar is triangular and about half the size of the molars. The necks of the lophs are coated in cementum like kangaroos. Unlike kangaroos, there is no connecting ridge between the lophs. The peaks of these lophs have a thick enamel coating, which thins towards the base. This could wear away with use and expose the dentine layer, and beneath that osteodentine. Like the first premolar of marsupials, the first molar of Diprotodon and wombats is the only tooth that is replaced.

Vertebrae

Diprotodon had five cervical (neck) vertebrae. The atlas, the first cervical (C1), has a pair of deep cavities for insertion of the occipital condyles. The diaphophyses (an upward-angled projection on either the side of the vertebra) of the atlas are relatively short and thick, and overall resemble those of wombats and koalas. The articular surface (the part that joints to another vertebra) of the axis (C2) is slightly concave on the front side, and flat on the back side. Like kangaroos, the axis has a low subtriangular hypophysis (projecting vertically from the underside of the vertebra) and a proportionally long odontoid (a projection from the axis which fits into the atlas), but the neural spine (which project vertically the topside of the vertebra) is more forwards. The remaining cervicals lack a hypophysis. Like kangaroos, C3 and C4 have a shorter and more compressed neural spine, which is supported by a low ridge along its midline in the front and the back. The neural spine of C5 is narrower but thicker, and is supported by stronger though shorter ridges.

Diprotodon skeletal reconstruction

Diprotodon probably had 13 dorsal vertebrae and 14 pairs of closely-spaced ribs. Like many other mammals, the dorsals initially decrease in breadth and then expand before connecting to the lumbar vertebrae. The front dorsals unusually match the short proportions of the cervicals, and the articular surface is flat. At the beginning of the series, the neural spine is broad and angled forward, and is also supported by a low ridge along its midline in the front and the back. Later on, the neural spine is angled backwards, and bifurcates (splits into two). Among mammals, bifurcation of the neural spine is only seen in elephants and humans, and even then, only in a few of the cervicals, not in the dorsals. Compared to wombats and kangaroos, the neural arch is proportionally taller. Like in elephants, the epiphysial plates (growth plates) and the neural arch (where the neural spine is attached to) are anchylosed (very rigid in regard to the vertebral centrum), which serve to support the animal's immense weight.

Like most marsupials, Diprotodon likely had six lumbar vertebrae. They retain a proportionally tall neural arch, but not the diapophyses, though L1 can retain a small protuberance on one side where a diapophysis would be in a dorsal vertebra; this has been documented in kangaroos among other mammals. The length of each vertebra increases along the series, so the lumbar series may have bent downward.

Like other marsupials, Diprotodon had two sacral vertebrae. The base of the neural spines of these two were ossified (fused) together.

Limbs

The general proportions of the scapula (shoulder blade) align more closely with more basal vertebrates such as monotremes, birds, reptiles, or fish, rather than marsupials or placentals. It is triangular and proportionally narrow, but unlike most mammals with a triangular scapula, the arm attaches to top of the scapula, and the subspinous fossa (the fossa below the spine of the scapula) increases towards the arm joint instead of decreasing. The glenoid cavity (where the arm connects) is oval shaped like in most mammals.

Unlike most marsupials, the humerus is rather straight (as opposed to S-shaped), and the trochlea of the humerus (at the elbow joint) is not perforated. The ridges for muscle attachments are poorly developed, which seems to have been compensated by the powerful forearms. Similarly, the condyles where the radius and ulna (the forearm bones) connect maintain their rounded shape and are quite similarly sized, unusually reminiscent of the condyles between the femur and the tibia and fibula in the leg of a kangaroo.

Unlike other marsupials, the ilia (the large wings of the pelvis) are lamelliform in being short and broad, and sporting a flat surface instead of a fossa (a depression). Lamelliform ilia have only been recorded in elephants, sloths, and apes, though these groups all have a much longer sacral vertebra series (marsupials are restricted to two sacral vertebrae). Nonetheless, the ilia provided strong muscle attachments which were probably oriented and used much the same as in an elephant. The sacroiliac joint (where the pelvis connects to the spine) is at a 35° angle in reference to the long axis of the ilium. The ischia (which form part of the hip socket) are thick and rounded tailwards, but tapers off and diverge towards the socket, unlike in kangaroos where the ischia proceed almost parallel to each other. They were not connected to the vertebra. The hip socket itself is well rounded and almost hemispherical.

Different views of a Diprotodon femur

Like elephants, the femur of Diprotodon is straight, compressed anteroposteriorly (from headside to tailside), and the walls of the femur are prodigiously thickened, strongly constricting the medullary cavity (where the bone marrow is). The proximal end (part closest to the hip joint) is notably long, broad, and deep. The femoral head projects up far from the greater trochanter. Like kangaroos, the greater trochanter is split into two lobes. The femoral neck is roughly the same diameter has the femoral head. Like kangaroos, the condyle for the fibula is excavated out, but the condyle for the tibia is well rounded and hemispheric. Like many other marsupials, the tibia is twisted and the tibial malleolus (on the ankle) is reduced. Like wombats and koalas, the jointing surface of the tibia is continuous with that of the fibula, which would permit the same flexibility in the hindpaws as in the forepaws. Wombats use their paw flexibility for digging, and koalas for climbing, but owing to the animal's shear bulk, Diprotodon was likely incapable of either.

Diprotodon has five digits on either paw. The wrist and ankle would have been largely rigid and inflexible. The forepaw was strong, and the shape of the wrist bones are quite similar to those kangaroos. The digits of the hindpaws turn inwards from the ankle at a 130° angle. The second and third metatarsals (the metatarsals connect the toes to the bones which connect to the ankle) are significantly reduced, which may mean these digits were syndactylous (fused) like in all modern diprotodontians. The first, fourth, and fifth digits are enlarged. The toes are each about the same length, except the fifth one which is much shorter. The digits overall are proportionally weak, so the paws probably had a lot of padding in life. Similarly, the digits do not seem to have been engaged much in weight bearing.

Size

1896 illustration of a Diprotodon and human skull

Diprotodon is the largest known marsupial to have ever lived. The bigger Diprotodon skeletons measure 1.6–1.8 m (5 ft 3 in – 5 ft 11 in) at the shoulders, and 2.75–3.4 m (9–11 ft) from head to tail. Accounting for cartilagenous intervertebral discs, Diprotodon may have been 20% longer than the reconstructed skeletons, exceeding 4 m (13 ft).

As researchers were formulating predictive body mass equations for fossil species, efforts were largely constrained to eutherian mammals as opposed to marsupials. The first attempt at estimating Diprotodon was Peter Murray in his 1991 review of the megafauna of Pleistocene Australia. He made an estimate of 1,150 kg (2,540 lb), though conceded it was probably not a very precise one. This made Diprotodon the largest herbivore in Australia. In 2001, Canadian biologist Gary Burness and colleagues did a linear regression between the largest herbivores and carnivores (living or extinct) from every continent (for Australia: Diprotodon, Varanus priscus, and Thylacoleo carnifex) vs. the landmass area of their continent, and another regression between the daily food intake of living creatures vs. the landmass of their continents. He calculated the food requirements of Diprotodon was 50–60% smaller than expected for what 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 compared to other continents. That is, the maximum attainable body size is capped much lower than on other continents.

In 2003, Australian palaeontologist Stephen Wroe and colleagues took a more analytical approach to body mass than Murray's guesstimate. They made a regression between the minimum circumference of the femora and humeri of 18 quadrupedal marsupials and 32 placentals vs. body mass, and then input 17 Diprotodon long bones into their predictive model. The outputs ranged from 2,272–3,417 kg (5,009–7,533 lb), for a mean of 2,786 kg (6,142 lb), though Wroe conceded that reconstructing the weight of extinct creatures which far outweigh living counterparts is problematic. For comparison, an American bison that they used in their study weighed 1,179 kg (2,599 lb), and a hippo 1,950 kg (4,300 lb). Using Burness's methods, Diprotodon is 25% larger than expected.

Paleobiology

Diet

Diprotodon sculpture at the Australian Museum

Like modern megaherbivores, most evidently the African elephant, the Pleistocene Australian megafauna likely had a profound effect on the vegetation, limiting the spread of forest cover and woody plants. Carbon isotope analysis suggests Diprotodon fed on a broad range of foods, and, like kangaroos, was consuming both C₃ (well-watered trees, shrubs, and grasses) and C₄ (arid grasses) plants. Still, the low seasonal δ¹³C values (its carbon isotope ratio remained about the same in both winter and summer, so it ate the same proportion of C₃ and C₄ plants regardless of their seasonal abundance) indicate that Diprotodon was a selective eater, at least more so than the modern wombat. The fossilised gut contents (incompletely digested) of one 53,000 year old individual from Lake Callabonna shows its last meal consisted of young leaves, stalks, and twigs.

The molars of Diprotodon are a simple bilophodont shape. Kangaroos use their bilophodont teeth to grind low-fibre (not too tough) plants as a browser, as well as grass as a grazer. Kangaroos which predominantly graze have specialised molars to resist the abrasiveness of grass, but such adaptations are not exhibited in Diprotodon, so Diprotodon may have had a mixed diet similar to a browsing wallaby. It may have chewed like them too, beginning with a vertical crunch before grinding transversely, as opposed to wombats which only grind transversely. Similar 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.

In 2016, Australian biologists Alana Sharpe and Thomas Rich estimated the maximum possible bite force of Diprotodon using finite element analysis. They calculated: 2,374 N (534 lb_f) at the incisors, and 4,118 to 11,134 N (926 to 2,503 lb_f) across the molar series. For reference, the American alligator can produce forces upwards of 9,500 N (2,100 lb_f). Though these are likely overestimates, the jaws of Diprotodon were still exceptionally strong, which would have allowed it to consume a broad range of plants, including tough, fibrous grasses.

Migration and sociality

One Diprotodon herd was making seasonal migrations along the Condamine River (above).

In 2017, Price and colleagues determined that Diprotodon made seasonal migrations, probably in search of food or watering holes. Price and colleagues did this by measuring the strontium isotope ratio (⁸⁷Sr/⁸⁶Sr) at various points along the Diprotodon incisor QMF3452 from the Darling Downs, and matching those ratios to the ratios of sites across that region. 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 making a 200 km (120 mi) northwest-southeast round-trip annually. This trek parallels the mammalian mass migrations of modern day East Africa.

Diprotodon is the only identified metatherian that seasonally migrated between two places. A few modern marsupials, such as the red kangaroo, have been documented making migrations should the need arise, but it is not a seasonal occurrence. Since Diprotodon could do it, it is not unlikely that other Pleistocene Australian megafauna did it as well.

Diprotodon apparently moved in large herds. These fossilised herds, most commonly unearthed in southeastern Australia, seem to be either predominantly or entirely female (sometimes in tow with juveniles). Such sexual segregation is normally seen in polygynous species, a common social organisation among modern megaherbivores which features an entirely female herd save for their young and the dominant male; the herd breeds exclusively with this male. Similarly, the skull is adapted to handling much higher stresses than what resulted from bite alone, so Diprotodon therefore may have subjected its teeth or jaws to more strenuous activities than just chewing, such as battling other Diprotodon for mates, or fending off predators, using the incisors. Like modern red and grey kangaroos (which also sexually segregate), bachelor herds of Diprotodon seem to have been less tolerant to drought conditions than female herds on account of their larger size and nutritional requirements.

Gait

Fossil tracks from the Victorian Volcanic Plain site: a) Protemnodon, b) Diprotodon pes, c) Diprotodon overlain by a vombatid, d) Thylacoleo

The locomotion of an extinct animal can be inferred using fossil trackways, which seldom ever preserve in Australia over the Cenozoic. Only the trackways of humans, kangaroos, vombatids, Diprotodon, and the diprotodontid Euowenia have been identified. Diprotodon trackways have been found at Lake Callabonna and the Victorian Volcanic Plain grasslands. The diprotodontid manus (forepaw) print is semicircular, and the pes (hindpaw) is reniform (kidney-shaped). Owing to proportionally small digits, most of the weight was borne on the carpus and tarsus (the bones connecting to respectively the wrist and the ankle). Diprotodontids seem to have had a much more erect gait, an adaptation to long-distance travel similar to elephants, rather than the more sprawling posture of wombats and zygomaturines, though there are no fossil trackways of the latter to verify its reconstructed standing posture.

At Lake Callabonna, the single Diprotodon responsible for the impressions had an average stride length of 1,500 mm (59 in), trackway width of 430 mm (17 in), and paw dimensions 295 mm × 202 mm (11.6 in × 8.0 in) in length x width. The gleno-acetabular length (the distance between the shoulders and pelvis) could have been about 1,125 mm (44.3 in), and assuming a hip height of 900 mm (35 in), the maker of tracks was probably moving at around 6.3 km/h (3.9 mph).

At the volcanic plains, the single Diprotodon responsible for the impressions had an average stride length of 1,310 mm (52 in), trackway width of 660 mm (26 in), and pes length of 450 mm (18 in). The gleno-acetabular length may have been about 1,080 mm (43 in), and assuming a hip height of 830 mm (33 in), the maker of tracks was probably moving at around 5.5 km/h (3.4 mph). Its posture was much more sprawled than that from Callabonna, aligning more with what might be expected of Zygomaturus, so the animal may have been a female carrying a large joey in her pouch, the added weight on the stomach altering the gait. The first trackway went on for 62.8 m (206 ft) in a southeasterly direction towards a palaeo-lake. The animal seems to have hesitated stepping down from the first sand bar on its path, with the right pes making three overlapping prints here while shuffling around. The trackway vanishes for a 20 m (66 ft) stretch, an reappears while the animal seemingly is stepping on wet sediment. Another diprotodontid trackway appears 50 m (160 ft) away, which may have been left by the same individual.

Life history

The marsupial metabolic rate is about 30% slower than that of placentals, due to a lower body temperature of 34 to 36 °C (93 to 97 °F). Marsupials also give birth at an earlier point in foetal development, relying on lactation to facilitate most of the joey's development; investing in lactation over pregnancy duration can be advantageous in a highly seasonal and unpredictable climate to minimise energy costs to the mother (as pregnancy is much more energetically expensive). Consequently, marsupials cannot support as large a litter size or as fast a generation time.

Using the relation between female body size and life history in kangaroos, a 1,000 kg (2,200 lb) Diprotodon female would have gestated for six to eight weeks, and given birth to a single 5 g (0.18 oz) joey. Given its massive size, Diprotodon may not have sat down to give birth as smaller marsupials do, possibly standing upright instead. Like koalas and wombats, the pouch may have faced backwards, so the joey could crawl down across its mother's abdomen to enter, and then attach itself to a teat until it could see (perhaps 260 days) and thermoregulate. It would have permanently left the pouch after 860 days, and suckled until reaching 270 kg (600 lb) after four or five years.

In large kangaroos, female usually reach sexual maturity and enter oestrus soon after weaning, and males need double the time. A similar pattern could have been exhibited in Diprotodon. Assuming a lifespan of up to 50 years, a female Diprotodon could have given birth eight times.

Palaeoecology

Diprotodon existed during the Pleistocene, and spread out across the entire Australian continent by the Late Pleistocene, most especially following MIS5 approximately 110,000 years ago. The onset of the Quaternary glaciation and the continuous advance and retreat of glaciers at the poles created fluctuating and extreme climatic swings elsewhere. In Australia, the warmer, wetter interglacials were received by forests and woodlands, and the colder, dryer glacial periods were more conducive to grasslands and deserts. Nonetheless, the continent progressively became dryer and dryer over time as the Asian monsoons became less influential over Australia, with the vast interior turning arid and sandy by 500,000 years ago; the mega-lakes (once prominent especially during interglacials over northwestern Australia) drying up; and the rainforests of eastern Australia gradually retreating. Aridity has hastened over the last 100,000 years, especially after 60,000 years ago with surging El Niño–Southern Oscillation.

The continent-wide distribution of Diprotodon indicates that herds trekked across most any habitat Australia had to offer, much like modern African elephants south of the Sahara. Diprotodon was a member of a diverse assemblage of megafauna endemic to Pleistocene Australia, such as the thylacine, modern kangaroos, sthenurines (giant short-faced kangaroos), a diversity of modern and giant koala and wombat species, the tapir-like Palorchestes, the giant turtle Meiolania, and the giant bird Genyornis. Diprotodon coexisted with the diprotodontid Zygomaturus trilobus, which appears to have clung to the forests, whereas Diprotodon foraged the expanding grasslands and woodlands. Other contemporary dipotodontids (Hulitherium, Z. nimborensia, and Maokopia) were insular forms restricted to the forests of New Guinea.

Reconstruction of a Thylacoleo attacking a Diprotodon

Given its massive size, Diprotodon would have been a tough adversary for native carnivores. It contended with the largest known marsupial predator, Thylacoleo carnifex, and while Diprotodon fossils gnawed or bitten by T. carnifex have been identified, it is unclear if the 100–130 kg (220–290 lb) marsupial predator was powerful enough to take down an animal surpassing 2,000 kg (4,400 lb). Given that the modern jaguar, at half the size of T. carnifex, can bring down a 500 kg (1,100 lb) bull, it is not impossible that T. carnifex could have killed more average or smaller sized Diprotodon. Similar to recent kangaroos with thylacines or quolls, juvenile Diprotodon may have been at high risk to T. carnifex predation, as T. carnifex and juvenile Diprotodon fossils have been recovered from the same caves. The largest predators of the continent were now-extinct reptiles, most notably the crocodiles Paludirex and Quinkana, and the giant lizard megalania (Varanus priscus). At 7 m (23 ft) in length, megalania was the largest carnivore of Pleistocene Australia.

Extinction

As part of the Quaternary extinction event, Diprotodon and every other land animal heavier than 100 kg (220 lb) went extinct from Australia. The timing, and thus the exact cause, are unclear because in general there is poor resolution on the ages of Australian fossil sites. Since their discovery, the Australian megafauna have been thought to have died out as the climate worsened, or because of overhunting by the first Aboriginal Australians. In 2001, Australian palaeontologist Richard Roberts and colleagues dated 28 major fossil sites across the continent, and were able to provide a precise date for megafaunal extinction. They found that most disappear from the fossil record by 80,000 years ago, but Diprotodon (in addition to the giant wombat Phascolonus, Thylacoleo, and the short-faced kangaroos Procoptodon, Protemnodon, and Simosthenurus) was identified at Ned's Gully, Queensland, and Kudjal Yolgah Cave, Western Australia, which they dated to respectively 47 and 46 thousand years ago. Thus, the megafauna all died out probably between about 41 to 50 thousand years ago. There also seems to have been a rather diverse assemblage of megafauna just before their extinction, and all populations across the continent (or at least in western and eastern Australia) died out at about the same time. As of 2021, there is still no solid evidence of megafauna surviving past approximately 40,000 years ago. Their latest occurrence (including Diprotodon) is recorded at South Walker Creek mine in the northeast, at about 40,100 ± 1,700 years ago.

At the time of Roberts et al., the earliest evidence of human activity was 56±4 thousand years old, which was rather close to their calculated date, so they hypothesised that human hunting had eradicated the last megafauna within about 10 thousand years of coexistence. Human hunting had earlier also been blamed for the extinction of North American and New Zealand megafauna. Human activity had then been generally regarded as the main driver of Australian megafaunal extinction, especially because the megafauna had survived multiple extreme drought periods during glacial periods, and at the time there did not seem to be any evidence of unusually extreme climate during this window. Given how slowly marsupials reproduce, even limited megafaunal hunting may have severely weakened their population.

The Australian Aboriginal practice of fire-stick farming (above depicts a kangaroo hunt) may be implicated in megafaunal extinction.

In 2005, American geologist Gifford Miller noticed that fire abruptly becomes much more common about 45,000 years ago, which he ascribed to aboriginal fire-stick farmers, who would have regularly started controlled burns to clear highly productive forests and grasslands. He believed this radically altered the vegetational landscape, and promulgated the expanse of the modern day fire-resilient scrub, at the expense of the megafauna. Many subsequent studies have had trouble firmly linking controlled burns and major ecological collapse. The frequency of fire could have also increased as a consequence of megafaunal extinction, as there was nothing around to eat the plants, leading to faster fuel buildup.

In 2017, the human-occupied Madjedbebe rockshelter by the northern Australian coast was dated to about 65,000 years, which meant that humans and megafauna had coexisted for over 20,000 years, casting strong doubt on the unilateral role of human activity in megafaunal extinction. Further, in the 2010s, several ecological studies were published in support of major drought conditions coinciding with the final megafaunal extinctions. Therefore, their demise may have been the result of a combination of ecological turmoil, hunting hunting, and human-driven landscape changes.

In culture

Fossil evidence

Despite the apparent role that the first Australian Aborigines are speculated to have had in the extinction of Diprotodon and other Australian megafauna, there is a dearth of evidence that humans utilised them at all over the 20,000 years of coexistence. There have been no megafaunal fossils suggestive of human butchery or cooking.

In 1984, an upper right Diprotodon incisor (²I) bearing 28 visible cut marks was discovered in Spring Creek in southwestern Victoria by Gail Paton, and formally studied by Ron Vanderwald and Richard Fullager. The incisor was split in half longitudinally, seemingly while the bone was still fresh, but it was glued together before Vanderwald and Fullager could inspect it. Each piece measures 40 cm (16 in) in length. The marks are aligned in a straight line, and measure 0.91–4.1 mm (0.036–0.161 in) in length, 0.14–0.8 mm (0.0055–0.0315 in) in width, and 0.02–0.24 mm (0.00079–0.00945 in) in depth. 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. This specimen became one of the most cited pieces of evidence that humans and megafauna directly interacted, until a 2020 re-analysis by Australian palaeoanthropologist Michelle Langley identified the engraver as most likely a tiger quoll.

In 2016, Australian archaeologist Giles Hamm and colleagues unearthed a partial right radius belonging to a young Diprotodon in the Warratyi rock shelter. Because it lacks carnivore damage and the rock shelter is a pretty steep climb, they believed that humans were responsible for the bone ending up where it did.

Tribal memory

The bunyip (above drawn by J. Mcfarlane, 1890) was associated with Diprotodon soon after discovery.

When the first massive fossils were dug up in Australia, it was not clear what animals they might have represented as there were no serious scientists on the continent, but locals guessed some may represent rhinos or elephants. The colonists, most vocally Reverend John Dunmore Lang, forwarded these fossils as evidence of the Genesis flood narrative. The aborigines also attempted to fit the finds into their own religious ideas, quickly associating Diprotodon with the bunyip, a large carnivorous lake monster. Many ethnologists and palaeontologists of the time consequently believed the bunyip is a tribal memory of the lumbering giant which probably frequented marshlands (though at the time it was not impossible that the creature and other megafauna were still alive, as the land exploration of the continent was still ongoing.) If Pleistocene megafauna are indeed the basis for some aboriginal mythology, it is unclear if the stories were based on the creatures when they were alive, or their bones discovered long after they had died out.

Rock art

See also: Indigenous Australian art

Australian Aborigines decorated caves with paintings and drawings of several creatures, but the identities of the subjects are oftentimes unclear. In 1907, Australian anthropologist Herbert Basedow found footprint petroglyphs in Yunta Springs and Wilkindinna, which he believed represented those of Diprotodon. In 1988, Australian historian Percy Trezise presented what he thought was a Quinkan depiction of Diprotodon to the First Congress of the Australian Rock Art Research Association. Both of these claims have their faults as the supposed depictions bear several features inconsistent with what is known about Diprotodon, and oftentimes, the subjects of aboriginal paintings are mythological beings from the Dreaming.

See also

• Australian megafauna

External links

• "Diprotodon optatum". Australian Museum. Australia's lost kingdom.
• "BBC 'WildFacts' page on Diprotodon optatum". BBC Science and Nature. British Broadcasting Corporation.
• "[page on the genera Diprotodont]". Regional Council of Goyder, South Australia. Archived from the original on 11 August 2007.
• "The Diprotodontids". Museum Victoria. Archived from the original on 4 January 2010.
• Online view of a Diprotodon skull. Archived from the original on 5 March 2016. Retrieved 23 June 2010. {{cite book}}: |website= ignored (help)
• "Diprotodon information" (PDF). South Australian Museum. Archived from the original (PDF) on 27 September 2007.
• "Description of Gilbert Price's research". abc.net.au. Australian Broadcasting Corporation. May 2005. Archived from the original on 5 December 2008.
• "3D model of the skull of Diprotodon". phenome10k.org.

Further reading

• Clode, Danielle (2009). Prehistoric Giants: The megafauna of Australia. Museum Victoria.
• Mahoney, J. A.; Ride, W. D. L. (1975). "Index to the genera and species described from Australia and New Guinea from 1838 to 1968" (PDF). Western Australian Museum Special Publication (6): 87.
• Owen, R. (1870). Restoration of an extinct elephantine marsupial (Diprotodon australis). Taylor and Francis.
• Vickers-Rich, P.; Monaghan, J. M.; Baird, R. F.; Rich, T. H., eds. (1991). Vertebrate palaeontology of Australasia. Pioneer Design Studio in cooperation with the Monash University Publications Committee. doi:10.5962/bhl.title.60647. ISBN 9780909674366.