Diprotodon optatum

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Diprotodon optatum


A life reconstruction of Diprotodon optatum. © @ Gabriel Ugueto.

Temporal range: Late Quaternary; Piacenzian to Late Pleistocene; Late Pliocene or Early Pleistocene[1] to Late Pleistocene (2.92-2.47 to 0.046 Ma)[2][3]

Scientific classification

Life
Domain: Eukaryota
(unranked): Unikonta
(unranked): Opisthokonta
(unranked): Holozoa
(unranked): Filozoa
Kingdom: Animalia
Phylum: Chordata
Subkingdom: Eumetazoa
(unranked): Bilateria
Superphylum: Deuterostomia
Phylum: Chordata
Infraphylum: Gnathostomata
Clade: Eugnathostomata
Clade: Teleostomi
Superclass: Tetrapoda
Clade: Reptiliomorpha
Clade: Amniota
Clade: Synapsida
Clade: Eupelycosauria
Clade: Sphenacodontia
Clade: Sphenacodontoidea
Order: Therapsida
Suborder: Cynodontia
Clade: Prozostrodontia
Clade: Mammaliaformes
Class: Mammalia
Legion: Cladotheria
Sublegion: Zatheria
Infralegion: Tribosphenida
Subclass: Theria
Clade: Metatheria
Infraclass: Marsupialia
Superorder: Australidelphia
Order: Diprotodontia
Suborder: Vombatiformes
Superfamily: †Diprotodontoidea
Family: †Diprotodontidae
Genus: †Diprotodon
Species: †D. optatum

Diprotodon is a genus of giant marsupial that lived in Australia from either the late Pliocene or early Pleistocene to the Late Pleistocene. Only one valid species is currently recognized: D. optatum.[4]

Geographic and temporal range:
Diprotodon remains have been uncovered from Australia, including Kangaroo Island.[5] The oldest remains have been uncovered in localities such as Fisherman’s Cliff, Moorna Formation, New South Wales.[1] This formation has been dated to the late Pliocene to early Pleistocene epochs, around 2.92 to 2.47 Ma.[2] The latest remains date to ~46,400 years ago.[3] Diprotodon was a member of the Malkuni Fauna.[6]

Description:
Diprotodon optatum was the largest marsupial to ever live. Body mass averaged 2,786 kilograms (with a 95% confidence interval of 2,272-3,417 kg).[7] To put this into perspective, male hippopotamuses average 1,480 kg and reach a maximum of 2,660 kg (for females, these figures are 1,365 kg and 2,025 kg, respectively).[8] Male and female white rhinoceroses weigh 2,300 and up to 1,600 kg, respectively[8][9], while the black rhinoceros averages 1,075 kg and weighs up to 1,350 kg.[10] This means that the average Diprotodon was slightly heavier than the largest hippos, substantially heavier than even an average male white rhinoceros, and twice as heavy as a large black rhinoceros.

To bear its great weight, Diprotodon evolved graviportal limbs. The evolution of this graviportal leg morphology was well underway in the early Pliocene diprotodontid Euowenia grata. These limbs were columnar with relatively immobile joints (with reduced mobility in the forelimb and cubo-navicular/cuneiform interface), muscle attachment areas that restricted the manipulative abilities of the forelimbs (the attachment areas vary in how well developed they are; given the backward-facing pouch of Diprotodon, manipulating the pouch with the forelimbs is unlikely), and a proportionally long humerus relative to the ulna. As such, Diprotodon’s limbs were adapted for weight bearing and movement at slow speeds.[11] The elephantine limbs of Diprotodon did not have strong development of bony processes (e.g. the deltopectoral crest was long but not especially prominent). This suggests that the muscles needed to produce large forces for digging and tearing were only modestly developed. The phalanges show no adaptations for unusual degrees of flexion or extension (with no evidence of large forces being applied through them). The unguals, while overall claw-shaped, are broad with no lateral compression and do not seem to have had sharp tips. As such, the claws of Diprotodon would have been of limited use and were not well adapted for most digging or defensive uses.[12]

The skull of Diprotodon was lightweight, made of thin cranial bone pneumatized by extensive cranial sinuses. These sinuses significantly lightened the skull while still providing structural support and strength. Bite force was estimated at 2,374 N at the incisors, and anywhere from 4,118 to 11,134 N at the cheek teeth (from the premolar to the fourth molar, respectively). Such exceptionally high bite forces suggest that Diprotodon was capable of consuming a wide variety of plant matter, including tough, fibrous grasses (indeed, isotope analysis indicates that Diprotodon fed on both C3 and C4 plants[13]). The low amount of stress along the cranium indicates that the skull of Diprotodon could withstand more force than its jaw adductors generated. As such, higher forces may have been produced by using the incisors for male-male competition (supported by inferred sexual dimorphism in Diprotodon[4]) and defense.[14]

The presence of two size classes of Diprotodon and differences in cheek teeth morphometrics (as in living sexually dimorphic marsupials) both suggest that Diprotodon was sexually dimorphic, with the large form being male and the small form being female. If this is the case, then this would have implications for the behavior of Diprotodon. Sexual dimorphism would suggest polygyny in Diprotodon (as in all sexually dimorphic extant megaherbivores), as well as gender segregation among social groups (the fact that some assemblages show bias towards one size class, and thus possibly one sex, may provide some support for this view).[4]

Footprints found at Lake Callabonna show remains of the foot pads themselves and fur impressions (albeit poorly preserved). These indicate that Diprotodon was covered in fur, not largely naked like modern elephants and rhinos.[15]

Extinction:
Diprotodon went extinct ~46,400 years ago, and was one of the many Australian megafauna to go extinct during the Pleistocene.[3] Although the true cause of extinction remains unclear, Sahul’s extinction patterns were unusual in that there was no clear relationship between extinction susceptibility and the chronological order in which taxa became extinct. This is evidence against non-selective human hunting as the cause of the continent-wide extinction event. Instead, it suggests that the extinction chronology was the result of finer-scale variation in climate change and/or human hunting (i.e. preferences for hunting or avoiding certain species).[16]

References:

[1] Marshall, L. G. (1973). Fossil vertebrate faunas from the Lake Victoria region, SW New South Wales, Australia. Memoirs of the National Museum of Victoria, 34(15), 1-17.
[2] Travouillon K. J. (2016). Oldest fossil remains of the enigmatic pig-footed bandicoot show rapid herbivorous evolution. Royal Society open science, 3(8), 160089. doi.org/10.1098/rsos.160089
[3] Roberts, R. G., Flannery, T. F., Ayliffe, L. K., Yoshida, H., Olley, J. M., Prideaux, G. J., Laslett, G. M., Baynes, A., Smith, M. A., Jones, R., & Smith, B. L. (2001). New ages for the last Australian megafauna: continent-wide extinction about 46,000 years ago. Science (New York, N.Y.), 292(5523), 1888–1892. doi.org/10.1126/science.1060264
[4] Price, G.J. (2008) Taxonomy and palaeobiology of the largest-ever marsupial, Diprotodon (Diprotodontidae, Marsupialia), Zoological Journal of the Linnean Society, Volume 153, Issue 2, June 2008, Pages 369–397, doi.org/10.1111/j.1096-3642.2008.00387.x
[5] www.abc.net.au/news/2017-07-23/kangaroo-island-fossil-footprints-reveal-ancient-wildlife/8735572
[6] Thomas H. Rich, Paul F. Lawson, Patricia Vickers-Rich & Richard H. Tedford (2019): R. A. Stirton: pioneer of Australian mammalian palaeontology, Transactions of the Royal Society of South Australia, DOI: 10.1080/03721426.2019.1602244
[7] Wroe, S., Crowther, M., Dortch, J., & Chong, J. (2004). The size of the largest marsupial and why it matters. Proceedings. Biological sciences, 271 Suppl 3(Suppl 3), S34–S36. doi.org/10.1098/rsbl.2003.0095
[8] Owen-Smith, R. N. (1988). Megaherbivores: the influence of very large body size on ecology. Cambridge university press. pp. 14-15.
[9] Hebbelmann, L. (2013). Changes in adult female white rhino seasonal home ranges in relation to variation in food quality and availability (Doctoral dissertation).
[10] Winkel, F., de Boer, W. F., & Lent, P. (2004). Diet choice of the black rhinoceros (Diceros bicornis) in the Doubledrift Game Reserve, Eastern Cape Province, South Africa. Resource Ecology Group.
[11] Camens, A. B. (2010). Systematic and palaeobiological implications of postcranial morphology in the Diprotodontidae (Marsupialia) (Doctoral dissertation).
[12] Coombs, M. C. (1983). Large mammalian clawed herbivores: a comparative study. Transactions of the American Philosophical Society, 73(7), 1-96.
[13] Gröcke, D. R. (1997). Distribution of C3 and C4 plants in the late Pleistocene of South Australia recorded by isotope biogeochemistry of collagen in megafauna. Australian Journal of Botany, 45(3), 607-617.
[14] Sharp, A. C., & Rich, T. H. (2016). Cranial biomechanics, bite force and function of the endocranial sinuses in Diprotodon optatum, the largest known marsupial. Journal of anatomy, 228(6), 984–995. doi.org/10.1111/joa.12456
[15] Flannery, T. (2002). The future eaters: an ecological history of the Australasian lands and people. Grove Press. p. 126.
[16] Bradshaw, C. J., Johnson, C. N., Llewelyn, J., Weisbecker, V., Strona, G., & Saltré, F. (2021). Relative demographic susceptibility does not explain the extinction chronology of Sahul's megafauna. eLife, 10, e63870. doi.org/10.7554/eLife.63870

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In a 1999 publication, Stephen Wroe and his colleagues say the following on Thylacoleo and potential predation on Diprotodon (Wroe et al., 1999).

It may be argued that T. carnifex, even at an average weight of 101–130 kg, would not have been capable of dispatching Australia's largest Pleistocene herbivore, Diprotodon australis (up to 2000 kg: Flannery 1997). With respect to this we suggest the following: the only obvious defensive 'card' in the repertoire of D. australis that might have deterred T. carnifex was size. On the other hand, Pleistocene–Recent predators of large-to-gigantic placental herbivores, were, and continue to be, faced by potentially lethal 'defensive' weapons, in the form of sharp horns, tusks and hooves (an additional obstacle faced or faces would-be predators of proboscideans and rhinoceroids extremely thick skin). Jaguars, P. onca, are capable of killing bulls of up to 500 kg in weight (Turner and Anton 1997). Given a maximum of 2000 kg for D. australis, the average was presumably much lower. We tender that if P. onca, around half the size of T. carnifex by our estimates, can dispatch an animal as formidably 'armed' and aggressive as a half tonne bull, then the claim that T. carnifex might kill D. australis of up to at least average size is not unreasonable.

With respect to this I suggest the following.

As I've recently demonstrated in the jaguar profile (link->), jaguars rarely kill mature half tonne bulls, and normally avoid them due to the risks involved. Accordingly, the domestic cattle they usually go after are young calves and cows. It must further be noted that the primary cattle breed are zebu cattle, which will flee instead of standing their ground against jaguars. There are bovinins that behave aggressively towards jaguars, namely water buffalo and San Martineros, and are armed with their horns and weight. As such, water buffalo are avoided by jaguars. Similarly, San Martineros display such effective antipredator behavior against jaguars that they are being considered by conservationists for use in reducing cattle casualties by jaguars (and therefore jaguar casualties by retaliating farmers). In short, the fact that rarely kill half tonne zebu bulls that flee from them (and don't even seem to necessarily have formidable horns for defense) doesn't exactly bode well for Thylacoleo's ability to successfully prey on adult Diprotodon.

Now consider this: the average body mass for male and female jaguars in the Pantanal (where they indeed target cattle) is 99.5 kg and 76.7 kg, respectively. A 500 kg zebu bull is thus 5-6.5 times heavier than a Pantanal jaguar. How does the size discrepancy between marsupial lion and Diprotodon compare?

As I've demonstrated above, Diprotodon was actually even larger than this excerpt claims (indeed, Stephen Wroe was one of the authors of this more recent paper, so he's obviously aware of this), and could weigh ~2.8 tonnes on average, with body masses >3 tonnes being plausible. It's clear that Diprotodon could get enormous, to the point where it is ~21-27.5 times larger than the average T. carnifex. Just Diprotodon's sheer size would be overwhelming against Thylacoleo. Wroe et al. mention hooves as among the defensive weapons that recent predators must face; I opine (deliberate understatement) that getting stomped on by a heavily-built, >20-fold heavier beast is no less deadly than getting kicked by hooves. Of course, as I've alluded to in the OP of this profile, Diprotodon actually does seem to have possessed defensive weapons in the form of its incisors, solidifying its ability to defend itself against the marsupial lion.

Now, I also noted above that Diprotodon appears to exhibit sexual dimorphism, represented by two size forms. However, even female Diprotodon were huge. One such specimen was estimated to weigh 1,048 kg (Sharp, 2016), making it 8-10 times heavier than the average T. carnifex. While I don't think it's downright impossible for T. carnifex to successfully kill such a creature, clearly it would still be very difficult to take down, and I doubt that even much smaller female Diprotodon were killed often either.

Wroe et al. (1999) also mention bite marks attributed to T. carnifex on Diprotodon bones. However, they do not make it clear how large the individuals in question were or whether any evidence points towards these bite marks being the result of actual predation or scavenging.

So Thylacoleo carnifex,


(This image is by Roman Uchytel, for anyone wondering; public domain, though.)

I suggest you leave the giant wombat takedowns to Varanus priscus.