Editing Diprotodon (section)

==Description==

===Skull===
{{Multiple image|
|align=left
|image1=Three-dimensional-digital-reconstruction-of-the-jaw-adductor-musculature-of-the-extinct-marsupial-peerj-02-514-g002.jpg
|image2=Three-dimensional-digital-reconstruction-of-the-jaw-adductor-musculature-of-the-extinct-marsupial-peerj-02-514-g003.jpg
|footer=''Diprotodon'' skull reconstructions showing the cranial bones (left) and the [[frontal sinus]]es (right)
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''Diprotodon'' has a long, narrow skull.{{sfn|Vickers-Rich|1991|loc=p. 1102}} Like other marsupials, the top of the skull of ''Diprotodon'' is flat or depressed over the small [[braincase]] and the [[sinus (anatomy)|sinuses]] of the [[frontal bone]].{{sfn|Owen|1870|loc=p. 523}} Like many other giant vombatiformes, the [[frontal sinus]]es are extensive; in a specimen from [[Bacchus Marsh]], they take up {{cvt|2675|cc}}—roughly 25% of skull volume—whereas the brain occupies {{cvt|477|cc}}—only 4% of the skull volume. Marsupials tend to have smaller [[Brain–body mass ratio|brain-to-body mass ratio]]s than [[placental]] mammals, becoming more disparate the bigger the animal, which could be a response to a need to conserve energy because the brain is a calorically expensive organ, or is proportional to the maternal metabolic rate, which is much less in marsupials due to the shorter gestation period. 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.<ref>{{cite journal|url=https://museumsvictoria.com.au/media/4258/331-342_mmv74_sharp_4_web.pdf |archive-url=https://web.archive.org/web/20180719123229/https://museumsvictoria.com.au/media/4258/331-342_mmv74_sharp_4_web.pdf |archive-date=2018-07-19 |url-status=live|first=A. C.|last=Sharp|year=2016|title=A quantitative comparative analysis of the size of the frontoparietal sinuses and brain in vombatiform marsupials|journal=Memoirs of the Museum of Victoria|volume=74|pages=331–342|doi=10.24199/j.mmv.2016.74.23}}</ref> They may also have helped dissipate [[stress (mechanics)|stresses]] produced by biting more efficiently across the skull.<ref name="Sharpe2016">{{Cite journal|last1=Sharpe |first1=A. C.|last2=Rich |first2=T. H.|year= 2016|title=Cranial biomechanics, bite force and function of the endocranial sinuses in ''Diprotodon optatum'', the largest known marsupial|journal=Journal of Anatomy|volume=228 |issue=6 |pages=984–995|doi=10.1111/joa.12456 |pmc=5341585 |pmid=26939052}}</ref>

The [[occipital bone]], the back of the skull, slopes forward at 45 degrees unlike most modern marsupials, where it is vertical. The base of the occipital is significantly thickened. The [[occipital condyle]]s, a pair of bones that connect the skull with the [[vertebral column]], are semi-circular and the bottom half is 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]]—the tube running through a vertebral centrum where the spinal cord passes through—than a foramen magnum.{{sfn|Owen|1870|loc=pp. 521–523}}

A [[sagittal crest]] extends across the midline of the skull from the supraoccipital—the top of the occipital bone—to the region between the eyes on the top of the head. The [[orbit (anatomy)|orbit]] (eye socket) is small and vertically oval-shaped. The [[nasal bone]]s slightly curve upwards until near their endpoint, where they begin to curve down, giving the bones a somewhat 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; the end of the nasals is the tallest point. The [[zygomatic arch]] (cheek bone) is strong and deep as in kangaroos but unlike those of koalas and wombats, and extends all the way from the supraoccipital.{{sfn|Owen|1870|loc=pp. 523–524}}

====Jaws====
[[File:Diprotodon, Natural History Museum, London, Mammals Gallery.JPG|thumb|''Diprotodon'' skull at the [[Natural History Museum, London]]]]

As in kangaroos and wombats, there is a gap between the jointing of the [[palate bone|palate]] (roof of the mouth) and the [[maxilla]] (upper jaw) behind the last molar, which is filled by the [[medial pterygoid plate]].{{sfn|Owen|1870|loc=p. 525}} This would have been the [[insertion (anatomy)|insertion]] for the [[medial pterygoid muscle]] that was involved in closing the jaw. Like many [[grazing (behaviour)|grazers]], the [[masseter muscle]], which is also responsible for closing the jaw, seems to have been the dominant jaw muscle. A probable large [[temporal muscle]] compared to the [[lateral pterygoid muscle]] may indicate, unlike in wombats, a limited range of side-to-side jaw motion means ''Diprotodon'' 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. ''Diprotodon''{{'s}} chewing strategy appears to align more with kangaroos than wombats: a powerful vertical crunch was followed by a transverse grinding motion.<ref name=Sharp2014>{{cite journal |last=Sharp |first=A. C. |year=2014 |title=Three dimensional digital reconstruction of the jaw adductor musculature of the extinct marsupial giant Diprotodon optatum |journal=PeerJ |volume=2 |pages=e514 |issn=2167-8359 |doi=10.7717/peerj.514 |pmid=25165628 |pmc=4137671 |doi-access=free}}</ref>

As in 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 from the last molar to the first. The strong [[mandibular symphysis]], which fuses the two halves of the mandible, begins at the front-most end of the third molar;{{sfn|Owen|1870|loc=pp. 526–527}} this would prevent either half of the mandible from moving independently of the other, unlike in kangaroos which use this ability to better control their incisors.<ref name=Sharp2014/>

====Teeth====
[[File:Diprotodon molars.jpg|thumb|''Diprotodon'' [[molar (tooth)|molars]]]]

The [[dental formula]] of ''Diprotodon'' is {{DentalFormula|upper=3.0.1.4|lower=1.0.1.4}}. In each half of either jaw are three incisors in the upper jaw and one in the lower jaw; there are one [[premolar]] and four molars in both jaws but no [[canine (tooth)|canines]]. A long [[diastema]] (gap) separates the incisors from the molars.{{sfn|Owen|1870|loc=p. 528}}

The incisors are scalpriform (chisel-like). Like those of wombats and [[rodent]]s, the first incisors in both jaws continuously grew throughout the animal's life but the other two upper incisors did not. This combination 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 {{cvt|11|in|order=flip}} of which {{cvt|8.5|in|order=flip}} is within the tooth socket; the second is {{cvt|4|in|order=flip}} and {{cvt|1|in|order=flip}} is in the socket; and the exposed part of the third is {{cvt|2.6|in|order=flip}}. The first incisor is convex and curves outwards but the other two are concave.{{sfn|Owen|1870|loc=pp. 528–530}} The lower incisor has a faint upward curve but is otherwise straight and has an oval cross-section. In the same old male specimen, the lower incisor  measures {{cvt|10|in|order=flip}}, of which {{frac|2|3}} is inside the socket.{{sfn|Owen|1870|loc=p. 533}}

The premolars and molars are [[bilophodont]], each having two distinct lophs (ridges). The premolar is triangular and about half the size of the molars.<ref name=Huxley1862>{{cite journal|last=Huxley|first=T. H.|authorlink=Thomas H. Huxley|year=1862|title=On the Premolar Teeth of ''Diprotodon'', and on a New Species of that Genus|journal=Quarterly Journal of the Geological Society|volume=18|issue=1–2|pages=422–427|doi=10.1144/gsl.jgs.1862.018.01-02.56|bibcode=1862QJGS...18..422H |s2cid=131284050 |url=https://zenodo.org/record/1793193 }}</ref> As in kangaroos, the necks of the lophs are coated in [[cementum]]. Unlike in kangaroos, there is no connecting ridge between the lophs. The peaks of these lophs have a thick [[tooth enamel|enamel]] coating that thins towards the base; this could wear away with use and expose the [[dentine]] layer, and beneath that osteodentine.{{sfn|Owen|1870|loc=pp. 530–532}} Like the first premolar of other marsupials, the first molar of ''Diprotodon'' and wombats is the only tooth that is [[deciduous teeth|replaced]].{{sfn|Owen|1870|loc=p. 539}} ''D. optatum'' premolars were highly morphologically variable even within the same individual.<ref>{{Cite journal |last1=Price |first1=Gilbert J. |last2=Sobbe |first2=Ian H. |date=7 September 2010 |title=Morphological variation within an individual Pleistocene Diprotodon optatum Owen, 1838 (Diprotodontinae; Marsupialia): implications for taxonomy within diprotodontoids |url=http://www.tandfonline.com/doi/abs/10.1080/03115511003793553 |journal=[[Alcheringa: An Australasian Journal of Palaeontology]] |language=en |volume=35 |issue=1 |pages=21–29 |doi=10.1080/03115511003793553 |issn=0311-5518 |access-date=6 May 2024 |via=Taylor and Francis Online}}</ref>

===Vertebrae===
''Diprotodon'' had seven cervical (neck) vertebrae.<ref>{{cite book |last1=Murray |first1=Peter |title=Diprotodon: background and interpretation of the display in the Central Australian Museum |date=1995 |publisher=Museum and Art Gallery of the Northern Territory |location=Northern Territory, Australia}}</ref> The [[atlas (anatomy)|atlas]], the first cervical (C1), has a pair of deep cavities for insertion of the occipital condyles. The diaphophyses of the atlas, an upward-angled projection on either the side of the vertebra, are relatively short and thick, and resemble those of wombats and koalas. The articular surface of the [[axis (anatomy)|axis]] (C2), the part that joints to another vertebra, is slightly concave on the front side and flat on the back side. As in 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 projects vertically the topside of the vertebra, is more forwards. The remaining cervicals lack a hypophysis. As in 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-but-shorter ridges.{{sfn|Owen|1870|loc=pp. 539–542}} C7 had a forked shape on top of the neural spine.<ref>{{cite book |last1=Murray |first1=Peter |title=Diprotodon: background and interpretation of the display in the Central Australian Museum |date=1995 |publisher=Museum and Art Gallery of the Northern Territory |location=Northern Territory, Australia}}</ref>

[[File:Lake Callabonna Diprotodon (3).jpg|thumb|left|Mounted skeleton, [[Museums Victoria]]]]

''Diprotodon'' probably had 13 [[dorsal vertebra]]e and 14 pairs of closely spaced [[rib]]s.{{sfn|Owen|1870|loc=p. 548}} Like many other mammals, the dorsals initially decrease in breadth and then expand before connecting to the [[lumbar vertebra]]e. Unusually, the front dorsals 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. In later examples, 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 only in a few of the cervicals and not in the dorsals. Compared to those of wombats and kangaroos, the neural arch is proportionally taller. As in elephants, the [[epiphysial plate]]s (growth plates) and the neural arch, to which  the neural spine is attached, are [[anchylosis|anchylosed]]—very rigid in regard to the vertebral centrum—which served to support the animal's immense weight.{{sfn|Owen|1870|loc=pp. 542–544}}

Like most marsupials, ''Diprotodon'' likely had six lumbar vertebrae.{{efn|Wombats have four lumbar vertebrae and koalas have five.{{sfn|Owen|1870|loc=p. 545}}}} 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 and other mammals. The length of each vertebra increases along the series so the lumbar series may have bent downward.{{sfn|Owen|1870|loc=pp. 545–546}}

Like other marsupials, ''Diprotodon'' had two [[sacrum|sacral vertebrae]]. The base of the neural spines of these two were [[ossified]] (fused) together.{{sfn|Owen|1870|loc=pp. 554–556}}

===Limbs===
====Girdles====
The general proportions of the [[scapula]] (shoulder blade) align more closely with more-basal vertebrates such as [[monotreme]]s, [[bird]]s, [[reptile]]s, and [[fish]] rather than marsupials and [[placental mammal]]s. 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 (anatomy)|fossa]], a depression below the [[spine of scapula|spine of the scapula]]) becomes bigger towards the arm joint rather than decreasing. The [[glenoid cavity]] where the arm connects is oval shaped as in most mammals.{{sfn|Owen|1870|loc=pp. 548–550}}

Unlike other marsupials, the [[ilium (bone)|ilia]], the large wings of the [[pelvis]], are lamelliform (short and broad, with a flat surface instead of an [[iliac fossa]]). Lamelliform ilia have only been recorded in [[elephant]]s, [[sloth]]s, and [[ape]]s, though these groups all have a much-longer sacral vertebra series whereas marsupials are restricted to two sacral vertebrae. The ilia provided strong muscle attachments that were probably oriented and used much the same as those in an elephant. The [[sacroiliac joint]] where the pelvis connects to the spine is at 35 degrees in reference to the long axis of the ilium. The [[ischium|ischia]], which form part of the [[hip socket]], are thick and rounded tailwards but taper and diverge towards the socket, unlike those 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.{{sfn|Owen|1870|loc=pp. 554–560}}

====Long bones====
Unlike those of most marsupials, the humerus of ''Diprotodon'' is almost straight rather than S-shaped, and the [[trochlea of humerus|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 for by the powerful forearms. Similarly, the condyles where the [[radius (bone)|radius]] and [[ulna]] (the forearm bones) connect maintain their rounded shape and are quite-similarly sized, and unusually reminiscent of the condyles between the [[femur]] and the [[tibia]] and [[fibula]] in the leg of a kangaroo.{{sfn|Owen|1870|loc=pp. 551–554}}

{{Multiple image
|image1 = Diprotodon femur interior.jpg
|image2 = Diprotodon femur exterior.jpg
|footer = Different views of a ''Diprotodon'' femur
}}
Like elephants, the femur of ''Diprotodon'' is straight and compressed anteroposteriorly (from headside to tailside). The walls of the femur are prodigiously thickened, strongly constricting the [[medullary cavity]] where the [[bone marrow]] is located. 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]]. As in kangaroos, the greater trochanter is split into two lobes. The [[femoral neck]] is roughly the same diameter as the femoral head. Also as in kangaroos, the condyle for the fibula is excavated out but the condyle for the tibia is well-rounded and hemispherical. Like those of many other marsupials, the tibia is twisted and the tibial [[malleolus]] (on the ankle) is reduced.{{sfn|Owen|1870|loc=pp. 560–566}}

====Paws====
''Diprotodon'' has five digits on either paw. Like other [[plantigrade]] walkers, where the paws were flat on the ground, the wrist and ankle would have been largely rigid and inflexible.<ref name=Stirling1899/><ref name=Weisbecker2008/> The digits are proportionally weak so the paws probably had a lot of padding.{{sfn|Vickers-Rich|1991|loc=p. 1102}} Similarly, the digits do not seem to have been much engaged in weight bearing.<ref name=Weisbecker2008/><ref name=Carey2011/>

The forepaw was strong and the shape of the wrist bones is quite similar to those of kangaroos. Like other vombatiformes, the [[metacarpal]]s, which connect the fingers to the wrist, are broadly similar to those of kangaroos and allies.<ref name=Weisbecker2008>{{cite journal|first1=V.|last1=Weisbecker|first2=M.|last2=Archer|year=2008|title=Parallel evolution of hand anatomy in kangaroos and vombatiform marsupials: Functional and evolutionary implications|journal=Palaeontology|volume=51|issue=2|pages=321–338|doi=10.1111/j.1475-4983.2007.00750.x|s2cid=82172054 |doi-access=free|bibcode=2008Palgy..51..321W }}</ref> The enlarged [[pisiform bone]] takes up half the jointing surface of the ulna. The fifth digit on the forepaw is the largest.<ref name=Stirling1899>{{cite book|first1=E. C.|last1=Stirling|first2=A. H. C.|last2=Zietz|author-link=Edward Charles Stirling|year=1899|title=Description of the manus and pes of Diprotodon australis|series=Fossil Remains of Lake Callabonna|publisher=Memoirs of the Royal Society of South Australia|pages=1–40}}</ref>

The digits of the hindpaws turn inwards from the ankle at 130 degrees. The [[second metatarsal|second]] and [[third metatarsal]]s (the [[metatarsal]]s connect the toes to the ankle) are significantly reduced, which may mean these digits were [[syndactyly|syndactylous]] (fused) like those of all modern diprotodontians. The first, fourth, and fifth digits are enlarged. The toes are each about the same length, except the fifth which is much stouter.<ref>{{cite journal|first1=A.|last1=Camens|first2=R.|last2=Wells|year=2009|title=Palaeobiology of Euowenia grata (Marsupialia: Diprotodontinae) and its Presence in Northern South Australia|journal=Journal of Mammalian Evolution|volume=17|issue=1|pages=9–10|doi=10.1007/s10914-009-9121-2|s2cid=42667860 }}</ref>

===Size===
[[File:Guide to fossil mammals and birds (1896) Diprotodon australis.png|thumb|1896 illustration of a ''Diprotodon'' and human skull]]

''Diprotodon'' is the largest-known marsupial to ever have lived.<ref name=Price2009/> In life, adult ''Diprotodon'' could have reached {{cvt|160–180|cm|ftin}} at the shoulders and {{cvt|275–340|cm|ft|0}} from head to tail.{{sfn|Vickers-Rich|1991|loc=[https://www.biodiversitylibrary.org/item/123394#page/1118/mode/1up p. 1102]}} Accounting for cartilaginous [[intervertebral disc]]s, ''Diprotodon'' may have been 20% longer than reconstructed skeletons, exceeding {{cvt|400|cm|ftin}}.<ref name = "Wroe2003">{{cite journal| last1 = Wroe | first1 = S. | last2 = Crowther | first2 = M.| last3 = Dortch | first3 = J. | last4 = Chong | first4 = J.| year=2004| title = The size of the largest marsupial and why it matters| journal = Proceedings of the Royal Society B: Biological Sciences| volume = 271 | issue = Suppl 3 | pages = S34–S36| pmc = 1810005 | pmid=15101412| doi = 10.1098/rsbl.2003.0095}}</ref>

As researchers were formulating predictive body-mass equations for fossil species, efforts were largely constrained to [[eutherian]] mammals rather than marsupials.<ref name = "Wroe2003"/> The first person to attempt to estimate the living weight of ''Diprotodon'' was Peter Murray in his 1991 review of the megafauna of Pleistocene Australia; Murray made an estimate of {{cvt|1150|kg}} using cranial and dental measurements, which he said was probably not a very precise figure.{{sfn|Vickers-Rich|1991|loc=p. 1156}} 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|Thylacoleo carnifex]]'') against the landmass area of their continent, and another regression between the daily food intake of living creatures against the landmass of their continents. 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. The maximum-attainable body size is capped much lower than those for other continents.<ref>{{cite journal|first1=G. P.|last1=Burness|first2=J.|last2=Diamond|author2-link=Jared Diamond|first3=T.|last3=Flannery|year=2001|title=Dinosaurs, dragons, and dwarfs: The evolution of maximal body size|journal=Proceedings of the National Academy of Sciences|volume=98|issue=25|pages=14518–14523
|doi=10.1073/pnas.251548698|pmid=11724953 |pmc=64714 |bibcode=2001PNAS...9814518B |doi-access=free }}</ref>

In 2003, Australian palaeontologist Stephen Wroe and colleagues took a more-sophisticated approach to body mass than Murray's estimate. 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. The results ranged from {{cvt|2,272–3,417|kg}}, for a mean of {{cvt|2,786|kg}}, though Wroe said reconstructing the weight of extinct creatures that far outweighed living counterparts{{efn|A bull [[red kangaroo]], the largest living marsupial, can weigh {{cvt|22–85|kg}}.<ref>{{cite web|title=Largest living marsupial|publisher=Guinness Book of World Records|access-date=31 August 2022|url=https://www.guinnessworldrecords.com/world-records/504773-largest-marsupial-living}}</ref>}} is problematic. For comparison, an [[American bison]] they used in their study weighed {{cvt|1179|kg}} and a hippo weighed {{cvt|1950|kg}}.<ref name = "Wroe2003"/>