Editing Diprotodon (section)

===Skull===
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|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>