Editing Tuatara (section)

===Skull===
[[File:Tuatara skull diagram.svg|thumb|Skull diagram in top down and side-on views]]
Unlike the vast majority of lizards, the tuatara has a complete lower temporal bar closing the lower [[temporal fenestra]] (an opening of the skull behind the eye socket), caused by the fusion of the [[Quadrate bone|quadrate]]/[[Quadratojugal bone|quadratojugal]] (which are fused into a single element in adult tuatara) and the [[jugal bone]]s of the skull. This is similar to the condition found in primitive [[diapsid]] reptiles. However, because more primitive rhynchocephalians have an open lower temporal fenestra with an incomplete temporal bar, this is thought to be [[Derived (phylogenetics)|derived]] characteristic of the tuatara and other members of the clade [[Sphenodontinae]], rather than a primitive trait retained from early diapsids. The complete bar is thought to stabilise the skull during biting.<ref name=":02">{{Cite journal |last1=Simões |first1=Tiago R. |last2=Kinney-Broderick |first2=Grace |last3=Pierce |first3=Stephanie E. |date=2022-03-03 |title=An exceptionally preserved Sphenodon-like sphenodontian reveals deep time conservation of the tuatara skeleton and ontogeny |journal=Communications Biology |language=en |volume=5 |issue=1 |page=195 |doi=10.1038/s42003-022-03144-y |issn=2399-3642 |pmc=8894340 |pmid=35241764}}</ref>

The tip of the upper jaw is chisel- or beak-like and separated from the remainder of the jaw by a notch,<ref name="Günther1867" /> this structure is formed from fused [[premaxilla]]ry teeth, and is also found in many other advanced rhynchocephalians.<ref>{{Cite journal |last1=Herrera-Flores |first1=Jorge A. |last2=Stubbs |first2=Thomas L. |last3=Elsler |first3=Armin |last4=Benton |first4=Michael J. |date=July 2018 |title=Taxonomic reassessment of Clevosaurus latidens Fraser, 1993 (Lepidosauria, Rhynchocephalia) and rhynchocephalian phylogeny based on parsimony and Bayesian inference |journal=Journal of Paleontology |language=en |volume=92 |issue=4 |pages=734–742 |doi=10.1017/jpa.2017.136 |issn=0022-3360|doi-access=free |bibcode=2018JPal...92..734H |hdl=1983/59126b60-16d8-46d2-b657-954693a39d4e |hdl-access=free }}</ref> The teeth of the tuatara, and almost all other rhynchocephalians, are described as [[acrodont]], as they are attached to the apex of the jaw bone. This contrast with the [[pleurodont]] condition found in the vast majority of lizards, where the teeth are attached to the inward-facing surface of the jaw. The teeth of the tuatara are extensively fused to the jawbone, making the boundary between the tooth and jaw difficult to discern, and the teeth lack roots and are not replaced during the lifetime of the animal, unlike those of pleurodont lizards.<ref>{{Cite journal |last1=Jenkins |first1=Kelsey M. |last2=Jones |first2=Marc E. H. |last3=Zikmund |first3=Tomas |last4=Boyde |first4=Alan |last5=Daza |first5=Juan D. |date=September 2017 |title=A Review of Tooth Implantation Among Rhynchocephalians (Lepidosauria) |url=http://www.bioone.org/doi/10.1670/16-146 |journal=Journal of Herpetology |language=en |volume=51 |issue=3 |pages=300–306 |doi=10.1670/16-146 |issn=0022-1511 |s2cid=90519352}}</ref> It is a common misconception that tuatara lack teeth and instead have sharp projections on the jaw bone;<ref name="lutzp27">{{Harvnb|Lutz|2005|p=27}}</ref> histology shows that they have true teeth with enamel and dentine with pulp cavities.<ref name="Kieser2009">{{cite journal |vauthors=Kieser JA, Tkatchenko T, Dean MC, Jones ME, Duncan W, Nelson NJ |year=2009 |title=Microstructure of dental hard tissues and bone in the Tuatara dentary, ''Sphenodon punctatus'' (Diapsida: Lepidosauria: Rhynchocephalia) |url=https://www.karger.com/Article/Abstract/242396 |journal=Frontiers of Oral Biology |volume=13 |pages=80–85 |doi=10.1159/000242396 |isbn=978-3-8055-9229-1 |pmid=19828975}}</ref> As their teeth wear down, older tuatara have to switch to softer prey, such as [[earthworm]]s, [[larva]]e, and [[slug]]s, and eventually have to chew their food between smooth jaw bones.<ref>{{cite news |author=Mlot, C. |date=8 November 1997 |title=Return of the Tuatara: A relic from the age of dinosaurs gets a human assist |website=Science News |url=http://www.sciencenews.org/pages/pdfs/data/1997/152-19/15219-21.pdf |access-date=24 May 2007}}</ref>

The tuatara possesses palatal dentition (teeth growing from the bones of the roof of the mouth), which is ancestrally present in reptiles (and [[tetrapod]]s generally).<ref name=":13">{{cite journal |vauthors=Matsumoto R, Evans SE |date=January 2017 |title=The palatal dentition of tetrapods and its functional significance |journal=[[Journal of Anatomy]] |volume=230 |issue=1 |pages=47–65 |doi=10.1111/joa.12534 |pmc=5192890 |pmid=27542892}}</ref> While many of the original palatal teeth present in reptiles have been lost,<ref name=":13" /> as in all other known rhynchocephalians, the row of teeth growing from the [[palatine bone]]s in the tuatara have been enlarged, and as in other members of Sphenodontinae the palatine teeth are orientated parallel to the teeth in the [[maxilla]]; during biting the teeth of the lower jaw slot between the two upper tooth rows.<ref name="Jones2008">{{cite journal |author=Jones, M.E. |date=August 2008 |title=Skull shape and feeding strategy in Sphenodon and other Rhynchocephalia (Diapsida: Lepidosauria) |journal=Journal of Morphology |volume=269 |issue=8 |pages=945–66 |doi=10.1002/jmor.10634 |pmid=18512698 |s2cid=16357353 |doi-access=free}}</ref> The structure of the jaw joint allows the lower jaw to slide forwards after it has closed between the two upper rows of teeth.<ref name="Jones et al. 2012">{{cite journal |vauthors=Jones ME, O'higgins P, Fagan MJ, Evans SE, Curtis N |date=July 2012 |title=Shearing mechanics and the influence of a flexible symphysis during oral food processing in Sphenodon (Lepidosauria: Rhynchocephalia) |journal=The Anatomical Record |volume=295 |issue=7 |pages=1075–91 |doi=10.1002/ar.22487 |pmid=22644955 |s2cid=45065504|doi-access=free }}</ref> This mechanism allows the jaws to shear through [[chitin]] and bone.<ref name="encyclo" />

The brain of ''Sphenodon'' fills only half of the volume of its [[endocranium]].<ref name="larsson2001">{{cite book |vauthors=Larsson HC |year=2001 |chapter=Endocranial anatomy of ''Carcharodontosaurus saharicus'' (Theropoda: Allosauroidea) and its implications for theropod brain evolution |veditors=Tanke DH, Carpenter K, Skrepnick MW |title=Mesozoic Vertebrate Life |place=Bloomington & Indianapolis |publisher=Indiana University Press |isbn=0-253-33907-3 |pages=19–33}}</ref> This proportion has been used by paleontologists trying to estimate the volume of dinosaur brains based on fossils.<ref name="larsson2001" /> However, the proportion of the tuatara endocranium occupied by its brain may not be a very good guide to the same proportion in Mesozoic dinosaurs since modern birds are surviving dinosaurs but have brains which occupy a much greater relative volume in the endocranium.<ref name="larsson2001" />

{{gallery|Tuatara Skull Lateral (50669113641).jpg|Skull of the tuatara in oblique view|File:Sphenodon punctatus (AM LH288) 601767.jpg|Skull of the tuatara in oblique view, with [[sclerotic ring]]s in eye sockets|Sphenodon punctatus (AM LH288) 601763 (cropped).jpg|Skull of tuatara from above|File:Tuatara Skull Doral (50668366013).jpg|Skull of tuatara from above|Tuatara skull diagram.jpg|Tuatara skull in various views with palatine tooth row visible on underside of the skull|||||||width=200|height=180|lines=|align=center}}