View source for Jugal bone

{{Short description|Skull bone found in most reptiles, amphibians, and birds}}
{{Refimprove|date=September 2014}}{{See also|Evolution of mammalian auditory ossicles}}[[Image:Gegenbaur 1870 skull homology color.png|thumb|Diagram showing homologous bones of the skulls of a [[monitor lizard]] and a [[crocodile]].  Jugal bone labelled ''Ju'', in pale green, at centre left.]]
The '''jugal''' is a [[skull]] bone found in most [[reptile]]s, [[amphibian]]s and [[bird]]s.  In [[mammal]]s, the jugal is often called the malar or [[zygomatic bone|zygomatic]].  It is connected to the [[quadratojugal]] and [[maxilla]], as well as other bones, which may vary by species.

== Anatomy ==
The jugal bone is located on either side of the skull in the [[Ocular scales|circumorbital]] region. It is the origin of several masticatory muscles in the skull.<ref>{{Cite journal|last1=Dechow|first1=Paul C.|last2=Wang|first2=Qian|date=2017|title=Evolution of the Jugal/Zygomatic Bones|journal=The Anatomical Record|language=en|volume=300|issue=1|pages=12–15|doi=10.1002/ar.23519|pmid=28000397|issn=1932-8494|doi-access=free}}</ref> The jugal and [[Lacrimal bone|lacrimal]] bones are the only two remaining from the ancestral circumorbital series: the prefrontal, postfrontal, postorbital, jugal, and lacrimal bones.<ref>{{cite book |last1=Romer |first1=Alfred Sherwood |last2=Parsons |first2=Thomas S. |author-link1=Alfred Romer |title=The Vertebrate Body |date=1978 |publisher=[[Saunders (imprint)|Saunders]] |location=Philadelphia |isbn=978-0-7216-7682-1 |edition=5th, shorter |oclc=3345587}}</ref>

During development, the jugal bone originates from [[dermal bone]].<ref>{{Cite book|last=Homberger, Dominique G.|title=Vertebrate dissection|date=2004|publisher=Thomson Brooks/Cole|others=Walker, Warren F. (Warren Franklin), Walker, Warren F. (Warren Franklin).|isbn=0-03-022522-1|edition=9th|location=Belmont, CA|oclc=53074665}}</ref>

=== In dinosaurs ===
This bone is considered key in the determination of general traits in cases in which the entire skull has not been found intact (for instance, as with [[dinosaur]]s in [[paleontology]]). In some dinosaur genera the jugal also forms part of the lower margin of either the [[antorbital fenestra]] or the infratemporal fenestra, or both.  Most commonly, this bone articulates with the quadratojugal, the postorbital, the [[lacrimal bone#In dinosaurs|lacrimal]], and the maxilla.<ref>Martin, A.J. (2006). Introduction to the Study of Dinosaurs. Second Edition. Oxford, Blackwell Publishing. pg. 299-300. {{ISBN|1-4051-3413-5}}.</ref>  In horned dinosaurs, like ''[[Pentaceratops]]'', the jugal bone is thick and comes to a point, which has led paleontologists to refer to it as the "jugal horn".<ref>Dodson, P. (1996). The Horned Dinosaurs. A Natural History.  346 pp. Princeton: Princeton University Press. {{ISBN|0 691 02882 6}}.</ref>

== Function ==
[[File:Gymnogyps californianus (Shaw, 1798).jpg|thumb|Skull of a California condor (''[[California condor|Gymnogyps californianus]]'')]]

=== In reptiles ===
{{Main|Quadratojugal bone#In reptiles and amphibians}}
The earliest reptiles primitively had a lower temporal bar (also referred to as a temporal arcade) where the posterior of the jugal bone contacts the [[quadratojugal]]. This structure has been repeatedly lost and regained in various groups.<ref>{{Cite journal |last=Müller |first=Johannes |date=2003-10-01 |title=Early loss and multiple return of the lower temporal arcade in diapsid reptiles |url=http://link.springer.com/10.1007/s00114-003-0461-0 |journal=Naturwissenschaften |volume=90 |issue=10 |pages=473–476 |doi=10.1007/s00114-003-0461-0 |pmid=14564408 |bibcode=2003NW.....90..473M |issn=0028-1042|url-access=subscription }}</ref>

=== In birds ===
While the jugal bone is thick and straplike in most other reptiles, the jugal bone is thin and strutlike in birds. This is thought to reduce the weight of the skull and facilitate [[cranial kinesis]].<ref>{{cite journal |last1=Wang |first1=Min |last2=Hu |first2=Han |year=2017 |title=A comparative morphological study of the jugal and quadratojugal in early birds and their dinosaurian relatives |journal=The Anatomical Record |volume=300 |issue=1 |pages=62–75 |doi=10.1002/ar.23446 |pmid=28000410|s2cid=3649504 |issn=1932-8494 |lang=en|doi-access=free }}</ref>

=== In mammals ===
[[File:Morganucodon skull.svg|thumb|Skull of stem-mammal ''[[Morganucodon]]'', with jugal labelled]]
In mammals, including humans, the jugal bone is more commonly referred to as the zygoma.<ref>{{Cite journal |last1=Dechow |first1=Paul C. |last2=Wang |first2=Qian |year=2017 |title=Evolution of the jugal/zygomatic bones |journal=The Anatomical Record |volume=300 |issue=1 |pages=12–15 |doi=10.1002/ar.23519 |doi-access=free |pmid=28000397 |issn=1932-8494 |lang=en}}</ref> It assists in constructing the facial contour, protecting the eye from damage, and providing attachment sites for facial muscles.<ref>{{Cite journal |last1=Gai |first1=Zhikun |last2=Yu |first2=Xiaobo |last3=Zhu |first3=Min |year=2017 |title=The evolution of the zygomatic bone from Agnatha to Tetrapoda |journal=The Anatomical Record |volume=300 |issue=1 |pages=16–29 |doi=10.1002/ar.23512 |pmid=28000409 |s2cid=3661931 |issn=1932-8494 |lang=en|doi-access=free }}</ref> The zygoma provides important functions as the origin of the [[masseter muscle]] and as a point of resistance for masticatory forces. Preliminary studies also indicate that variation in zygomatic structure may be useful in determining ancestral origins of modern human populations.<ref>{{cite journal |last1=Dechow |first1=Paul C. |last2=Wang |first2=Qian |year=2017 |title=Evolution of the jugal/zygomatic bones |journal=The Anatomical Record |volume=300 |issue=1 |pages=12–15 |doi=10.1002/ar.23519 |doi-access=free |pmid=28000397 |issn=1932-8494 |lang=en}}</ref>

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==References==
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{{Tetrapod osteology|S.}}

[[Category:Skull bones]]
[[Category:Bird anatomy]]

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