# Epipubic bone

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{{Short description|Pair of frontal pelvic bones found in certain orders of mammals}}
[[Image:Macropus major hind legs.png|thumb|Skeleton of [eastern grey kangaroo](/source/eastern_grey_kangaroo) hind legs. Epipubic bones labeled as 10]]
[[Image:Paris - Université Pierre-et-Marie-Curie - Macropus rufogriseus 17.jpg|thumb|Skeleton of a [red-necked wallaby](/source/red-necked_wallaby), centered on the epipubic bones.]]

'''Epipubic bones''' are a pair of bones projecting forward from the [pelvic bones](/source/pelvis) of modern [marsupials](/source/marsupials), [monotremes](/source/monotremes) and fossil mammals like [multituberculates](/source/Multituberculata), and even [basal](/source/basal_(phylogenetics)) [eutheria](/source/eutheria)ns (the ancestors of [placental](/source/placental)s, who lack them).<ref>
{{cite journal
 | last1=Novacek   | first1=M.J.     | last2=Rougier  | first2=G.W.
 | last3=Wible     | first3=J.R.     | last4=McKenna  | first4=M.C.
 | last5=Dashzeveg | first5=D.       | last6=Horovitz | first6=I.
 | year=1997
 | title=Epipubic bones in eutherian mammals from the late Cretaceous of Mongolia
 | journal=Nature
 | volume=389 | issue=6650 | pages=440–1
 |pmid=9333234 | doi=10.1038/39020
 | bibcode=1997Natur.389..483N | s2cid=205026882
}}
</ref>
They first occur in non-mammalian [cynodonts](/source/Cynodont) such as [tritylodontids](/source/Tritylodontidae), suggesting that they are a [synapomorphy](/source/synapomorphy) between them and [Mammaliformes](/source/Mammaliformes).

They were first described as early as 1698, but to date, their function(s) remain unresolved.<ref name=Reilly-White-2003-01-17/> Epipubic bones are often called ''marsupial bones'' because they support the mother's [pouch](/source/Pouch_(marsupial)) in modern marsupials ("''marsupium''" is Latin for "pouch").<ref name=White-1989/>

==Function==
Some writers have suggested<ref name=Reilly-White-2003-01-17>
{{cite journal
 | author1 = Reilly, S.M.  | author2 = White, T.D.
 | date = 2003-01-17
 | title = Hypaxial motor patterns and the function of epipubic bones in primitive mammals
 | journal = Science
 | volume = 299 | issue = 5605 | pages = 400–402
 | doi=10.1126/science.1074905 | bibcode = 2003Sci...299..400R
 | s2cid = 41470665 | pmid=12532019
}}
</ref>
that the epipubic bones are a part of a kinetic link stretching from the femur on one side, to the ribs on the opposite side. This linkage is formed by a series of muscles: Each epipubic bone is connected to the femur by the [pectineus muscle](/source/pectineus_muscle), and to the ribs and vertebrae by the [pyramidalis](/source/Pyramidalis_muscle), [rectus abdominis](/source/Rectus_abdominis_muscle), and [external](/source/Abdominal_external_oblique_muscle) and [internal obliques](/source/Abdominal_internal_oblique_muscle). According to this hypothesis, the epipubic bones act as levers to stiffen the trunk during locomotion, and aid in breathing.<ref>
{{cite journal
 | author1 = Reilly, Stephen M.
 | author2 = Mcelroy, Eric
 | author3 = White, Thomas D.
 | year = 2009
 | title = Abdominal muscle function in ventilation and locomotion in new world opossums and basal eutherians: Breathing and running with and without epipubic bones
 | journal = Journal of Morphology
 | volume = 270  | issue = 8  | pages = 1014–1028
 | doi = 10.1002/jmor.10735 | pmid = 19274744
 | s2cid = 13458526
}}
</ref>
Others have suggested that epipubic bones may constrain asymmetrical gaits, although this appears not to be the case.<ref>
{{cite journal
 |author1=Schilling, Nadja
 |author2=Hackert, Rémi
 |year=2006
 |title=Sagittal spine movements of small therian mammals during asymmetrical gaits
 |journal=Journal of Experimental Biology
 |volume=209 |issue=19
 |pages=3925–3939
 |doi=10.1242/jeb.02400
|pmid=16985208
 |s2cid=258375
 |url=https://hal.archives-ouvertes.fr/hal-00365971/file/3925.pdf
 }}
</ref>

==Occurrence==
Only placentals, and possibly the early mammaliformes ''[Megazostrodon](/source/Megazostrodon)'' and ''[Erythrotherium](/source/Erythrotherium)'', lack them;<ref>
{{cite book
 |author1=Lillegraven, Jason A.
 |author2=Kielan-Jaworowska, Zofia
 |author3=Clemens, William A.
 |date=1979-12-17
 |title=Mesozoic Mammals: The first two-thirds of mammalian history
 |publisher=University of California Press
 |page=321
}}
</ref>
in [thylacines](/source/thylacines) and [sparassodonts](/source/sparassodonts), they appear to have become primarily cartilaginous and the osseous element has become strongly reduced or even absent.<ref>
{{cite book
 |last=Marshall |first=L.
 |year=1978
 |title=Evolution of the Borhyaenidae – extinct South American predaceous marsupials
 |place=Berkeley, CA
 |publisher=University of California Press
}}
</ref><ref>
{{cite web
 |title=Thylacine skeleton
 |website=naturalworlds.org
 |url=http://www.naturalworlds.org/thylacine/biology/anatomy/skullandskeleton/skeleton/skeleton_1.htm
}}
</ref>

''[Trichosurus](/source/Trichosurus)'' mimicked placentals in shifting [hypaxial](/source/hypaxial) muscles attachment sites from the epipubic to the [pelvis](/source/pelvis), losing the [respiratory](/source/respiration_(physiology)) benefits (see below), but otherwise retains large epipubics.<ref>
{{cite journal
 | vauthors = Reilly SM, McElroy EJ, White TD, Biknevicius AR, Bennett MB
 | date = April 2010
 | title = Abdominal muscle and epipubic bone function during locomotion in Australian possums: Insights to basal mammalian conditions and Eutherian-like tendencies in Trichosurus
 | journal = J. Morphol.
 | volume = 271 | issue = 4 | pages = 438–450
 | doi = 10.1002/jmor.10808
 | pmid = 19862837 | s2cid = 18709271
| doi-access = free
 }}
</ref>
Epipubic bones show sexual size dimorphism.<ref>
{{cite journal
 |last=Nasoori |first=Alireza
 |year=2020
 |title=Formation, structure, and function of extra‐skeletal bones in mammals
 |journal=Biological Reviews
 |volume=95  |issue=4  |pages=986–1019
 |doi=10.1111/brv.12597
 |pmid=32338826  |s2cid=216556342
 |url=https://archive.org/details/formation-structure-and-function-of-extra-skeletal-bones-in-mammals
}}
</ref>

In modern marsupials, the epipubic bones are often called "marsupial bones" because they support the mother's pouch ("''marsupium''" is Latin for "pouch"), but their presence on other groups of mammals indicates that this was not their original function, which some researchers think was to assist locomotion by supporting some of the muscles that [flex](/source/flexion) the thigh.<ref name=White-1989>
{{cite journal
 | author=White, T.D.
 | date=9 August 1989
 | title=An analysis of epipubic bone function in mammals using scaling theory
 | journal=Journal of Theoretical Biology
 | volume=139  | issue=3  | pages=343–57
 | pmid=2615378  | doi=10.1016/S0022-5193(89)80213-9
 | bibcode=1989JThBi.139..343W
}}
</ref>

Placentals are the only mammal lineage that lacks epipubic bones, and this absence has been considered to be correlated to the development of the [placenta](/source/placenta) itself; epipubic bones stiffen the torso, preventing the expansion necessary for prolonged pregnancy.<ref>
{{cite book
 |author1 = Power, Michael L.
 |author2 = Schulkin, Jay
 |date=2012-10-01
 |title=The Evolution of the Human Placenta
 |isbn=9781421406435
 |url=https://books.google.com/books?id=G1exWxU3QHIC&dq=epipubic+placenta&pg=PA68
 |page=68
}}
</ref>
This however apparently did not prevent large litter sizes; ''[Kayentatherium](/source/Kayentatherium)'' is now known to have given birth to litters of 38&nbsp;undeveloped young, a considerably higher number than living monotremes or marsupials.<ref>
{{cite journal
 | author1 = Hoffman, Eva A.
 | author2 = Rowe, Timothy B.
 | year = 2018
 | title = Jurassic stem-mammal perinates and the origin of mammalian reproduction and growth
 | journal = Nature
 | volume = 561 | issue = 7721 | pages = 104–108
 | doi = 10.1038/s41586-018-0441-3 | pmid = 30158701
 | bibcode = 2018Natur.561..104H | s2cid = 205570021
}}
</ref>
However, vestiges of the epipubic bone may survive in a common placental characteristic, the [baculum](/source/baculum).<ref name="Szalay2006">
{{cite book
 |author=Szalay, Frederick S.
 |date=11 May 2006
 |title=Evolutionary History of the Marsupials and an Analysis of Osteological Characters
 |publisher=Cambridge University Press
 |isbn=978-0-521-02592-8
 |url=https://books.google.com/books?id=HmQTTChULOMC&dq=marsupial+baculum&pg=PA293
 |page=293
}}
</ref> 

Additionally, the capacity for epipubic bones to prevent long term pregnancies has been debated in a 2022 study on [multituberculate](/source/multituberculate)s, which proved that they were capable of producing young as developed as those of placentals despite their epipubics.<ref>Urton, James (25 July 2022). [https://www.washington.edu/news/2022/07/25/primitive-mammal-reproduction/ "New Study Challenges Old Views on What's 'Primitive' in Mammalian Reproduction"]. UW News.
* {{Cite journal|last1=Weaver |first1=Lucas N. |last2=Fulghum |first2=Henry Z. |last3=Grossnickle |first3=David M. |last4=Brightly |first4=William H. |last5=Kulik |first5=Zoe T. |last6=Wilson Mantilla |first6=Gregory P. |last7=Whitney |first7=Megan R. |date=September 2022 |title=Multituberculate Mammals Show Evidence of a Life History Strategy Similar to That of Placentals, Not Marsupials |url=https://www.journals.uchicago.edu/doi/10.1086/720410 |url-access=subscription |journal=The American Naturalist |volume=200 |issue=3 |pages= |doi=10.1086/720410 |access-date=14 August 2025}}</ref>

== See also ==
* [Evolution of mammals](/source/Evolution_of_mammals)
* [Obturator process](/source/Obturator_process)
* [Pelvic digit](/source/Pelvic_digit)
* [Proximodorsal process](/source/Proximodorsal_process)

==References==
{{Reflist|25em}}

== External links ==
* {{Wiktionary-inline}}

Category:Marsupial anatomy

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Adapted from the Wikipedia article [Epipubic bone](https://en.wikipedia.org/wiki/Epipubic_bone) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Epipubic_bone?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.