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Epipubic bones are a pair of bones projecting forward from the
pelvic bones The hip bone (os coxae, innominate bone, pelvic bone or coxal bone) is a large flat bone, constricted in the center and expanded above and below. In some vertebrates (including humans before puberty) it is composed of three parts: the ilium, ischi ...
of modern marsupials, monotremes and fossil mammals like
multituberculates Multituberculata (commonly known as multituberculates, named for the multiple tubercles of their teeth) is an extinct order of rodent-like mammals with a fossil record spanning over 130 million years. They first appeared in the Middle Jurassic, ...
, and even basal
eutheria Eutheria (; from Greek , 'good, right' and , 'beast'; ) is the clade consisting of all therian mammals that are more closely related to placentals than to marsupials. Eutherians are distinguished from noneutherians by various phenotypic tra ...
ns (the ancestors of placental mammals, who lack them). They first occur in non-mammalian
cynodonts The cynodonts () (clade Cynodontia) are a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 mya), and extensively diversified after the Permian–Triassic extinction event. Cynodonts had a wide variety ...
such as
tritylodontids Tritylodontidae ("three-knob teeth", named after the shape of their cheek teeth) is an extinct family of small to medium-sized, highly specialized mammal-like cynodonts, bearing several mammalian traits like erect limbs, endothermy and details o ...
, suggesting that they are a synapomorphy between them and
Mammaliformes Mammaliaformes ("mammalian forms") is a clade that contains the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts. It is defined as the clade originating from the most recent ...
. They were first described as early as 1698, but to date, their function(s) remain unresolved. Epipubic bones are often called ''marsupial bones'' because they support the mother's pouch in modern marsupials ("''marsupium''" is Latin for "pouch").


Function

Some writers have suggested 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 The pectineus muscle (, from the Latin word ''pecten'', meaning comb) is a flat, quadrangular muscle, situated at the anterior (front) part of the upper and medial (inner) aspect of the thigh. The pectineus muscle is the most anterior adductor o ...
, and to the ribs and vertebrae by the pyramidalis,
rectus abdominis The rectus abdominis muscle, ( la, straight abdominal) also known as the "abdominal muscle" or simply the "abs", is a paired straight muscle. It is a paired muscle, separated by a midline band of connective tissue called the linea alba. It exte ...
, and external and internal obliques. According to this hypothesis, the epipubic bones act as levers to stiffen the trunk during locomotion, and aid in breathing. Others have suggested that epipubic bones may constrain asymmetrical gaits, although this appears not to be the case.


Occurrence

Only placentals, and possibly the early mammaliformes ''
Megazostrodon ''Megazostrodon'' is an extinct genus of basal mammaliaforms belonging to the order Morganucodonta. It is approximately 200 million years old.
'' and ''
Erythrotherium ''Erythrotherium'' (meaning "red beast") is an extinct genus of basal mammaliaforms from the Late Triassic to Lower Jurassic. It is related to ''Morganucodon''. Only one species is recorded, ''Erythrotherium parringtoni'', from Red Beds, Stormbe ...
'', lack them; in
thylacines The thylacine ( , or , also ) (''Thylacinus cynocephalus'') is an extinct carnivorous marsupial that was native to the Australian mainland and the islands of Tasmania and New Guinea. The last known live animal was captured in 1930 in Tasman ...
and
sparassodonts Sparassodonta (from Greek to tear, rend; and , gen. , ' tooth) is an extinct order of carnivorous metatherian mammals native to South America, related to modern marsupials. They were once considered to be true marsupials, but are now thoug ...
, they appear to have become primarily cartilaginous and the osseous element has become strongly reduced or even absent. ''
Trichosurus The brushtail possums are the members of the genus ''Trichosurus'' in the Phalangeridae, a family of marsupials. They are native to Australia (including Tasmania) and some small nearby islands. Unique among marsupials, they have shifted the hypa ...
'' mimicked placentals in shifting
hypaxial In adult vertebrates, trunk muscles can be broadly divided into hypaxial muscles, which lie ventral to the horizontal septum of the vertebrae and epaxial muscles, which lie dorsal to the septum. Hypaxial muscles include some vertebral muscles, th ...
muscles attachment sites from the epipubic to the pelvis, losing the
respiratory The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies gre ...
benefits (see below), but otherwise retains large epipubics. Epipubic bones show sexual size dimorphism. 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 Flex or FLEX may refer to: Computing * Flex (language), developed by Alan Kay * FLEX (operating system), a single-tasking operating system for the Motorola 6800 * FlexOS, an operating system developed by Digital Research * FLEX (protocol), a com ...
the thigh. 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 itself; epipubic bones stiffen the torso, preventing the expansion necessary for prolonged pregnancy. This however apparently did not prevent large litter sizes; ''
Kayentatherium ''Kayentatherium'' is an extinct genus of tritylodontid cynodonts that lived during the Early Jurassic. It is one of two tritylodonts from the Kayenta Formation of northern Arizona, United States. ''Kayentatherium'' means "Kayenta Beast", and ...
'' is now known to have given birth to litters of 38 undeveloped young, a considerably higher number than living monotremes or marsupials. However, vestiges of the epipubic bone may survive in a common placental characteristic, the
baculum The baculum (also penis bone, penile bone, or ''os penis'', ''os genitale'' or ''os priapi'') is a bone found in the penis of many placental mammals. It is absent from the human penis, but present in the penises of some primates, such as the ...
.


See also

* Evolution of mammals *
Obturator process The obturator process is an anatomical feature on the pelvis of archosaurs. It is a raised area of the ischium bone of the pelvis.Romer, Alfred S. (1923) "The Pelvic Musculature of Saurischian Dinosaurs" "Bulletin of the American Museum of Natural ...
*
Proximodorsal process The proximodorsal process is a feature of the skeleton of archosaurs. It may be a pair of tabs or blade - shaped flanges on the pelvis, and serves as an anchor point for the attachment of leg muscles. This process is of particular importance in the ...
* Pelvic digit


References

{{reflist, 25em Marsupial anatomy