Epipubic bones are a pair of bones projecting forward from the
pelvic bones of modern
marsupials
Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a po ...
,
monotremes and fossil mammals like
multituberculates, and even
basal eutherians (the ancestors of
placental mammals, who lack them).
They first occur in non-mammalian
cynodonts such as
tritylodontids, suggesting that they are a
synapomorphy between them and
Mammaliformes.
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, and to the ribs and vertebrae by the pyramidalis, rectus abdominis, 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'' and '' Erythrotherium'', lack them;
in thylacines and sparassodonts, they appear to have become primarily cartilaginous and the osseous element has become strongly reduced or even absent.
'' Trichosurus'' mimicked placentals in shifting hypaxial muscles attachment sites from the epipubic to the pelvis
The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton).
The ...
, losing the respiratory 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
The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate mate ...
itself; epipubic bones stiffen the torso, preventing the expansion necessary for prolonged pregnancy.
This however apparently did not prevent large litter sizes; '' Kayentatherium'' 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
The evolution of mammals has passed through many stages since the first appearance of their synapsid ancestors in the Pennsylvanian sub-period of the late Carboniferous period. By the mid-Triassic, there were many synapsid species that looked l ...
* Obturator process
* Proximodorsal process
* Pelvic digit
References
{{reflist, 25em
Marsupial anatomy