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Epipubic bones labeled as 10 Skeleton of a red-necked wallaby, centered on the epipubic bones. Epipubic bones are a pair of bones projecting forward from the pelvic bones of modern marsupials , monotremes and fossil mammals like multituberculates , and even basal eutherians (the ancestors of placentals , who lack them). [ 1 ]
Placental mammals are anatomically distinguished from other mammals by: a sufficiently wide opening at the bottom of the pelvis to allow the birth of a large baby relative to the size of the mother. [4] the absence of epipubic bones extending forward from the pelvis, which are found in all other mammals. [5]
Epipubic bones extend forwards from the pelvis; [1] these are not found in any placental, but are found in all other mammals, including non-placental eutherians, marsupials, monotremes and other Mesozoic mammals as well as in the cynodont therapsids that are closest to mammals.
However, some primitive mammals, as well as fossil ancestors of the Cretaceous higher mammals also exhibit these bones. It can therefore be assumed that the epipubic bones were an ancestral trait of mammals that has been reduced in today's placentals, and that no morphological evidence exists for the Marsupionta hypothesis.
Hadrocodium lacks the multiple bones in its lower jaw seen in reptiles. These are still retained, however, in earlier mammaliaforms. [18] With the possible exception of Megazostrodon and Erythrotherium (as well as placental mammals), [19] all mammaliforms possess epipubic bones, a possibly synapomorphy with tritylodontids, which also have them ...
All extant eutherians lack epipubic bones, which are present in all other living mammals (marsupials and monotremes). This allows for expansion of the abdomen during pregnancy, [ 1 ] though epipubic bones are present in many primitive eutherians. [ 2 ]
The surangular is a long, curved bone at the top of the lower jaw, just behind the teeth, present in nearly every vertebrate living or extinct, apart from mammals. Muscles attached to this bone ...
Non-placental mammals possess osteological projections of the pubis known as epipubic bones. These evolved first among derived cynodonts , and evolved as a means of support for muscles flexing the thigh, facilitating the development of an erect gait. [ 5 ]