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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 ]
Baculum of a dog's penis; the arrow shows the urethral sulcus, which is the groove in which the urethra lies. Fossil baculum of a bear from the Miocene. The baculum (pl.: bacula), also known as the penis bone, penile bone, os penis, os genitale, [1] or os priapi, [2] is a bone in the penis of many placental mammals.
Figure 1:In mammals, the quadrate and articular bones are small and part of the middle ear; the lower jaw consists only of dentary bone.. While living mammal species can be identified by the presence of milk-producing mammary glands in the females, other features are required when classifying fossils, because mammary glands and other soft-tissue features are not visible in fossils.
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]
Since these are present in males and pouchless species, it is believed that they originally had nothing to do with reproduction, but served in the muscular approach to the movement of the hind limbs. This could be explained by an original feature of mammals, as these epipubic bones are also found in monotremes. Marsupial reproductive organs ...
The pelvis is made up of three paired bones and a sacrum. The three paired bones are called the ilium, the ischium, and the pubis. On the ischium there may be an obturator process and/or a proximodorsal process. The more primitive condition is for there to be no proximodorsal process, but a large obturator process. In primitive birds the ischia ...
This is the plesiomorphic condition among viviparous mammals; the presence of epipubic bones in all non-placentals prevents the expansion of the torso needed for full pregnancy. [80] Even non-placental eutherians probably reproduced this way. [41]
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 ]