![]() They are fed milk produced by mammary glands the milk is secreted onto the skin within the pouch and sucked or lapped up by the babies. They are protected in a temporary pouch in echidnas but not platypuses. The young, which are tiny and at a very early stage of development when they hatch, break out of the eggs using a "milk tooth. ![]() Like the eggs of birds, monotreme eggs are incubated and hatched outside the body of the mother. Only the left ovary is functional in the platypus, but both produce eggs in the echidna. ![]() They contain a large yolk, which is concentrated at one end of the egg very much like the yolk of a bird's egg. The number of eggs laid is small, usually 1-3, and they are placed in the mother's pouch. The eggs layed by monotremes are small (13-15 mm diameter) and covered by a leathery shell. Monotremes are endothermic, but they have unusually low metabolic rates and maintain a body temperature that is lower than that of most other mammals. It is now thought that epipubic bones are a vestige of the skeleton of therapsids, providing members of that group with extra attachments for abdominal muscles to support the weight of the hindquarters. They also occur in all species of marsupials, whether a pouch is present or not (not all marsupials have a pouch). Epipubic bones were originally thought to be related to having a pouch, but they are found in both males and females. Vertebrae as well as the chest (thoracic) vertebrae in all other modern mammals, they are restricted to the thoracic region.Īnother interesting skeletal characteristic of monotremes is the largeĮpipubic bones in the pelvic region. Humerus are held roughly parallel to the ground when the animal walks, more in the fashion of therapsids and most modern reptiles than like modern mammals. The shoulder girdle is much more rigidly attached to the axillary skeleton than in other mammals. The scapula, however, is simplified, lacking a supraspinous fossa. Shoulder girdles are complex, including the standard components of modern mammals (Ĭlavicle), but also additional elements including It is a fascinating mosaic of primitive characteristics inherited from therapsids but found in no other living mammals, and modifications probably related to the burrowing habits of modern monetremes. ![]() Postcranially, the skeleton of monotremes is also unique among mammals. On the skulls, theĭentary is a slender bone with only a vestige of aĪngle of the dentary is not inflected medially (unlike that of marsupials),Īuditory bullae are missing (part of the middle ear is enclosed by tympanic rings), and much of the wall of the braincase is made up by the petrosal rather than theĪlisphenoid (unlike all other modern mammals). Monotremes have several important mammalian characters, however, including fur (but they lack vibrissae), a four chambered heart, a single dentary bone, three middle ear bones, and the ability to lactate.īesides the absence of teeth, lacrimals, and obvious sutures, monotremes share a number of skeletal characteristics. Modern monotremes lack teeth as adults sutures are hard to see the rostrum is elongate, beak-like, and covered by a leathery sheath and lacrimal bones are absent. The skulls of monotremes are almost birdlike in appearance, with a long rostrum and smooth external appearance. They retain many characters of their therapsid ancestors (for example, a complex pectoral girdle, laying of eggs rather than bearing live young, limbs oriented with humerus and femur held lateral to body, and a cloaca). They are often placed in a separate subclass from other mammals, Prototheria. Monotremes probably split from the lineage leading to other mammals sometime in the Mesozoic. ![]()
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