Besides the absence of teeth, lacrimals, and obvious sutures, prototherians share a number of skeletal characteristics. On the skulls, the jugals are reduced or absent, the dentary is a slender bone with only a vestige of a coronoid process, the angle of the dentary is not inflected medially (unlike that of marsupials), auditory 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 the alisphenoid (unlike all other modern mammals). Postcranially, the skeleton of prototherians is also unique among mammals. 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 prototherians. Their shoulder girdles are complex, including the standard components of modern mammals ( scapula and clavicle), but also additional elements including coracoid, epicoracoid, and interclavicle. The scapula, however, is simplified, lacking a supraspinous fossa. The shoulder girdle is much more rigidly attached to the axillary skeleton than in other mammals. Femur and 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. Ribs are found on the neck (cervical) vertebrae as well as the chest (thoracic) vertebrae; in all other modern mammals, they are restricted to the thoracic region.
Another interesting skeletal characteristic of prototherians is the large epipubic bones in the pelvic region. Epipubic bones were originally thought to be related to having a pouch, but they are found in both males and females. They also occur in all species of marsupials, whether a pouch is present or not (not all marsupials have a pouch). 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.
Prototherians are endothermic, but they have unusually low metabolic rates and maintain a body temperature that is lower than that of most other mammals.
All male prototherians have spurs on their ankles that are presumed to be used in fighting and in defense. In one family (Ornithorhynchidae), a groove along the spur carries poison secreted by adjacent glands.
Other Physical Features: endothermic ; homoiothermic; bilateral symmetry ; venomous