A history of land mammals in the western hemisphere

by William Berryman Scott

†Polymastodon, up. Cretac. and Paleoc., N. A. †Ptilodus, do. †Neoplagiaulax, Paleoc., N. A.

VI. †Polydolopidæ.

†Propolymastodon, Casa Mayor. †Polydolops, do. †Amphidolops, do.

Despite all their diversity of appearance and habits, the unity of structure among the marsupials is such that the formation of groups of higher than family rank is very difficult, and it is by no means certain that the suborders currently accepted correspond to the facts of actual relationship.

Except in certain extinct South American genera, there is very little change of teeth, only the last premolar in each jaw being replaced. Sometimes the temporary tooth is long retained in function and, more rarely, it is shed very early; while in several genera no replacement of teeth has been observed. There is a difference of opinion among naturalists as to the proper interpretation of the marsupial dentition. According to one view, all except the replaced premolar belong to the milk-series and the permanent series has been lost; the alternative and more probable belief is that the milk-dentition has been almost or completely suppressed. Whichever one of these interpretations be the right one, there is strong reason to maintain that the very limited amount of change is not a primitive condition, but a secondary one, for a series of rudimentary teeth is formed before those which are to become functional. The only reasonable explanation of such a condition is that it has been derived from one in which the normal succession and replacement of the teeth took place. Something of the same sort has been observed in the simplicidentate rodents. The marsupial dentition differs from the placental one in the usual number of four molars, instead of three, and frequently also in exceeding the normal total number of 44. The incisors are almost always of a different number in the upper and the lower jaw and are frequently more numerous than in the placentals.

The skeleton has several diagnostic characters, which are present throughout the order, though one or other of these features may be absent in particular instances. The skull has a very small brain-capacity and elongate face and jaws. In the placental mammals, the sutures between adjoining bones of the skull tend to close by coössification, and the separate bones are clearly distinguishable only in young animals; but in the marsupials the sutures remain open for a much longer period. The lachrymal is expanded on the face and the foramen is outside of the orbit. The tympanic is a mere ring and permanently separate from the other bones of the cranium, while a false bulla is formed by the inflation of part of the alisphenoid. In almost all marsupials there are large openings or vacuities in the bony palate. One of the most characteristic and constant features of the marsupial skull is in the conformation of the angle of the lower jaw, which is turned inward, or inflected, at nearly a right angle with the body of the jaw. It is true that one existing Australian genus has lost this character; and in some of the placental orders, especially the Rodentia, a somewhat similar structure may occasionally be found, but it is never quite the same as in the marsupials, in which it goes back to a remote antiquity.

There are very constantly 19 trunk-vertebræ, of which usually 13 are dorsals. The tail differs greatly in length in the various genera, but most of them have well-developed tails. An additional pair of elements, besides the three which are found in the placentals, enter into the composition of the hip-bones; these are the marsupial bones, slender, flattened rods, directed forward in the abdominal wall and diverging in V-shape. Save in a few genera, clavicles are present and of full size. The humerus may or may not have the epicondylar foramen, but the femur never has the third trochanter. The feet vary greatly in form and structure, in accordance with the habits, but there is a very widespread adaptation to an arboreal life, and even in terrestrial and burrowing forms more or less distinct traces of this arboreal adaptation may be noted. This fact has led to a generally accepted inference that all existing marsupials had an arboreal ancestry.

The soft parts and more especially the organs of reproduction are likewise very characteristic, and one or two of these peculiarities may be mentioned. (1) In the female, the vagina is double and on the abdomen is the pouch, or marsupium (which gives its name to the order), a hair-lined bag, opening either forward or backward, which serves to carry the young and into which the teats open. A considerable number of species have lost the marsupium, while other species of the same genera retain it, and there can be little question that its absence is a secondary condition. (2) Except in one modern Australian genus, the marsupials have no true placenta, and the young are born in a very immature state, incapable of even swallowing. The new-born young are transferred to the nipples of the mother and are attached to these and fed by the pumping of milk into their mouths by muscular action of the mother. A special, though temporary, arrangement of the gullet and windpipe is provided, so that the helpless young animal shall not be suffocated by the entrance of milk into the lungs.

Suborder Polyprotodonta

This suborder, as is indicated by its name, is characterized by its numerous incisors, which are 5/4, or 4/3, and none of them is especially enlarged; by the large canines in both jaws, simple premolars and tritubercular upper molars. The members of this group are carnivorous or insectivorous in habit, and all the existing ones are of small or moderate size, though some very large extinct forms are known. Except in one Australian family, the feet are not “syndactyl,” a term which means the enclosure of two or more digits in one fold of skin. The only existing American representatives of the suborder are the opossums, the great majority of which are Neotropical in distribution.