A history of land mammals in the western hemisphere

by William Berryman Scott

1. Didelphiidæ. Opossums

In this family the dental formula is: i 5/4, c 1/1, p 3/3, m 4/4, × 2 = 50. The incisors are small and closely crowded together, the canines large and tusk-like, the premolars simple and of compressed-conical form; in existing species, the upper molars are triangular, each of the three main cusps is V-shaped and there are additional minute cusps along the outer border; the lower molars have a high anterior triangle of three pointed cusps and a low heel with several distinct cusps. The humerus has an epicondylar foramen and the feet are five-toed; in the manus all the digits are armed with claws and the thumb is but partially opposable, while in the pes the hallux is without a claw and completely opposable, making the foot much like that of a monkey.

The division of the existing opossums into genera has caused much difference of opinion and practice among naturalists; there are five groups, which by some are regarded as genera, and by others as subgenera, all modern members of the family being very much alike. The species Didelphis marsupialis, which is common in the eastern United States and extends through temperate North America, Central America and tropical South America, has a complete pouch and is chiefly arboreal and insectivorous in habit. In the woolly opossums (Caluromys) there is no pouch, and the young, when sufficiently advanced, are carried on the mother’s back, winding their tails around hers. In both of these genera the tail is long, naked and prehensile, but in the tiny species of Peramys the tail is short and hairy. Another Neotropical genus, Chironectes, the Yapock or Water Opossum, is the only existing instance of an aquatic marsupial. It has light grey fur, striped with brown, and webbed hind feet; living chiefly in the water, it feeds upon crayfish, water-insects and small fish.

The derivation of the modern North American opossums is a matter of great uncertainty. The present distribution of the family, with by far the greater number of its species confined to the Neotropical region, is certainly suggestive of a South American origin, but such considerations are very untrustworthy guides in tracing the history of animal groups. No opossum has been found in any North American formation between the Pleistocene and the lower Oligocene, though in the case of such small animals, negative evidence must be accepted with caution. In the White River Oligocene many minute opossums have been found and referred to the European genus †Peratherium, though it so closely resembles the modern Didelphis that many systematists do not make the distinction. In the Eocene, Paleocene and upper Cretaceous, opossums were represented doubtfully; the material is too incomplete for assured determination; in Europe they existed in the Oligocene and upper Eocene. In South America the family went back uninterruptedly to the oldest mammal-bearing beds of Patagonia, which may be upper Cretaceous. The opossums are thus the remnants of an exceedingly ancient group, whose beginnings are to be sought in the Mesozoic era and which was probably spread over all the continents. To all appearances, the whole group vanished completely from the northern hemisphere, but reëntered North America from the south at some time during the Pliocene or early Pleistocene and permanently established itself here.

The opossums are the most primitive of existing marsupials, especially the little South American genus, Marmosa, and are regarded, by some of the most competent students of the order, as closely representing the ancestral type of all the Recent families and genera, both of the Polyprotodonta and Diprotodonta.

2. Thylacynidæ. Predaceous Marsupials

By many naturalists this group of flesh-eating forms is included in the Dasyuridæ. The family never entered North America, but played a very important part in the Tertiary of South America. Three existing genera of the Australian region throw considerable light upon the South American types, and therefore some account of them will not be out of place here.

The largest of modern predaceous marsupials is the animal (Thylacynus cynocephalus) erroneously, but very naturally, called the “Tasmanian Wolf,” now confined to Tasmania, but occurring also in the Pleistocene of Australia. As “wolf” applied to a marsupial is misleading, it will be less confusing to employ the anglicized form of the generic name “Thylacine.” This animal is of the size of the small Prairie Wolf or Coyote (Canis latrans) and has very wolf-like appearance and habits. The muzzle is long and pointed, the ears erect and rather small, the tail long, very thick at the base and tapering to the end, not bushy, but covered with short, close-set hairs; the colour is greyish brown, with dark, transverse stripes on the posterior half of the back and base of the tail. Apparently the creature is in process of losing its stripes and acquiring the solid body-colour. The dental formula is: i 4/3, c 1/1, p 2/2, m 4/4, × 2 = 46; the incisors are small, the canines large fangs, and the premolars simple; the upper molars are tritubercular, with large inner cusp and postero-external cutting ridge, and the lower molars are trenchant, with low heel. The whole dentition is remarkably like that of many Eocene †creodonts, such as †Sinopa and †Tritemnodon (see [p. 566]). The milk-premolar is small and functionless and is shed very early. The skull is very wolf-like in appearance, but thoroughly marsupial in structure, and has the large palatal vacuities common in the order. The marsupial bones do not ossify and are evidently on the point of disappearance. There are five digits in the manus, four in the pes, the hallux being completely suppressed. In habits, the Thylacine is carnivorous and so destructive to sheep that the farmers have nearly exterminated it.