This order of marsupials contains one family (Microbiotheriidae) with one living species (Dromiciops australis, common name "monito del monte"). It also includes a genus of fossil species, Microbiotherium, with six known species that lived during Oligocene and Miocene times. The order is known only from South America; currently, Dromiciops is found in the Andes of Chile and Argentina.
While not diverse, microbiotheres are especially interesting because they may be more closely related to Australian marsupials (Cohort Australidelphia) than to any South American family. This tentative conclusion is based on both anatomical and molecular investigations (summarized by Aplin and Archer, 1987). Research on the relationships of microbiotheres and other marsupials, however, is an active and fascinating field, and the last word on the phylogeny of these animals probably has yet to be written.
These evolutionary ties between the South American Microbiotheriidae and the Australian marsupials help to illuminate the biogeographic history of the Metatheria. The current hypothesis of metatherian biogeography holds that metatheres (marsupials) originated in North America, dispersed to South America (where they underwent a spectacular radiation), moved across Antarctica and into Australia where they enjoyed a second radiation. The Microbiotheriidae are either a relict of the line leading to the Australian marsupials, or they are the result of a reinvasion of South America by Australian forms. Additional fossil evidence and comparative studies may help to resolve this question.
Dromiciops is a small animal with a long, moderately prehensile tail and silky, dense pelage. Its ears are short and rounded. The tail is long and may serve as a site for storing fat to maintain the animal during hibernation. The pouch or marsupium is well developed. Dromiciops has a peculiar skull, especially compared to that of the didelphids (with which it was once classified). The premaxillae is elongated; the nasals are expanded posteriorly; and paroccipital processes are absent. The auditory bullae are unique: large, inflated, ossified, with the anterior third of the bullar wall formed by a large process of the alisphenoid. The teeth are also unusual. The upper and lower incisors are broad and spatulate. The uppers form a semicircle or U-shaped dental arcade, while the lowers are splayed outward. The cusps of the cheekteeth are low and rounded compared to those of didelphids. The dental formula, however, is identical to that of the Didelphidae (5/4, 1/1, 3/3, 4/4 = 50), and like didelphids, Dromiciops is polyprotodont and not syndactylous.
Monitos del monte live in dense, humid vegetation in the mountains of southern Chile and Argentina. They have a special liking for thickets of bamboo. Monitos del monte are primarily insectivorous, feeding in the trees as well as on the ground, but they may occasionally consume vegetation.
The fossil genus Microbiotherium is known from the late Oligocene. An early Paleocene mammal, Khasia, may also have belonged to this family.
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