Dromiciops gliroides Thomas, 1894, last of the Microbiotheria (Marsupialia), with a review of the family Microbiotheriidae / Philip Hershkovitz.

Abstract

The tiny Chilean and bordering Argentinian endemic Dromiciops gliroides Thomas, lone survivor of the South American Cohort Microbiotheriomorphia, is basal to all known marsupials. Indications are that the ancestral microbiothere may have originated in South America during the Late Mesozoic in a cool, humid, low-latitude biome dominated by a beech-bamboo (Nothofagus-Chusquea) plant association, the bamboo being its primary nesting material. A postulated worldwide climatic warming initiated a southward migration of cool-loving elements of the Southern Hemisphere. The philopatric microbiotheres evidently clung to their Nothofa- gus-Chusquea niche and nesting material as the association shifted from subequatorial into Patagonian and Antarctic latitudes. A climatic reversal during the Tertiary returned the Nothofagus-Chusquea-vmcvobioihcvc association to Patagonian latitudes. An expanding arid scrub savanna farther north, however, halted the Nothofagus community shift in that direction. At the same time, habitat was being lost to the increasingly colder climate advancing from the south. Extinction followed. The most northern progression of the association was in the west, where rains intercepted by the rising Andes provided a favorable environment. The NothofagusChusquea-microbiothere association now survives in the cool, humid Valdivian region of Chile and its narrow Argentine extension as a relictual enclave sharply delimited by the warmer arid environment to the north and the colder, drier environment to the south. The present ecogeographically restricted Cohort Microbiotheriomorphia is described and compared with its sister Cohort Didelphimorphia, which, by virtue of its adaptability, fecundity, and diversity, had dispersed into all continents. The narrow climatically controlled distribution and what little is known of the life history and anatomy of the single surviving microbiothere, Dromiciops gliroides, are reviewed.