Dromiciops gliroides Thomas, 1894, Last of the
Microbiotheria (Marsupialia), with a Review of the
Family Microbiotheriidae
Philip Hershkovitzf
Abstract
The tiny Chilean and bordering Argentinian endemic Dromiciops gliroides Thomas, lone
survivor of the South American Cohort Microbiotheriomorphia, is basal to all known marsu-
pials. 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 Noth-
ofagus-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 Nothofagus-
Chusquea-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.
Introduction
The monito del monte, Dromiciops gliroides
Thomas (frontispiece; Plates 1, 2), a mouse opos-
sum with a simian vernacular name, is the lone
survivor of the South American marsupial family
Microbiotheriidae Ameghino, 1887. The monito
del monte and a dozen or so known extinct spe-
cies of the same family compose the monophy-
letic Cohort Microbiotheriomorphia Ameghino.
Together with Cohort Didelphimorphia Gill, 1872,
it constitutes the Infraclass Marsupialia, also
known as Metatheria. Reig (1955) was the first to
recognize the relationship between the living
Dromiciops and the extinct microbiotheriid mar-
supial then known only from the Miocene.
The Microbiotheriomorphia, with its single
known family Microbiotheriidae, was reviewed
by Marshall (1982), with attention given to cra-
nial, dental, and external characters. The clade
was treated as a family of Didelphoidea, its dis-
tinctive characters unperceived except for men-
tion of small canine teeth. It has since been de-
termined (Hershkovitz, 1992a) that microbiothe-
riids are monophyletic and must have arisen at
some time before the divergence of didelphoids,
FIELDIANA: ZOOLOGY, N.S., NO. 93, MAY 28, 1999, PP. 1-60
BioStor
