Microbiotheria

Microbiotheres Fossil range: Early Paleocene–Recent PreЄ Є O S D C P T J K Pg N
Dromiciops gliroides
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Mammalia Infraclass: Marsupialia Superorder: Australidelphia Order: Microbiotheria Ameghino, 1889 Family: Microbiotheriidae Ameghino, 1887
Genus
†Khasia †Mirandatherium †Eomicrobiotherium †Ideodelphys †Pitheculus Dromiciops

Microbiotheria is an order of New World marsupials of which the only living species is the monito del monte or colocolo (Dromiciops gliroides), a mouse-sized, tree climber species found in southern and central Chile and southwestern Argentina. Microbiotheres were once considered to be members of another New World marsupial order, Didelphimorphia (opossums), but were separated based on several morphological differences and recent evidence now suggests they may be more closely related to Australasian marsupials than those of the New World.

Overview and description

While most mammals are placentals, the opossum is a marsupial, belonging to the order (or infraclass) Marsupialia. Unlike placental mammals (Placentalia), almost all marsupials lack the placenta that connects the mother with the developing fetus in the womb. Some marsupials have a rudimentary placenta that functions for only a short time, such as the bandicoot. Marsupial females typically have an external pouch in which the immature young are raised after birth until early infancy. The newborn typically crawl to this pouch after birth, and attach themselves to milk-secreting teats (nipples), and are nursed until they can survive outside the pouch. This time period in the pouch is similar to the later stages of a placental mammal's development in the womb.

Marsupials also differ from placental mammals in their reproductive traits. The female has two vaginas (the tubular tract leading from the uterus to the exterior of the body). Both of the vaginas open externally through one orifice, but lead to different compartments within the uterus. Males usually have a two-pronged penis, which corresponds to the females' two vaginas. The penis only passes sperm. Marsupials have a cloaca (posterior opening that serves as the only such opening for the intestinal and urinary tracts) that is connected to a urogenital sac in both sexes. Waste is stored there before expulsion.

Besides the opossum, well-known marsupials include kangaroos, bandicoots, and wombats. Marsupials are native to Australia, New Guinea, Tasmania, and the Americas. Of the over 330 species of marsupials, most (over 200 species) are native to Australia and nearby islands to the north. Indeed, they are the dominant mammals in Australia, which have almost no native placental mammals. However, there also are many extant (living) marsupial species in South America, including the opossum.

The order Didelphimorphia, the opossums, include one extant family, Didelphidae. Opossums probably diverged from the basic South American marsupials in the late Cretaceous or early Paleocene. A sister group is Paucituberculata (shrew opossums). They are commonly also called "possums," though that term is also applied to Australian fauna of the suborder Phalangeriformes. Colloquially, the Virginia opossum is frequently called simply possum.

The monito del monte is the only extant member of its family (Microbiotheriidae) and the only surviving member of an ancient order, the Microbiotheria. The oldest microbiothere currently recognised is Khasia cordillerensis, based on fossil teeth from Early Palaeocene deposits at Tiupampa, Bolivia. Numerous genera are known from various Palaeogene and Neogene fossil sites in South America. A number of possible microbiotheres, again represented by isolated teeth, have also been recovered from the Middle Eocene La Meseta Formation of Seymour Island, Western Antarctica. Finally, several undescribed microbiotheres have been reported from the Early Eocene Tingamarra Local Fauna in Northeastern Australia; if this is indeed the case, then these Australian fossils have important implications for understanding marsupial evolution and biogeography.

Although once thought to be members of the order Didelphimorphia (the order that contains the Virginia opossum), an accumulation of both anatomical and genetic evidence in recent years has led to the conclusion that microbiotheres are not didelphids at all, but are instead most closely related to the Australasian marsupials; together, the microbiotheres and the Australian orders form the clade Australidelphia. The distant ancestors of the monito del monte, it is thought, remained in what is now South America while others entered Antarctica and eventually Australia during the time when all three continents were joined as part of Gondwana.

Monito del monte

The monito del monte (Spanish for "little bush monkey"), Dromiciops gliroides, also called chumaihuén in Mapudungun, is a diminutive marsupial native only to southwestern South America (Chile and Argentina). It is the only extant species in the ancient order Microbiotheria, and the sole New World representative of the superorder Australidelphia (all other New World marsupials are members of Ameridelphia). The species is nocturnal and arboreal, and lives in thickets of South American mountain bamboo in the Valdivian temperate rain forests of the southern Andes, aided by its partially prehensile tail. It eats primarily insects and other small invertebrates, supplemented with fruit.

Phylogeny and biogeography

It has long been suspected that South American marsupials were ancestral to those of Australia, consistent with the fact that the two continents were connected via Antarctica in the early Cenozoic. Australia’s earliest known marsupial is Djarthia, a primitive mouse-like animal that lived about 55 million years ago. Djarthia had been identified as the earliest known australidelphian, and this research suggested that the monito del monte was the last of a clade which included Djarthia. This implied that the ancestors of the Monito del Monte might have reached South America via a back-migration from Australia. The time of divergence between the Monito del Monte and Australian marsupials was estimated to have been 46 million years ago. However, in 2010, analysis of retrotransposon insertion sites in the nuclear DNA of a variety of marsupials, while confirming the placement of the Monito del Monte in Australidelphia, showed that its lineage is the most basal of that superorder. The study also confirmed that the most basal of all marsupial orders are the other two South American lineages (Didelphimorphia and Paucituberculata, with the former probably branching first). This indicates that Australidelphia arose in South America (along with the ancestors of all other living marsupials), and probably reached Australia in a single dispersal event after Microbiotheria split off.

Size

Body length is 11–12.5 cm. Tail length is 9–10 cm.^{[citation needed]}

Reproduction

The monito del monte normally reproduces in the spring and can have a litter size varying anywhere from one to four young. The females have a pseudovagina, and a fur-lined pouch containing four mammae. When the young are mature enough to leave the pouch they are nursed in a nest, and then carried on the mother’s back. The young remain in association with the mother after weaning. Males and females both reach sexual maturity after 2 years. They are known to reproduce aggressively, sometimes leaving blood on the reproductive organs.

Habitat

Monitos del monte mainly live in trees, where they construct spherical nests of water resistant colihue leaves. These leaves are then lined with moss or grass, and placed in well protected areas of the tree. The nests are sometimes covered with grey moss as a form of camouflage. These nests provide the monito del monte with some protection from the cold, both when it is active and when it hibernates. It stores fat in the base of its tail for winter hibernation. It lives in the dense, humid forests of highland Chile and Argentina.

Role as a seed disperser

A study performed in the temperate forests of southern Argentina showed a mutualistic seed dispersal relationship between D. gliroides and Tristerix corymbosus, also known as the Loranthacous mistletoe. The monito del monte is the single dispersal agent for this plant, and without it the plant would likely become extinct. The monito del monte eats the fruit of T. corymbosus, and thus disperses the seeds. Scientists speculate that the coevolution of these two species could have begun 60–70 million years ago.

References

ISBN links support NWE through referral fees

Siciliano Martina, L. 2014. "Microbiotheria" (On-line), Animal Diversity Web. Accessed May 12, 2014 at http://animaldiversity.ummz.umich.edu/accounts/Microbiotheria/

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