Marsupial - Evolution

Evolution

The relationships between the three extant divisions of mammals (monotremes, marsupials, and placental mammals) was long a matter of debate among taxonomists. Most morphological evidence comparing traits such as number and arrangement of teeth and structure of the reproductive and waste elimination systems favors a closer evolutionary relationship between marsupials and placental mammals than either with the monotremes. Most genetic and molecular evidence also supports grouping marsupials and placental mammals as a single clade, subclass Theria.

Marsupials and placental mammals split from the monotremes during the Cretaceous period. In the absence of soft tissues, such as the pouch and reproductive system, fossil marsupials can be distinguished from placentals by the form of their teeth; primitive marsupials possess four pairs of molar teeth in each jaw, whereas placental mammals never have more than three pairs. Using this criterion, the earliest known marsupial is Sinodelphys szalayi, which lived in China around 125 million years ago (mya). This makes it almost contemporary to the earliest eutherian fossils, which have been found in the same area.

The oldest metatherian fossils (Metatheria being a larger clade that groups marsupials with some of their extinct relatives) are found in present-day China.About 100 million years ago (mya), the supercontinent Pangaea was in the process of splitting into the northern continent Laurasia and the southern continent Gondwana, with what would become China and Australia already separated by the Tethys Ocean. Marsupials spread westward into modern North America (still attached to Eurasia) and then to South America, which was connected to North America until around 65 mya. Laurasian marsupials eventually died off, possibly due to competition from placental mammals for their ecological niches.

In South America, the opossums retained a strong presence, and the Tertiary saw the evolution of shrew opossums (Paucituberculata) and metatherian predators such as the borhyaenids and the saber-toothed Thylacosmilus. South American niches for mammalian carnivores were dominated by these marsupial and sparassodont metatherians. While placental predators were absent, the metatherians did have to contend with avian (terror bird) and terrestrial crocodilian competition. South America and Antarctica remained connected until 35 mya, as shown by the unique fossils found there. North and South America were disconnected until about three million years ago, when the Isthmus of Panama formed. This led to the Great American Interchange. Competition from placental mammals from the north drove sparassodonts to extinction, while didelphimorphs (opossums) invaded Central America, with the Virginia opossum reaching as far north as Canada.

Marsupials reached Australia via Antarctica about 50 mya, shortly after Australia had split off. This suggests a single dispersion event of just one species, most likely a relative to South America's monito del monte (a microbiothere, the only New World australidelphian). This progenitor may have rafted across the widening, but still narrow, gap between Australia and Antarctica. In Australia, they radiated into the wide variety seen today. Modern marsupials appear to have reached the islands of Borneo and Sulawesi relatively recently via Australia. A 2010 analysis of retrotransposon insertion sites in the nuclear DNA of a variety of marsupials has confirmed all living marsupials have South American ancestors. The branching sequence of marsupial orders indicated by the study puts Didelphimorphia in the most basal position, followed by Paucituberculata, then Microbiotheria, and ending with the radiation of Australian marsupials. This indicates that Australidelphia arose in South America, and reached Australia after Microbiotheria split off.

In Australia, terrestrial placental mammals disappeared early in the Cenozoic (their most recent known fossils being 55 million-year-old teeth resembling those of condylarths) for reasons that are not clear, allowing marsupials to dominate the Australian ecosystem. Extant native Australian terrestrial placental mammals (such as hopping mice) are relatively recent immigrants, arriving via island hopping from Southeast Asia.