Evolution and systematics

The order Paucituberculata is represented by just a single living family, the Caenolestidae, with just two genera. The handful of living species is all that remains of what appears to have been a once-abundant marsupial dynasty during the Oligocene epoch, approximately 25 million years ago. There are seven extinct families, some of which were described from fossils before the living caenolestids were discovered, including the Groberiidae. Some of the earliest fossils, members of the family Polydolopidae, date back to the Palaeocene, more than 60 million years ago.

The decline of the paucituberculates began in the Oligocene, and it gathered pace in the Miocene, when the continents of North and South America were briefly joined. There was an influx of placental mammals from the north into what had been for many millions of years a bastion of marsupial diversity. The newcomers included rodents and primates whose descendants have since thrived at the expense of many ousted marsupials.

A number of similarities with other American marsupials has led some authorities to consider the shrew opossums to be no more than a subgroup of the order Didelphimorphia. However, molecular evidence supports the classification used hereā€”it may be that at one time the Paucituberculata were as diverse as the extant Australian order Diprotodontia.

Until recently, there were thought to be three living caenolestid genera. The third, Lestoros, contained the species, L. inca, and is now considered part of the larger genus, Caenolestes.

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