The different groups of small mammals show distinct patterns of geographical distribution across the globe, reflecting their independent evolutionary histories. Tree-shrews have the most restricted global distributions, with both living and fossil species only known from tropical forest environments in southern and south-east Asia. Eulipotyphlans and rodents both have much more cosmopolitan distributions, with rodents occurring on all of the world’s continents excluding Antarctica. However, different taxonomic groups also show important patterns of spatial variation in their geographical occurrence.
Within the Eulipotyphlans, solenodons and the recently extinct nesophontid island-shrews are completely restricted to the insular Caribbean, whereas other families have broader geographical distributions. Both talpids and shrews are found across Eurasia and North America, with shrews occurring in the Americas as far south as the Andes. Erinaceids do not occur in the Americas, with gymnures and moonrats only found in south-east Asia. Shrews and hedgehogs also occur in Africa, where they coexist with small-bodied Afrotheres such as elephant shrews or sengis.
Many distantly related rodent families are restricted to different continents, but can display superficial similarities in locomotion and appearance as a result of convergent evolution to similar ecological roles. For example, North American kangaroo rats, African springhares and Eurasian jerboas have all independently evolved saltatory or jumping locomotion, whereas North American pocket gophers, South American tuco-tucos and African mole rats all display different levels of adaptation towards an obligate burrowing lifestyle. The great majority of South American and Caribbean rodent families – including hutias, capybaras, maras, chinchillas and agoutis – are hystricognaths, descended from overwater colonists from Africa that reached the then-isolated island continent over 30 million years ago. More recent colonizations of other landmasses between five and seven million years ago by other rodent lineages – colonization of New Guinea by anisomyines, Australia by conilurines, and South America by muroids – led to further rapid, massive adaptive radiations that together account for nearly 10% of all current-day mammalian species diversity.