Although they are referred to collectively as “small mammals”, these three mammal groups have extremely different evolutionary histories, and do not represent each others’ closest relatives.
Higher-order relationships between the major mammal groups continue to be revised on the basis of new molecular, morphological and fossil data. Rodents and tree-shrews fall within the major placental mammal clade Euarchontoglires; within this clade, the tree-shrews probably represent the sister group to the colugos (Dermaptera) and fall within the Euarchonta together with primates, whereas rodents and lagomorphs (rabbits, hares and pikas) make up the Glires. Lipotyphlans instead represent the most basal divergence within the Laurasiatheria, another major placental mammal clade, which also contains the pangolins, carnivores, bats, ungulates and cetaceans. These major placental clades all emerged close to the extinction of the dinosaurs, although different studies debate the exact timing of the main mammalian evolutionary radiations.
Within these major clades, different small mammal lineages have shown extremely different patterns of divergence and diversification. Several lineages – such as the Caribbean solenodons (Solenodontidae), the North American mountain beaver (Aplodontidae), and the enigmatic kha-nyou of Lao PDR (Diatomyidae) – are very ancient and species-poor, and may be considered “living fossils”. Other groups have undergone extremely species-rich recent evolutionary radiations. For example, the tuco-tucos comprise a single genus (Ctenomys) of at least 38 species which all diverged rapidly around three million years ago. Even more strikingly, the Muridae – the single most diverse family of mammals, containing over 1300 recognized species – diverged less than 25 million years ago, and experienced a series of extremely large-scale radiations within the past few million years.