Not only is burrowing widespread among mammals (82% of non-aquatic, non-flying families), but it is particularly common among the groups that are considered the most primitive. (Both families of egg-laying mammals, and six out of seven families of marsupial mammals). The discovery of a Thrinaxodon burrow from the Lower Triassic of South Africa suggests that burrowing was present in the group of therapsid reptiles from which the mammals evolved. Thrinaxodon belongs to cynodonts, a group considered ancestral to mammals. According to some reconstructions, Thrinaxodon possessed hair and was capable of thermoregulation. It has been suggested that burrowing facilitated thermoregulation in these proto-mammals.
Damiani and coworkers (2003) point out that the basal members of the major placental lineages are burrowers. It is easy to envision how burrowing animals are more likely to survive extinctions and give rise to specialized groups. Burrowing is a primarily a behavioural adaptation, which may leave no effect on the anatomy. In addition, an animal burrowing for shelter is quite independent of other organisms inhabiting ist environment. In contrast, animals that find shelter aboveground are more likely to be dependent on other species. For instance, many mustelids (weasels, martin, fisher) find shelter in hollow stumps and logs, and are at present affected by the reduction of the old-growth forest.
Shelter has been often neglected as factor both in ecology and in evolution, partly because it can not be deduced from the study of morphology. In most situations, there is little competition for shelter, and so evolution probably proceeds differently, allowing inefficient and unspecialized solutions to persist.
