A fossorial () animal is one adapted to digging which lives primarily, but not solely, underground. Some examples are badgers, naked mole-rats, clams, meerkats, and mole salamanders, as well as many beetles, wasps, bees and spiders.
Prehistoric evidence
The physical adaptation of fossoriality is widely accepted as being widespread among many prehistoric phyla and taxa, such as bacteria and early eukaryotes. Furthermore, fossoriality has evolved independently multiple times, even within a single family. Fossorial animals appeared simultaneously with the colonization of land by arthropods in the late Ordovician period (over 440 million years ago). Other notable early burrowers include Eocaecilia and possibly Dinilysia. The oldest example of burrowing in synapsids, the lineage which includes modern mammals and their ancestors, is a cynodont, Thrinaxodon liorhinus, found in the Karoo of South Africa, estimated to be 251 million years old. Evidence shows that this adaptation occurred due to dramatic mass extinctions in the Permian period.
Physical adaptations in vertebrates
There are six major external modifications, as described by H. W. Shimer in 1903, that are shared in all mammalian burrowing species: Fusiform, a spindle-shaped body tapering at both ends, adapted for the dense subsurface environment. Lesser developed or missing eyesight, considering subsurface darkness. Small or missing external ears, to reduce naturally occurring friction during burrowing. Short and stout limbs, since swiftness or speed of movement is less important than the strength to dig. Broad and stout forelimbs (manus), including long claws, designed to loosen the burrowing material for the hind feet to disperse in the back. This trait is disputed by Jorge Cubo, who states that the skull is the main tool during excavation, but that the most active parts are the forelimbs for digging and that the hind-limbs are used for stability. Short or missing tail, which has little to no locomotor activity or burrowing use to most fossorial mammals.
Other important physical features include a subsurface adjusted skeleton: a triangularly shaped skull, a prenasal ossicle, chisel-shaped teeth, effectively fused and short lumbar vertebrae, well-developed sternum, strong forelimb and weaker hind limb bones. Due to the lack of light, one of the most important features of fossorial animals are the development of physical, sensory traits that allow them to communicate and navigate in the dark subsurface environment. Considering that sound travels slower in the air and faster through solid earth, the use of seismic (percussive) waves on a small scale is more advantageous in these environments. Several different uses are well documented. The Cape mole rat (Georychus capensis) uses drumming behavior to send messages to its kin through conspecific signaling. The Namib Desert golden mole (Eremitalpa granti namibensis) can detect termite colonies and similar prey underground due to the development of a hypertrophied malleus. This adaptation allows for better detection of low-frequency signals. The most likely explanation of the actual transmission of these seismic inputs, captured by the auditory system, is the use of bone conduction; whenever vibrations are applied to the skull, the signals travel through many routes to the inner ear.
For animals that burrow by compressing soil, the work required increases exponentially with body diameter. In amphisbaenians, an ancient group of burrowing lizard-like squamates, specializations include the pennation of the longissimus dorsi, the main muscle associated with burrowing, to increase muscle cross-sectional area. Constrained to small body diameters by the soil, amphisbaenians can increase muscle mass by increasing body length, not body diameter. In most amphisbaenians, limbs were lost as part of fossorial lifestyle. However the mole lizard Bipes, unlike other amphisbaenians, retains robust digging forelimbs comparable to those of moles and mole crickets.
Physiological modifications
Many fossorial and sub-fossorial mammals that live in temperate zones with partially frozen grounds tend to hibernate due to the seasonal lack of soft, succulent herbage and other sources of nutrition.