"Fossorial" amusingly has nothing to do with fossils, but rather is the technical term generally applied to vertebrates that burrow (Although sometimes it is also applied to burrowing invertebrates) . It derives from the Latin fossor or one who digs. It most commonly applies to vertebrates because the implication is that the organism in question digs with its limbs. So, think rabbits, badgers, or to the annoyance of gardeners in California, ground squirrels and gophers. Generally, the assumption is that a fossorial animal spends some time above ground during the day and thus is not truly "subterranean", that is, living fully underground.