Many burrowing animals have existed, including numerous mammals, insects, amphibians, reptiles (including small dinosaurs), crustaceans, worms, and even a few fish and birds. The start of the modern era of life, the Phanerozoic, is defined by the appearance of complex burrows in the fossil record 542 million years ago. These burrowing animals broke up the previously hard-packed and anoxic ocean floor, allowing much greater biological diversity as well as interspecies competition. Burrowing is thought to have evolved as a defense against predation. Many ecological arms races between predators and prey can be characterized as burrowing animals versus predators trying to get animals out of their burrows.
The most famous burrowing animals are mammals, including rabbits, chipmunks, moles, gophers, and groundhogs. The burrow of a single groundhog occupies 1 cubic meter, while the complex warrens of rabbits may occupy hundreds of cubic meters. Some animals, such as the marsupial mole, have adapted so extensively to burrowing that they have lost their eyes and hunt prey using only their senses of smell and touch. In Australia, burrowing rabbits were introduced in the late 18th century and have since reproduced to be out of control, destroying large tracts of the bush and leading to the extinction of many other species.
Though we are most familiar with mammalian burrowing animals, non-mammalian burrowers are also common, especially in the sea. Entire animal phyla, such as phoronids and mud dragons, spend their life in self-constructed burrows, living entirely using tiny cilia that reach out into the water. Some sea animals can secrete special chemicals that allow them to burrow directly into hard rock, albeit at a slow rate. Some of the most prolific burrowers in the sea are the polychaete worms, aquatic annelids that are expert bottom-scavengers. These burrows help them escape the jaws of bottom-dwelling predatory fish.
Some burrowers that evolved from surface animals have developed highly unusual adaptations to the dark, subterranean life. One animal, the star-nosed mole, has a sense organ composed of incredibly sensitive nasal tentacles called Eimer's organs. These are used by the mole to detect very small prey animals. The star-nosed mole is also known as nature's fastest eater, taking as little as 120 milliseconds (faster than the human eye can follow) to identify and consume prey items.
