Found on mountain peaks penetrating the ice sheet in East Antarctica to the Chihuahuan desert Mexico, as well as the on moss growing stubbornly in your lawn (from which the specimen pictured was isolated), tardigrades are remarkable creatures. They have been exposed to -272.95°C for 20 hours, to 130°G, to 6,000 atmospheres of pressure, and to excessive concentrations of most common gases and x-rays, and were still able to return to active life.
To survive under these under such extreme environmental stresses, tardigrades enter a state known as cryptobiosis. The mechanism of cryptobiosis is poorly understood; however a study on dessication (anhydrobiosis) of tardigrades in the University of Stuttgart in 2009 showed that tardigrades have evolved efficient DNA repair mechanisms which enable them to recover even after long periods of anhydrobiosis. Most impressively of all, in the FOTON-M3 space mission in September 2007, tardigrades were seen to survive exposure to both space vacuum and solar radiation during a ten day, low Earth orbit (258-281 km above sea level).
Morphologically, they have a bilaterally symmetrical body, covered with a chitinous cuticle, and four pairs of legs terminating in claws. Division into classes is based on the presence or absence of external sensory receptors. These classes are Heterotardigrada, Eutardigrada and Mesotardigrada. However, the grouping of Mesotardigrada is considered dubious. They are represented by only one species, Thermozodium esakii, found in a thermal spring in Japan which has since been destroyed by an earthquake.
As a group, tardigrades are distinct enough to make their closest living relatives unclear and have been assigned phylum status. They are considered a sister group of the arthropods, forming (along with the onychoporans, the velvet worms) the Panarthropoda, which is considered as either a clade of the Ecdysozoa (molting animals) or the Articulata (animals with segmented bodies).
Fossil evidence indicates that tardigrades as we know them today have existed since the Cambrian period. Finds are few, but among others, four specimens were extracted from about 530 million year old Middle Cambrian “Orsten”-type rock in Siberia.