In the present study a character data set suitable for cladistic analysis at the family level was developed. A data matrix consisting of 50 morphological characters from 15 families of tardigrades was analyzed by maximum parsimony. Kinorhynchs, loriciferans, and gastrotrichs were used as outgroups. The results agree with the currently accepted hypothesis that Eutardigrada and Heterotardigrada are distinct monophyletic groups. Among the eutardigrades, Eohypsibiidae was found to be a sister group to Macrobiotidae+Hypsibiidae, while Milnesiidae was the basal eutardigrade family. The basal heterotardigrade family was found to be Oreellidae. Echiniscoideans grouped with some traditional Arthrotardigrada (Renaudarctidae, Coronarctidae+Batillipedidae) suggesting that the arthrotardigrades are not monophyletic. The 18S rRNA gene sequence of Batillipes mirus Richters, 1909 and Calohypsibius schusteri Nelson & McGlothlin, 1996 were obtained and their addition to a previously published dataset supports the monophyly of Heterotardigrada and of Parachela versus Apochela within the Eutardigrada.

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Tardigrada is a phylum closely allied with the arthropods. They are usually less than 0.5 mm in length, have four pairs of lobe-like legs and are either carnivorous or feed on plant material. Most of the 900+ described tardigrade species are limnoterrestrial and live in the thin film of water on the surface of moss, lichens, algae, and other plants and depend on water to remain active and complete their life cycle. In this review of 910 tardigrade species, only 62 species representing13 genera are truly aquatic and not found in limnoterrestrial habitats although many other genera contain limnoterrestrial species occasionally found in freshwater.