The place of tardigrades in arthropod evolution

Abstract

The phylum Tardigrada is an engaging but enigmatic group composed of minute metazoans with four pairs of stubby lobopodous appendages. Although tardigrades are thought to share important derived characters with arthropods, many of the designated attributes are actually plesiomorphies and, as a consequence, the phylogenetic position of the taxon remains uncertain. Embryological data indicating, for example, an enterocoelous formation of mesoderm, are questionable (Marcus, 1929; Pollock, 1975; Nelson, 1982; Bertolani, 1989), and the fossil record from Cretaceous amber (Cooper, 1964) and Quaternary travertine (Durante and Maucci, 1972) reveals only representatives from well-established taxonomic categories. Although similarities between tardigrades and the Middle Cambrian lobopod Aysheaia have been recognized (Renaud-Mornant, 1982; Grimaldi de Zio et al., 1987a; Simonetta and Delle Cave, 1991), the relationship of Aysheaia to other lobopods (Ramsköld, 1992; Hou and Bergström, 1995) and of both to arthropods remains elusive.