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Taxonomy

Scientists have conducted morphological and molecular studies to understand how tardigrades relate to other lineages of ecdysozoan animals. Two plausible placements have been proposed: tardigrades are either most closely related to Arthropoda and Onychophora, or to nematodes. Evidence for the former is a common result of morphological studies; evidence for the latter is found in genomic analysis.

The minute sizes of tardigrades and their membranous integuments make their fossilization both difficult to detect and highly unusual. The only known fossil specimens are those from mid- Cambrian deposits in Siberia (Orsten fauna) and a few rare specimens from Cretaceous and Neogene amber.

The Siberian tardigrade fossils differ from living tardigrades in several ways. They have three pairs of legs rather than four, they have a simplified head morphology, and they have no posterior head appendages, but they share with modern tardigrades their columnar cuticle construction. Scientists think they represent a stem group of living tardigrades.