Paleontology and Molecular Dating

Few fossil tardigrade records are known, probably due to their minute size
and absence of consistent hard body parts. A possible but not certain tardigrade
ancestor was found as an “Orsten”-type secondarily phosphatized fossil from the
Middle Cambrian limestone. It is characterized by only three pairs of legs as a
possible consequence of a plesiomorphic anameric ontogeny that led to four leg
segments in the extant tardigrades. Its other characters, related to claw and cuticle
morphologies, resemble those of heterotardigrades. The first findings of sure tardigrades
come from Cretaceous amber, in which the species Beorn leggi and
Milnesium swolenskyi were found. These species, and especially the latter, evidence
that more than 90 million years ago, “modern” tardigrades already existed, and only
few morphological differences, mainly related to morphometric tracts, occur with
respect to known modern species. Quaternary subfossils of tardigrades have been
found with palynological studies during paleoecology researches. They have been
recorded mainly in polar regions and classified within the non-pollen palynomorphs
category. Cuticular remains of animals and eggs can persist in the substrates for very
long time, so tardigrades can be used as paleoclimatic bioindicators, but despite their
potentiality, a few studies have focused to find these organisms in microfossil
samples and in sediments for paleoecological studies.
For estimating the dates of phylogenetic events related to the origin of tardigrades
and/or their evolutionary lineages, molecular clock analyses have been used. Molecular
data indicate that the origin of the phylum should be posed during or before the
Cambrian period, placing the origin of Tardigrada in marine environment and their
terrestrialization not before that of other ecdysozoans.