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.

Paleontology and Molecular Dating / Guidetti, Roberto; Bertolani, Roberto. - (2018), pp. 131-143.

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