Cretaceous amber inclusions illuminate the evolutionary origin of tardigrades

Tardigrades are a diverse phylum of microscopic invertebrates renowned for extreme survival capabilities. Molecular clocks suggest tardigrades diverged from other panarthropods before the Cambrian, yet their fossil record is sparse. Only Milnesium swolenskyi (Late Cretaceous) and Paradoryphoribius chronocaribbeus (Miocene) have secure taxonomic positions, limiting fossil calibrations for estimating the origin of the phylum. Using confocal fluorescence microscopy, we revise two crown-group tardigrades from Canadian Cretaceous amber, resolving key morphological characters. A formal redescription of Beorn leggi reveals Hypsibius-type claws, and we describe Aerobius dactylus gen. et sp. nov. based on a unique combination of claw characters. Phylogenetic analyses place Beorn leggi and Aerobius dactylus within the eutardigrade superfamily Hypsibioidea, adding a critical fossil calibration for investigating tardigrade origins. Molecular clock estimates suggest an early Paleozoic diversification of crown-group Tardigrada and highlight Beorn leggi as a calibration that directly impacts estimates of shallow nodes. Results further suggest independent terrestrialization of eutardigrades and heterotardigrades near the end-Carboniferous and Lower Jurassic, respectively, providing minimum ages for convergent acquisition of cryptobiosis.

Marc A. Mapalo, Joanna M. Wolfe, Javier Ortega-Hernández