Two new species of fossil Paratydeidae (Acari: Trombidiformes) from the late Eocene amber highlight ultraslow morphological evolution in a soil-inhabiting arthropod lineage

We describe two species of soil mite of the family Paratydeidae from the late Eocene Rovno amber: Scolotydaeus vlaskini sp. nov. and Tanytydeus pogrebnyaki sp. nov. This is the first formal description of fossils from the family Paratydeidae. These exceptionally well-preserved fossils have very similar morphologies compared to extant species, suggesting that Paratydeidae is a lineage that has experienced extremely slow rates of phenotypic evolution. To infer rates of morphological evolution in this group, we used a combination of tip dating (from the fossils) and internal node dating (based on molecular outgroup time-calibrated analysis) in a Bayesian phylogenetic framework. This analysis inferred Paratydeidae as an ancient lineage, originating 242?Ma (stem group) and diversifying 150?Ma (crown group), having ultraslow average rates of morphological evolution, i.e. 4.53e-04 changes per site per million years, which is equivalent to one change in any of the characters per 46.0 million years, per each branch of the tree. Based on our divergence time estimates, we suggest that the present global distribution of the group could be explained by their early evolution on the Pangaean supercontinent. Analysis of character state distributions in the fossil specimens suggests later extinction of ancestral taxa in the Palaearctic.