Molecular clock estimations
Age estimation of the nodes is burdened with relatively wide 95%
Highest Posterior Density (95% HPD; partly transparent blue horizontal
bars in Fig. 2), indicating considerable uncertainty in node dating (see
also Table 3), which was expected taking into account the limited input
tree priors with only two available calibration points. The split
between Eutardigrada and Heterotardigrada corresponds to the range
introduced as a prior, based on the date calculated by Guidetti et al.
(2017). The split between Apochela and Parachela dates back to 432
million years ago (Mya) with 95% HPD ranging from 323 to 540 Mya, thus
it most likely took place in the Palaeozoic but the exact geological
period is not possible to ascertain with the current data. The most
recent common ancestor of Milnesium observed in our tree is
estimated to have lived 162 Mya, with 95% HPD ranging from 116 to 207
Mya, thus possibly correlating with the initial stages of the Gondwana
breakdown (160–180 Mya, Jokat, Boebel, König, & Meyer, 2003; partly
transparent vertical grey bar in Fig. 2). Due to the polytomy present in
the basal part of the tree (Fig. 2), it is difficult to estimate the
exact ages of particular clades; however the lack of support for these
basal nodes suggests a relatively rapid diversification after the first
split between the extant lineages in the genus, recovered in our
analysis. The split between clades A–D and the remaining species took
place ca. 138 Mya, with 95% HPD ranging from 98 to 178 Mya.