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.