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Particularly, the in our opinion unrealistically young root age prior of 800 Ma did not result in a major shift of age estimates towards after the Marinoan glaciation.In fact, the inference that Metazoa and most of its major subgroups originated prior to (or, regarding some crown clades, during) the Cryogenian Snowball Earth periods was not affected by assumptions about the age of crown-group Opisthokonta.In contrast to previous studies, however, our results suggest that not only crown-group Metazoa, but all non-bilaterian phyla (and at least stem-lineages of all classes), as well as total-group Bilateria, originated before the Sturtian, probably within a geologically relatively short time span.Importantly, this main conclusion is robust to a number of major assumptions that can drastically influence the outcome of molecular clock studies..Only three of the nodes of greatest interest were estimated to be Phanerozoic: crown-group Calcarea, Hexactinellida (glass sponges), and Ctenophora.Mean estimates for Calcarea were mid-Cambrian under all three calibration sets, with Cr Is narrowest for A and widest for C.However, in-depth molecular clock studies utilizing this dataset have thus far been lacking.Here, we present molecular clock analyses of the Philippe et al. Our results confirm previous molecular-clock estimates of an early-mid Neoproterozoic (Tonian; 1000–720 Ma) origin of crown-group Metazoa, before the onset of long-lasting global glaciations, the Sturtian and Marinoan “Snowball Earths” of the Cryogenian (720–635 Ma).

Estimates for crown Demospongiae were all consistent with a glacial origin, although the mean estimates were pre-Sturtian under A, Sturtian under C, and end-Sturtian under B.Given that the autocorrelated model had a better fit according to preliminary model selection analyses (see Supplementary Material) and yielded younger – i.e., in better agreement with the fossil record – and more precise age estimates for most nodes, we did not consider the uncorrelated model further. For crown Bilateria, Protostomia, and Deuterostomia, the ranking was the same, but estimates fell within the Sturtian, Marinoan, or interglacial intervals under A and B.Age estimates for major animal groups obtained under different molecular clock models. Under A and B, Cr Is for crown Medusozoa spanned the general glaciation interval, with means falling within or close to the interglacial period.In order to assess the robustness of our age estimates to a number of prior assumptions, we ran analyses under (1) an autocorrelated and an uncorrelated relaxed molecular-clock model – the former assuming that the rate of molecular evolution of a lineage is correlated with the rate of its mother lineage, the latter allowing completely independent rates from lineage to lineage; (2) three different sets of fossil calibrations for internal nodes – one aiming at a maximum breadth of calibrations (Set A), another one excluding some potentially controversial fossils (Set B), and a third one adopted from Erwin et al.(Set C); (3) different prior assumptions about the age of the root of the phylogeny (=origin of crown-group Opisthokonta) – 800, 1000, and 1360 Ma; and 4) three different alternative assumptions about the phylogenetic placement of Ctenophora (sister to Cnidaria, sister to the remaining Metazoa, and sister to Placozoa Cnidaria Bilateria), which is currently a matter of debate Mean age estimates for the nodes of greatest interest were generally older, sometimes considerably so, under the uncorrelated model (Fig. The only exceptions were the crown-groups of Medusozoa (jellyfish and their kin), Anthozoa (corals, anemones etc.), Demospongiae (common sponges), and Calcarea (calcareous sponges), but the 95% credibility intervals (Cr Is) obtained under the autocorrelated model for these nodes fell completely or almost completely within the Cr Is obtained under the uncorrelated model.

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