Early divergence dates of demosponges based on mitogenomics and evaluated fossil calibrations

Demosponges are among the most primitive biomineralized metazoans to appear first in the fossil record with hard skeletons; their confirmed earliest fossils are from the lower Cambrian rocks about 520 Ma, with putative demosponge biomarkers reported from 713 to 635 Ma sediments. In this study, we use mitogenomic data to approach the early divergence timescale of demosponges using relaxed molecular clock techniques and likelihood-evaluated fossil calibration strategies. We found that among various molecular dating models, the correlated rate model yielded time estimates of demosponges in this analysis which is most congruent with the fossil appearance dates of demosponges. Our dating analyses show that crown groups of Demospongiae appeared at about 704 (674–741) Ma, and the silicification in demosponges (divergence of spicular sponges) began about 633 (616–648) Ma indicating a gap of over 100 million years between the origin of silicification and their first unequivocal appearance of siliceous spicules in the fossil record (520–525 Ma); demosponges with tetraxon-type spicules (Tetractinellida) are dated here at about 514 (498–530) Ma, an estimate comparable with the earliest tetraxial megasclere fossil records (510–520 Ma, Ordian Age, middle Cambrian).