A timescale for the evolutionary history of sea spiders (Arthropoda: Pycnogonida)

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Abstract Sea spiders (Arthropoda: Pycnogonida) are an ancient lineage of chelicerates represented by a single living order, Pantopoda, and a patchy fossil record that provides limited information of their evolutionary timescale. The sudden appearance of Pantopoda in the Middle Jurassic has led several authors to propose a recent (Early Jurassic) origin and rapid diversification of modern pycnogonids. In contrast, a recent molecular clock study suggested Pantopoda originated before the Devonian, and diversified slowly, though the calibrations on which it was based have since been revised. Here, we conduct timetree inference analyses using a set of 13 mitochondrial protein-coding genes, 18S rRNA sequences and 98 Ultra Conserved Elements from 198 pycnogonid taxa, with calibrations reflecting the most recent interpretations of the sea spider fossil record. Our analyses estimate that pycnogonids diverged from other arthropods during the Cambrian (539-510 Ma), indicating about 100 Myr between Pycnogonida origin and pantopod diversification. The pycnogonid crown group (=Pantopoda) diversified during the Late Palaeozoic, in an interval spanning the Early Silurian and the Late Devonian (435-367 Ma), preceding the appearance of the first pantopod fossil by ~206.5 Myr. Our analysis also implies that pantopod families originated between the Late Devonian and the Late Jurassic (378-154 Ma). This leads to very different estimates to previous studies based on the fossil record. However, previous molecular divergence time analyses yielded a similar evolutionary timescale, despite a notably different set of calibrations. This effective corroboration of previous evidence indicates that the underpinning molecular evidence is informative and that the inferred long ghost lineages faithfully reflect the patchy nature of the pycnogonid fossil record which results from their low fossilisation potential. Regardless, our results predict further fossil discoveries from different environments and periods, which might at least partially rewrite pycnogonid history.