Arthropod molecular divergence times and the Cambrian origin of pentastomids

Pentastomida are parasites of tetrapods (especially reptiles) and have had long contentious relationships. Traditionally seen as a separate phylum with arthropod affinities, recent ultrastructural and molecular evidence unites them to branchiuran crustaceans (fish lice). However, the discovery of Cambrian pentastomids has been interpreted to refute the view that pentastomids are closely related to advanced crustaceans (of presumably recent origins). Bayesian phylogenetic and relaxed-clock analyses of molecular data (18S and 28S ribosomal RNA, elongation factor 1 alpha and 2, RNA polymerase II subunit) help reconcile these apparently contradictory views: a pentastomid-branchiuran grouping is recovered, but this clade is the sister group to other Tetraconata, whereas all other crustaceans form a monophyletic group that is sister to hexapods. Crustacean monophyly is widely supported by morphology, but has rarely been retrieved in other molecular studies. Estimated molecular dates for pentastomid origins range from similar to 490 to similar to 520 million years ago, consistent with Cambrian fossils. The basal position of branchiurans within tetraconatans means pentastomids can be simultaneously related to branchiurans, and also have a deep fossil record. All Cambrian pentastomids are tiny, resembling the larvae of living pentastomids that infect intermediate hosts including fish; as definitive hosts (tetrapods) of modern pentastomids were absent in the Cambrian, it is possible that the small Cambrian forms were adults rather than larvae, and their entire life cycle was confined to small fish-like vertebrates that were then present.