MESOZOIC ORIGIN OF THE BONE-EATING OSEDAX WORMS AND INSIGHTS ON THE ORIGIN OF THE SIBOGLINIDAE (ANNELIDA)

When a new genus of siboglinid tubeworms, Osedax, was discovered over a decade ago living on the skeletons of dead whales, it was hypothesized that it might have evolved to exploit the carcasses of early cetaceans. Subsequent discoveries revealed a highly speciose clade with a near global geographic distribution, but the origins and causes of this diversity have remained an unresolved aspect of Osedax biology and evolution. Molecular clock age estimations have been equivocal and, depending on the calibration used, suggest that either Osedax split from its relatives about 45 million years ago, coincident with the origins of large archeocete cetaceans during the Eocene, or in the Cretaceous, when it could have been supported by the bones of large marine Mesozic reptiles.

Here we report the first fossil evidence of Osedax traces on early Late Cretaceous (~ 100 Myr) plesiosaur and sea turtle bones, which supports a pre-Cenozoic origin of the clade. Whereas plesiosaurs went extinct at the Cretaceous-Tertiary boundary, chelonioids survived to the event, representing available substrates for Osedax in the 20 Myr gap preceding the radiation of cetaceans. This finding shows that marine reptile carcasses, before whales, played a key role in the evolution and dispersal of Osedax and confirms that its generalist ability of colonizing different vertebrate substrates, like fishes and marine birds, besides whale bones, is an ancestral trait. A Cretaceous age for Osedax trace fossils also dates back to the Mesozoic the origin of the entire siboglinid family, which includes chemosynthetic tubeworms living at hydrothermal vents and seeps, contrary to phylogenetic estimations of a Late Mesozoic–Cenozoic origin (approx. 50–100 Myr).