THE EARLIEST APPEARANCE OF PEARLS IN THE PALEONTOLOGICAL RECORD: CLUES FOR COMPLEX PARASITE LIFE CYCLES OR FOSSIL BIASES?

Recent molecular studies have suggested that the basal parasitic flatworms (Neodermata) had a simple life cycle, while more derived parasitic flatworms (Cestoda, Trematoda) developed complex life cycles. The intermediate stages of cestodes and trematodes have commonly been implicated in the formation of pearls or other shell secretions in recent bivalves. In absence of reliable parasitic flatworm body fossils, late Paleozoic blister and early Mesozoic free pearls have therefore been used as an indication for the presence of complex parasite life cycles.

We compiled a database of blister and free pearls in extant and fossil bivalves. Their occurrences were investigated through geological time in an up to date ecological and phylogenetic framework. Their fossil record proves to be extremely biased, particularly within the Paleozoic and Mesozoic. No clear correlation between the earliest occurrence of blisters and pearls with the appearance of new taxonomic or ecological bivalve groups could be found. Furthermore, their wide occurrence in distantly related orders as well as various other mollusk phyla suggests an ancient origin of pearl-like structures in the earliest shelled mollusks. Our survey also underlines that pearls in extant relatives are not only caused by parasitic flatworms, but a variety of other irritants between the mantle and shell, including other parasites and shell-burrowing organisms. In conclusion, pearl-like structures can not be used to indicate the appearance of complex flatworm life cycle in the geological past until their paleontological record and the diagnostic characters of the mantle-shell reactions with parasitic causes in recent bivalves are better known.