Parasitic infestation in a Middle Ordovician <i>Illaenus</i> (Trilobita)

Abstract Evidence for parasites in the fossil record is rare. As such, any examples present insight into parasitism in deep time. Trilobites have often been used for documenting parasites in the Paleozoic. Here we examine an Illaenus sp. pygidium from the Middle Ordovician of Estonia that displays thirteen small structures with domical to crater-like shapes. These morphologies are consistent with circular depressions on the pygidium inner surface. We propose that these structures formed while the trilobite was alive and record an infestation located within soft tissue. The trace maker seems to have influenced pygidial mineralization and caused a pathological reaction. The symbiont may have been capable of bioerosion, excavating these depressions by dissolving the trilobite’s mineral tissues; however, this scenario is less likely considering comparisons with syndromes and pathologies known in modern arthropods. The parasitic organism may have fed on the trilobite’s tissues or utilized nutrients within the trilobite’s body for growth. These observations are consistent with a parasitic organism.