From living to fossil: parasitological evidence in modern and Pleistocene shells of <i>Zidona dufresnii</i>

Abstract Ecological live–live interactions in fossil invertebrates, although providing valuable information about the past environment and ecology, are still underexplored. Trematode parasites have a complex life cycle, often utilizing mollusks as their intermediate hosts. However, since the trematodes themselves lack a rigid skeleton, these interactions can be detected only when there is anomalous growth in the shell, the sole preserved part of the mollusk. When malformations produced by the host are preserved, it may be concluded that the mollusk was alive at the time of interaction. This study aims to assess the evidence of different ecological interactions preserved in the shells of Zidona dufresnii (Donovan, 1823) and determine which were results of a life-to-life relationship, as well as which organisms were involved in that interaction. A total of 894 shells collected from Quaternary deposits in southern Brazil were analyzed regarding the presence and morphology of bioerosions and malformations. As a result, 10 ichnogenera of bioerosion traces were identified. In addition, seven types of malformations were observed. These include pits and igloo-like structures, which are results of infestation by parasitic trematodes. Blister and sheet forms indicate response to the presence of spionid species that perforated the shell. Verrucose malformations, we suggest here, are also indicative of trematode infestation. ¹⁴C dating shows that the gastropod shells are derived from both Holocene and Late Pleistocene deposits, indicating significant time averaging. While evidence of life–life interaction is more common in Holocene shells, Pleistocene shells display a greater prevalence of bioerosion traces.