Exceptionally Preserved Fossils of Minute Infective Larvae - 90 Million Years Old Window Into the Evolution of the Parasitism of Epicaridean Isopods

Abstract : Isopoda is an ingroup of Peracarida and must be considered an overall evoluttionary successful group. Isopods have conquered numerous habitats fromthe deep sea to dry desert and exhibit various feeding strategies. In most isopods the early post-embryonic stages strongly re semble the adults. The obligate parasitic epicarideans form an exception. Their life cycle is complex and includes a host switch and extreme sexual dimorphism. This requires two highly mobile stages. One of them is the cryptoniscus larva. After detaching from its copepod larval host, it becomes plan ktic and searches for its definitive host. Within Epi caridea there are different strategies to feed on the definitive hosts right up to endoparasitism. The most specialised form is the inhabitation of the gill chamber which can be indirectly traced back to the Jurassic period, i.e. 200-142 million years ago, based on specific deformations of the hosts. Here, we present the so far oldest record of epicaridean body fossils from a rare sample of French Cretaceous amber of about 90 million years in age. This represents a rare kind of preservation where marine organisms were trapped in resin in a coastal environment. With the help of high-resolution composite fluorescence microscopy, we shed light on their highly specialised morphology that was already present in the Cretaceous and discuss implications of this find on the evolution of the synecological relationship between parasites and their hosts.