Widespread mermithid nematode parasitism of Cretaceous insects
Mermithid nematodes are obligate invertebrate parasites dating back to the Early Cretaceous. Their fossil record is sparse, especially before the Cenozoic, thus little is known about their early host associations. This study reports 16 new mermithids associated with their insect hosts from mid-Cretaceous Kachin amber, 12 of which include previously unknown hosts. These fossils indicate that mermithid parasitism of invertebrates was already widespread and played an important role in the mid-Cretaceous terrestrial ecosystem. Remarkably, three hosts (bristletails, barklice and perforissid planthoppers) were previously unknown to be parasitized by mermithids both past and present. Furthermore, our statistical analyses show that in contrast to their Cenozoic counterparts, Cretaceous nematodes including mermithids are more abundant in heterometabolous insect hosts. This result suggests that nematodes have not completely exploited the dominant Holometabola as their hosts until the Cenozoic. This study reveals what appears to be a vanished history of nematodes that parasitized Cretaceous insects.