INSECT-FEEDING TRACES FROM EOCENE PATAGONIA REVEAL UNRECOGNIZED EVOLUTIONARY HISTORY OF HERBIVOROUS INSECTS ON AUSTRALIA’S ICONIC EUCALYPTUS

Plants and their insect herbivores represent the bulk of terrestrial (non-microbial) biodiversity, accounting for over 50% of all described species. Understanding how insect herbivore communities assemble on plant host lineages through evolutionary time is key to comprehend the source of this great biodiversity. However, direct fossil evidence of long-term associations originated in deep time and persisting to the modern day on the same host lineage is scarce. Here, we tested whether insect herbivore assemblages tracked the speciose genus Eucalyptus (Myrtaceae) for 52 Ma by comparing the insect herbivore damage on 284 fossil Eucalyptus frenguelliana leaves from the Laguna del Hunco fossil rainforest locality in Argentinean Patagonia with that of 36 extant, rainforest-associated Eucalyptus species. In the fossil material­—which represents the oldest macrofossil evidence of the genus­—we identified a diverse suite of 29 damage types, including twelve types of external feeding associations, two of piercing-and-sucking marks, five of galls, and ten of mines that, to the best of our knowledge, represent the highest richness of mines reported for a single plant host in the fossil record. Nearly identical suites of insect herbivore damage were observed in extant Eucalyptus herbarium specimens (>10,000 sheets reviewed), suggesting that the associated insect herbivore assemblages tracked and radiated on multiple species of their host genus through time and space. Our literature survey showed that although hundreds of insect herbivore species are associated with Eucalyptus hosts, most of the extant analogs for the herbivore damage seen in the fossils are made by still-unknown culprits, pointing to previously unrecognized biodiversity and evolutionary history of herbivorous insects on Australia’s iconic Eucalyptus. The undescribed insect culprits can be sought at the surviving locations where the herbarium vouchers were collected.