A RECORD OF EVOLUTIONARY CHANGE IN AN EOCENE PLANT-HERBIVORE ASSEMBLAGE

How have insect-plant relationships changed over time? To address this issue we utilized a sample of fossil plants and traces of insect-mediated damage on fossil leaves from two stratigraphic intervals from the same section of the Green River Formation (46Ma, Middle Eocene) found at Douglas Pass, Colorado. The fossils found at Douglas Pass predominantly contain subtropical species, but also contain a few warm-temperate taxa. By studying these fossil assemblages we attempted to determine which plants and which feeding guilds were common in the two paleocommunities.

For our analyses, climate and insect-feeding data were collected. We examined two assemblages separated by a stratigraphic distance of five meters. Our sample includes 204 leaves from the younger site and 49 leaves from the older site. Leaves from both sites were separated into morphospecies, and we used leaf margin analysis to estimate mean annual temperature (MAT) and leaf area to estimate mean annual precipitation (MAP). All leaves were examined for the presence or absence of insect damage. The type and number of functional feeding groups were recorded. Our data on herbivory levels and climate were then compared between the two intervals.

Both assemblages had similar MAT and MAP, estimated at 23oC and 45cm respectively. We found that 22 % of leaves from the older site had insect damage as compared to 30 % in the younger site. The majority (82%) of damaged leaves from the older site had only one type of insect damage, whereas 83% of damaged leaves from the younger site had one damage type per leaf. Although preliminary, it appears that insect damage levels do not change dramatically if climate does not change.