LEAF DAMAGE INTENSITY AND DIVERSITY IN THE PALEOGENE CHICKALOON AND ARKOSE RIDGE FORMATIONS, SOUTH-CENTRAL ALASKA

South-central Alaska’s Matanuska Valley-Talkeetna Mountains basin developed among accretionary terranes between a volcanic arc and mélange of the Chugach Terrane. The basin’s Paleogene non-marine strata includes the Chickaloon and Arkose Ridge formations unconformably overlying a Cretaceous marine succession. Both formations are similar in age and have yielded newly collected paleofloral assemblages from fluvial and lacustrine facies. These plant remains were preserved during the warm climate phase in the late Paleocene/early Eocene. Fossil leaves exhibiting plant-insect interactions can provide paleoecological information about these past environments. In this study, quantitative collections of fossil leaves were analyzed from both formations and leaf damage was scored according on the presence/absence of eight functional feeding groups. Feeding groups include hole feeding, margin feeding, leaf mining, galling, surface feeding, piercing and sucking, and oviposition. Leaf damage intensity and diversity were assessed using two methods: (1) scoring all damage found on leaf fragments >1cm² in size, and (2) scoring damage on leaves with more than half the blade fossilized intact on the sample. Newly calculated and previously published damage intensities are as follows- Arkose Ridge by 1cm² method: 9.2%, Arkose Ridge by half-leaf method: 13.4%, Chickaloon by 1cm²: 9.4%, Chickaloon by half leaf method: 7.6%. When compared to lower-latitude, similar-age paleofloral assemblages, we noted that leaf herbivory diversity and intensity are very low for the study units considering leaf-derived mean annual temperature estimates. Skeletonization, hole-feeding, and margin-feeding rarely occur among fossil leaves from either formation, and galling and leaf mining are entirely absent. Chickaloon fossil leaf mass per unit areas (avg. 63.6 g/m²) are well below values associated with impeded herbivory; thus they do not explain the low incidence of herbivory. Possible causes for the low leaf damage intensity and diversity may include varied plant defenses, light seasonality, or some other environmental effect on plant or insect life cycles due to the anomalously warm climate at high latitudes.