RECONSTRUCTING PLANT-INSECT INTERACTIONS ACROSS THE PALEOGENE-NEOGENE BOUNDARY IN ETHIOPIA

The early Miocene Mush Valley lagerstätte in north-central Ethiopia (ca. 160 km north-east of Addis Ababa) provides a rare view of life, landscape, and climate in tropical Africa at the start of the Neogene. Fossil plant and animal compressions are preserved in lacustrine carbonaceous shales that are between 21.736 +/- 0.054 Ma and 21.73 +/- 0.01 Ma, as measured by ²³⁸Ur/²⁰⁶Pb content in zircons from ashes below and above the fossiliferous strata. Fossil leaves are exceptionally complete, retain original cuticle, and clearly preserve insect herbivore damage. Comparisons of the Mush Valley paleofloras with those from the late Oligocene Chilga Basin, also in Ethiopia, allow us to document changes in plant and insect herbivore assemblages across the Paleogene-Neogene boundary, a pivotal interval for environmental and biotic change in Africa.

Over 2100 fossil leaves from six stratigraphic levels within the Mush Valley lacustrine shales were scored for insect herbivore damage. A total of 35 damage morphotypes was observed, including seven gall types and four mine types; body fossils of four species of scale insects are also preserved on leaf surfaces. The percent of leaves with herbivore damage at Mush is comparable to that at Chilga. Specialized damage frequency at Mush is most similar to that at Chilga floral assemblages thought to represent early successional forests and bears less resemblance to that of late successional Chilga paleofloras. In particular, galling is much less abundant at Mush, which can be indicative of wetter conditions or a recently disturbed habitat.