Inferring ecological disturbance in the fossil record: A case study from the late Oligocene of Ethiopia

Abstract

Environmental disturbances profoundly impact the structure, composition, and diversity of modern forest communities. A review of modern studies demonstrates that important characteristics used to describe fossil angiosperm assemblages, including leaf margin type, plant form, plant diversity, insect herbivore diversity and specialization, and variation in herbivory among plant species, differ between early and late successional forests. Therefore, sequences of fossil floras that include a mix of early and later successional communities may not be appropriate to study long-term temporal trends or biotic effects of climate, latitude, or other variables. We conducted sedimentological, paleobotanical, and insect damage analyses at two contemporaneous late Oligocene (27–28 Ma) leaf localities in the Chilga Basin, northwest Ethiopia, to test the hypothesis that successional stage explains variation between the assemblages. The Guang River and Bull's Bellow fossil plant localities are stratigraphically equivalent and only 1.5 km apart, but they have no plant species in common. Sedimentary structures at Bull's Bellow suggest multiple episodes of deposition and a more disturbed environment than the single, featureless mudstone that composes the Guang River fossil unit. Bull's Bellow has a significantly higher percentage of plant species with toothed margins, lower plant and insect herbivore damage diversity, and less specialized herbivore damage than Guang. Furthermore, the nearest living relatives of many of the Bull's Bellow plant species are associated with early successional forests, whereas the Guang River plants are a mix of early and late successional species. Thus, the physiological and ecological attributes of the Guang flora are consistent with a more mature forest community, and the differences between the two floras emphasize the importance of collecting multiple floras from the same stratigraphic level in order to account for landscape-level ecological processes. More specifically, accurate estimates of diversity and feeding specialization in tropical, angiosperm-rich fossil assemblages are essential to address adequately the origin and persistence of relatively high species diversity in the modern tropics, and diversity differences between regions such as modern tropical Africa and South America.