NEW δ13C DATA FROM THE LATE OLIGOCENE OF THE CHILGA, ETHIOPIA, EVIDENCE FOR A CLOSED-CANOPY TROPICAL FOREST

Previous analyses of late Oligocene (~27 mya) sediments and fossils from Chilga, Ethiopia document rich, heterogeneous ecosystems, primarily consisting of moist tropical forests. In order to test this hypothesis, and to test for the presence of a closed-canopy forest structure, we collected new δ¹³C data derived from bulk organic matter from 12 lignites (x = -26.42‰, 1S = 0.80‰) and from a single tissue type preserved in 18 fossil Annonaceae seeds (x = -25.67‰, 1S = 1.60‰) representing a single taxon, possibly a single genus or species. All data fall within the expected range of C₃ plants growing in a moist tropical forest. Fifteen of the seeds were collected from a single stratum across ten meters laterally, and yielded a range in δ¹³C values of >5‰. Considering that these samples represent a single tissue type from a single taxon and a narrow range of time and space, this most likely indicates the existence of a closed canopy in at least some areas. Taken further, the range in values represents a gradient of decreasing canopy-effect as the seeds developed at different heights within the forest or in different areas with varying canopy cover.