LATERAL VARIATION IN INSECT DAMAGE ALONG INDIVIDUAL CARBONACEOUS SHALE HORIZONS IN THE EARLY EOCENE IN THE BIGHORN BASIN, WYOMING

Although temporal variations in insect damage on fossil angiosperm leaves from the Paleocene and Eocene have been well-studied in the Western Interior, USA (e.g. Wilf and Labandeira 2006), little attention has been paid to spatial variation in insect damage within a single bed. Temporal variations in damage have been attributed to climate change, with higher diversity and frequency of damage during global warming events like the Paleocene-Eocene Thermal Maximum. The primary objective of this study is to gain a more complete understanding of the strength of the correlation between global warming and increases in insect damage by analyzing heterogeneity within a single time horizon. Additionally, this study examines the geographic area captured by sampling from a single quarry.

Lateral variation in insect damage was studied at two sites in the Bighorn Basin, Wyoming: Elk Creek (54.9 Ma) and 15 Mile Creek (52.4 Ma, Eocene Thermal Maximum). Fossil leaves are preserved in laterally extensive carbonaceous shale beds that can be traced for kilometers. Deposition was largely parautochthonous, with fossil layers representing a season to a few years (Davies-Vollum and Wing 1999). At Elk Creek, 995 leaves (dominated by Alnus and Averrhoites) were collected from 5 quarries, successively spaced by 30-150m. Seven quarries consecutively separated by 10–60m. were dug at 15 Mile Creek, and 1854 leaves (dominated by Alnus, Platycarya, and Dombeya) were analyzed. Every identifiable dicot leaf was scored for the presence/absence of the 150+ damage morphotypes currently recognized in fossils. Damage diversity, damage frequency, and relative abundance of individual damage types on both the bulk flora and individual plant species were statistically compared across the quarries at each site to determine whether a single quarry adequately captures the insect damage for a stratigraphic level. These analyses provide error bars on insect damage at each time horizon.