TRANSIENT INSECT REBOUND LESS THAN 2 MYR AFTER THE CRETACEOUS-PALEOGENE BOUNDARY: DIVERSE INSECT DAMAGE ON A LOW-DIVERSITY EARLY PALEOCENE FLORA FROM SOUTHEASTERN MONTANA

Insect herbivore extinction across the Cretaceous-Paleogene boundary (K-T, 65.5 Ma) in the Williston Basin of North Dakota and increased herbivore diversity with warming temperatures across the Paleocene-Eocene boundary (55.8 Ma) in southwestern Wyoming are known from feeding damage on fossil leaves (Wilf and Labandeira 1999; Labandeira, Johnson, and Wilf 2002). However, the pace and pattern of recovery during the intervening ~10 Myr in the Western Interior USA has not been studied. An expectation from modern analogs is that the low-diversity floras typical of the Paleocene in this region supported low herbivore diversity. Here, we report, and integrate with previous work, new data from quantitatively collected Paleocene floras analyzed for insect damage on identified dicot leaves. The six new Paleocene sites range in age from ~64-58 Ma and come from the Fort Union Formation of northwestern Wyoming (Polecat Bench), southwestern Wyoming (Rock Springs Uplift), and southeastern Montana (Powder River Basin). These sites produced 4-16 dicot leaf species each and a total of 7,445 analyzed specimens (10,750 including previous Paleocene collections). Although all other Paleocene sites in our study have low feeding richness, Mexican Hat (~64 Ma, Williston Basin) shows a striking richness of insect damage and is an exception to the expected pattern of coupled, low plant and insect diversity. The diversity and abundance of insect mines is particularly noteworthy and indicates host-specialized feeding associations. The six distinct mine morphotypes are more than twice the number in preceding or succeeding deposits, and mines are found on 10% of insect-damaged leaves, more than double any other Paleocene site. Five of the six dominant host plants have mines on over 3% of insect-damaged leaves. One sycamore species that comprises 53% of specimens has mines on 3.5% of total leaves, on par with most ambient modern values and below that of pest outbreak levels. All six dominant hosts are in the top quartile for the Paleocene in overall insect-damage diversity. The unusually rich insect damage at Mexican Hat supports a significant but apparently ephemeral insect diversification or immigration unconnected to plant diversification or turnover.