ANOMALOUS INSECT-PLANT DAMAGE DIVERSITY AT MEXICAN HAT, MONTANA LINKED TO AN INFLUX OF PALEOCENE INSECTS

Plant diversity in the western U.S.A. decreased significantly at the Cretaceous-Paleogene (K-Pg) boundary and was coupled with a decrease in insect–plant damage diversity during the Paleocene. However, Mexican Hat, a southeastern Montana fossil plant site with a typical, low-diversity "disaster flora", anomalously exhibits remarkably high damage diversity compared to more than 100 other Paleocene sites. The site is part of the Lebo Member of the Fort Union Formation (⁴⁰Ar/³⁹Ar ages ca. 64.0-64.7 Ma). The flora is dominated by four species: Platanus raynoldsii, Juglandiphyllites glabra, Zizyphoides flabella, and Cercidiphyllum genetrix. To assess whether the high insect damage diversity at Mexican Hat could be attributed to survivorship of Cretaceous insects versus Paleocene immigrant or newly evolved taxa, we compared damage from Mexican Hat to that in the terminal Cretaceous Hell Creek flora in nearby, southwestern North Dakota. Thirty-one damage types (DTs) are found at Mexican Hat and represent four functional feeding groups (external feeding, piercing and sucking, mining, and galling). Preliminary results suggest that ~74% of the DTs observed at Mexican Hat span the K-Pg boundary, and ~26% first appear in the Paleocene. However, most of the persisting DTs are generalized forms made by various groups of insects and thus carry little information about species diversity or extinction. Host-specialized interactions, such as leaf mines, can be attributed to specific insect lineages in some cases. Seven of 10 specialized DTs at Mexican Hat, including 6 mine DTs, have not been found at Cretaceous or Paleocene sites, linking high damage diversity to an abundance of specialized interactions. We also compared damage on species of Platanaceae (sycamores), common in the Hell Creek flora, to diverse damage on P. raynoldsii. In this broader Platanaceae sample, damage unique to P. raynoldsii at Mexican Hat includes elliptical piercing and sucking marks, and larval mines from sawflies, microlepidopteran moths, and agromyzid flies. Overall, comparisons to Hell Creek insect damage more strongly supports the high damage diversity on the depauperate Mexican Hat flora as being caused by an influx of novel insect herbivores during the Paleocene, rather than survivorship of Cretaceous taxa from refugia.