Novel insect leaf-mining at Mexican Hat, Montana (early Paleocene) and the demise of Cretaceous leaf miners
Open Access
- Author:
- Donovan, Michael Philip
- Graduate Program:
- Geosciences
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- June 14, 2013
- Committee Members:
- Peter Daniel Wilf, Thesis Advisor/Co-Advisor
- Keywords:
- plant-insect interactions
Cretaceous-Paleogene extinction
herbivory
paleoecology
Fort Union Formation
Powder River Basin - Abstract:
- Plant and associated insect-damage diversity in the western U.S.A. decreased significantly at the Cretaceous-Paleogene (K-Pg) boundary. Insect-feeding richness remained low on fossil leaf floras until the late Paleocene. However, Mexican Hat, a fossil plant locality in southeastern Montana with a typical, low-diversity "disaster flora," uniquely exhibits high damage diversity compared to all known local and regional early Paleocene sites, on nearly all its plant hosts. The locality is in the Lebo Member of the Fort Union Formation (ca. 65 Ma), and the flora is dominated by four widespread, unrelated leaf species that show minimal damage elsewhere: Platanus raynoldsii, Juglandiphyllites glabra, Zizyphoides flabella, and Cercidiphyllum genetrix. We investigated whether the high insect damage diversity at Mexican Hat was more likely related to the survival of Cretaceous insects from refugia or to an influx of novel Paleocene taxa. We compared damage on 1073 leaf fossils from Mexican Hat to over 9000 terminal Cretaceous leaf fossils from the Hell Creek Formation of nearby southwestern North Dakota. A supporting data set of over 9000 local and regional early and late Paleocene leaf fossils from the Fort Union Formation of North Dakota, Montana, and Wyoming was also used for comparison. We described the entire insect-feeding ichnofauna at Mexican Hat and focused on leaf mines because they are typically host-specialized and preserve a number of morphological features that allow for detailed comparisons and potential identification of mining culprits. Eight mine morphotypes attributable to three orders of insects are found at Mexican Hat. We found all of them to be unique to the site except for one, a lepidopteran mine also found on C. genetrix at late Paleocene Wyoming sites. Not only is there no evidence linking any Cretaceous mines with those found at Mexican Hat, there is also no conclusive evidence for the survival of any Cretaceous leaf miners over the K-Pg boundary regionally, even on the well-sampled, surviving plant families Platanaceae and Cercidiphyllaceae. Overall, our results strongly relate the high damage diversity on the depauperate Mexican Hat flora to an influx of novel insect herbivores during the Paleocene, possibly caused by an undocumented, transient warming event, and suggest drastic extinction rather than survivorship of Cretaceous insect taxa from refugia.