Preliminary assessment of insect herbivory across the Cretaceous-Tertiary boundary: Major extinction and minimum rebound

Abstract

Forty-nine insect damage types morphologically characterize plant-insect associations spanning a 136 m composite boundary interval from the uppermost Cretaceous Hell Creek Formation to the lowest Paleocene Fort Union Formation. The time duration of this interval is ∼2.2 m.y., 1.4 m.y. of which is latest Cretaceous. These data originate from 80 localities from the Williston Basin of southwestern North Dakota and are allocated to four assemblage zones consisting of 385 megafloral morphotypes of bryophytes, ferns, conifers, cycads, Ginkgo, monocots, and 333 dicots. All 49 foliar damage types were produced by external foliage feeding, mining, galling, and piercing and sucking insects, each of which was assigned one of the following host specificity levels: generalized, intermediate, or specialized. A distinctive pattern emerged when these damage types were plotted stratigraphically: of the 22 damage types that survive the Cretaceous-Tertiary boundary (K-T), 55% are generalized; of the 15 damage types that are extinguished immediately below the boundary, all are specialized or intermediate, as are the 12 damage types that disappear in stepped fashion somewhat before the boundary. Within this interval only two damage types are unique to the Fort Union Formation, occurring during the lowest Paleocene. These data indicate that highly and moderately specialized associations were preferentially culled at the K-T boundary, disproportionately enriching early Paleocene floras in generalized herbivores as dominant colonizers. These data are local, and the Signor-Lipps effect may partially obscure the true pattern, thus restricting extrapolation to the Western Interior of North America. Nevertheless, (1) the selective extinction of specialist herbivores at the boundary, (2) the absence of significant evidence for immigrant colonization, (3) the prolonged occurrence of species-depauperate floras during the Paleocene, and (4) the high percentage of last occurrences at and not before the boundary, despite the Signor-Lipps effect, suggest a role for end-Cretaceous abiotic perturbation and consequent biotic response in the evolution of modern plant and insect associations.