PATTERNS OF PLANT-INSECT ASSOCIATIONS FROM THE CRETACEOUS-PALEOCENE INTERVAL OF THE DENVER BASIN

The latest Cretaceous to earliest Paleogene (K-P) succession of floras in the Denver Basin of Colorado provides a detailed history of plant-insect associations through a 5 m.y. interval spanning the upper Laramie, Denver, and Dawson Formations, encompassing a major turnover of terrestrial organisms. Twenty-one bulk-collected and sample-unbiased floras, each representing from 175 to 2289 specimens, were ordinated by pollen zonation into K (Maastrichtian, 3 floras), P1 (Puercan, 5 floras), P2 (Puercan/Torrejonian, 1 flora), and P3 (Torrejonian, 11 floras). We analyzed damage diversity (using rarefaction) and damage frequency for these floras, indicating a highly elevated level of K herbivory, a modest level for P1 and perhaps P2, and a uniform decrease during P3. However, for the P1 sites, this gross analysis may disproportionately inflate generalized damage occurring multiply on the same leaf since these sites have few specialized DTs.

Newly discovered associations are almost entirely confined to a few K floras, and exhibit the targeting of particular tissues, host monospecificity, and typical confinement to single sites. The most prominent pattern is five DTs on a particular Laramie palm species found at sites 2174, 2362 and 3213, whose damage is controlled by venation. These palm associations include two DTs of ellipsoidal scale insect impression scars; elongate, black oviposition scars; tubular, frass filled mines constructed probably by beetles; and small, ellipsoidal punctures likely made by a heteropteran piercer-and-sucker. In addition, three mine DTs include a distinctive, robust, full-depth leaf mine on Platanites marginata (locality 2302) revealing an incidence rate of 27.8 % (N= 209 leaves); a unique mine whose terminal stages have removed midrib vascular tissue from “Ficus” planicostata (probable Lauraceae); and a thick, tubular mine occurring on the pinnule margin of the fern Allantodiopsis erosa.

Denver Basin DT diversity attests to a high intensity of insect herbivory on varied vascular plants during the latest K and a dramatic decrease by P3 times. This pattern is generally similar to that of the Williston Basin, 750 km to the north, with the possible exception of somewhat higher levels during P1 + P2.