A 64.1 MILLION YEAR OLD TROPICAL RAINFOREST FROM CASTLE ROCK, COLORADO

An extremely diverse, early Paleocene (64.1 Ma) fossil leaf site located along the east side of I-25 in Castle Rock, Colorado represents the oldest well-documented tropical rainforest. The site, which occurs on the western margin of the Denver Basin in synorogenic sediments associated with the rise of the Laramide Front Range, was dated using three different methods and is ~ 280 m above the K-T boundary. Leaves are found at two horizons that are separated by 1 m of section along an outcrop that is over 100 m long. The upper horizon contains a single leaf layer and the lower horizon contains 1-4 distinct leaf layers. A paleosol directly below the lowest leaf horizon contains an in situ cycad plant complete with roots and a 1.8 m diameter tree trunk with 3 m long roots. Heavy equipment was used to excavate numerous ~1.5 x 2 m quarries from both horizons. The leaves from these quarries were censused and scored for species, size, margin and type of insect damage. We documented over 100 different varieties of angiosperm, cycad, fern, and conifer leaves. In the lower horizon, the species of leaves show little taxonomic variation vertically, but dramatic variation laterally. Insect damage is surprisingly rare and consists primarily of hole feeding.

The flora from the lower horizon has numerous features in common with the leaf litter of extant equatorial rainforests, including dominance by angiosperms, large leaves, often exhibiting smooth margins and drip tips, and high spatial heterogeneity from quarry to quarry. Analysis of leaf physiognomy indicates that the mean annual temperature was 22.2 ºC and the mean annual precipitation was 225 cm. We believe that the lower leaf horizon was created through the rapid deposition of sand and mud onto a mature rainforest floor via over bank flooding. Several smaller floods occurred in a short span of time followed by a massive flood, which probably buried and killed the forest.