An Early Paleocene Carpoflora from the Denver Basin of Colorado, USA, and Its Implications for Plant-Animal Interactions and Fruit Size Evolution

Abstract

Premise of research. Fossil fruit and seed floras are valuable because of the wealth of taxonomic characters that they can provide, but they are rare in the Cenozoic of North America. Here, we document a fossil fruit and seed flora from the Paleocene Denver Formation of the Denver Basin, Littleton, Colorado, representing the first account of an early Paleocene carpoflora in North America. This locality has previously yielded a rich and well-studied therian and sauropsid fauna confirming a Puercan age. In conjunction with these vertebrate records, the Littleton flora serves as an ecological snapshot of a transitional time in both angiosperm and vertebrate lineages following the K-T mass extinction event.Methodology. Disseminules were studied with reflected light and X-ray micro–computed tomography.Pivotal results. The flora contains at least 15 different morphotypes, including four members of the Cornales (Amersinia littletonensis sp. n., Langtonia parva sp. n., Mastixicarpum hoodii sp. n., and Portnallia alexanderi sp. n.). The occurrence of Kingsboroughia rostellata (Lesquereux) comb. n., a close relative of Meliosma (Sabiaceae), is among the earliest records for this genus in North America. Although the morphology of the Littleton fruits and seeds is consistent with animal dispersal and in agreement with dietary preferences suggested by mammalian dentition, wear from predation is not evident in any of these specimens.Conclusions. The fruits and seeds reveal components of the early Paleocene Denver Basin flora that were not known from prior investigations of leaf assemblages. However, modern-day relatives of the Littleton taxa may occupy environments similar to those inferred for other Paleocene floras in the Denver Basin. Although the Littleton disseminules were found to have larger fruits and a wider range of sizes than other Paleocene floras, they fit into previously recognized trends of increasing seed size from the late Cretaceous through the early Tertiary.