Onoclea sensibilis in the Paleocene of North America, a dramatic example of structural and ecological stasis

Abstract

Thousands of specimens of the filicalean fern Onoclea sensibilis, including large segments of whole plants in growth position, have been recovered from nonmarine sediments of the Paleocene, Paskapoo Formation in central Alberta, Canada. The fossils are preserved by coalified compression/impression within a community dominated by a betulaceous species of Paleocarpinus and Metasequoia. Four relatively complete and rooted Onoclea plants were exposed at the outcrop. Such deposition indicates that the community was periodically inundated by flooding and that at such times, the fronds were bent over and buried in place. In all recognizable features of habitat, community structure, plant size, growth form, morphology of the vegetative fronds, morphology of the fertile spikes, sporangia and spores, the fossils conform to the living species Onoclea sensibilis. These data demonstrate that essentially modern onocleoid ferns had evolved by the Paleocene and that at least one species has remained virtually unchanged throughout the Cenozoic.