Osmunda cinnamomea (Osmundaceae) in the Upper Cretaceous of Western North America: Additional Evidence for Exceptional Species Longevity among Filicalean Ferns

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Abstract
The discovery of numerous anatomically preserved fossils in Upper Cretaceous sediments reveals that essentially modern osmundaceous ferns have inhabited southern Alberta, Canada, since the end of the Mesozoic. The Cretaceous fossils consist of small stems that are surrounded by leaf bases and adventitious roots. All of the features of the fossils fall within the ranges of variation for characters of living Osmunda cinnamomea L., and the fossils display all of the specifically diagnostic anatomical characters for this species. These include an ectophloic, dictyoxylic solenostele that lacks leaf gaps in the phloem, C‐shaped frond traces, frond bases with lateral stipular expansions, features of the endodermis, and disposition of sclerenchyma tissues in the stem and frond bases. A reexamination of extant specimens and of previously described fossils from Neogene and Paleocene deposits clarifies the range of variation for specifically diagnostic characters and reveals the more or less continuous presence of O. cinnamomea L. in western North America for at least 70 million years. This article provides an additional example of a well‐characterized fossil filicalean fern that can be confidently assigned to a species with living representatives. It is becoming increasingly clear that species longevity for homosporous pteridophytes can be far greater, and species turnover may be far lower, than expected from evolutionary models developed for flowering plants.