The morphology and relationships of Equisetum fluviatoides sp.nov. from the Paleocene Ravenscrag Formation of Saskatchewan, Canada

Abstract

Vegetative and associated fertile remains of Equisetum have been recovered from early Tertiary sediments of the Ravenscrag Formation, Saskatchewan, Canada. The morphology of both reproductive and vegetative organs of this fossil species is remarkably similar to that of extant Equisetum fluviatile, the swamp horsetail. Aerial axes of the fossil are 3.0–19.0 mm in diameter, with internodal lengths of up to 30.0 mm. The stems are hollow and the central cavity is large. Branches, apparently simple, are borne in whorls at the nodes. Leaf collars at the nodes are up to 23.0 mm long, longer than broad, with leaves fused in the lower four-fifths of the collar. The leaf apices are long attenuate. Cones are up to 14.0 mm long, bearing peltate, six-sided sporangiophores in whorls of five. The fossil record suggests stasigenesis in the evolutionary history of some members of the genus Equisetum since the beginning of the Tertiary, and perhaps longer.