Ferns (division Pteridophyta or Filicophyta) range from the Devonian to recent times. Ferns, like clubmosses and horsetails are vascular plants with true roots, stems, and leaves. While many ferns are smaller herbaceous plants, larger tree-like ferns still exist.

The leaves of ferns are formed as fronds. Fronds may be simple or compound and are attached to a vertical stem or underground rhizome by a stipe (which corresponds to the petiole in angiosperms). Compound fronds are sub-divided numerous times into pinnae (singular: pinna), which are attached to a long central axis called the rachis. Pinnae may be subdivided into ultimate units called pinnules, which may be further sub-divided.

Ferns like to grow in moist shady places. Like clubmosses and horsetails, ferns require a thin film of water covering their small gametophytes to reproduce sexually. Like clubmosses and horsetails, ferns also reproduce asexually with spores. The spores in clubmosses and horsetails are produced in cones on the sporophyte. In ferns, spores are produced in round structures called sori (sorus: singular) on the underside of pinnules.

The First Forests

Middle Devonian fossil trunks from Gilboa, New York provide a window into the earliest forests. Stumps with roots stretching out into a paleosol (fossil soil) have been preserved as casts at a site known as Riverside Quarry. The casts reveal only the outer structure of these tree stumps. From the 1870's and until recently the stumps have been assigned to various plant groups including Psaronius (the tree fern), Eospermatopteris (the accepted name) and a progymnosperm (Nudds & Selden, 2008, pp. 98 & 99). The mystery of Eospermatopteris identity was not solved until recently. In 2007 Stein et al. described the discovery of fossil trees from Shoharie County, New York uniting the crown of Wattieza with the trunk of Eospermatopteris. Wattieza is a genus of prehistoric tree that belongs to the class Cladoxylopsida. This class is currently placed within the division Pteridophyta. So, fossil stumps at Gilboa are cladoxylopsid trees related to ferns.

Tree Ferns

Ferns occupied many habitats in the Carboniferous. Fern species grew as epiphytes, ground cover, understory and canopy in these ancient forests. Psaronius was the largest arborescent fern found in the coal measure swamps (Willis & McElvain, 2002, p. 108). Psaronius was up to 10 m tall and occupied the drier areas of the swamps. The trunk had no secondary wood for strength. The center of the trunk was composed of a narrow cylinder of vascular tissue. Enclosing this cylinder was a mantle of leaf petioles and aerial roots, which created a fibrous, tough, lightweight structure (Kenrick & Davis, 2004, p. 71). Today, epiphytes like orchids and bromeliads grown in tree fern root mantles. During the Carboniferous certain species of small ferns and horsetails grew in association with the roots of Psaronius (Rothwell, 2002, p 42).The trunk of Psaronius was unbranched and clothed in a thick mass of descending roots near the bottom. Along the trunk leaf scars marked former positions of fronds. These circular scars are diagnostic of species. At the top, Psaronius bore fronds forming an apical crown. The fronds of Psaronius are known as Pecopteris. Towards the end of the Carboniferous arborescent ferns started to dominate the canopy as the climate became dryer.

Science Olympiad Fossil Event

The 2016 Science Olympiad Fossil List includes the genus Calamites (Horsetail) and the plant leaf Annularia within the phylum Pteridophyta (Ferns).

Pecopteris Frond
Note Pinnae and Rachis
Upper Carboniferous, Westfalien
Piesberg/Osnabruck, Germany
Plate 10.5 cm x 6.7 cm

Kenrick, P. and Davis, P. (2004). Fossil Plants. Smithsonian Books: Washington.

Nudds, J.R. & Selden P.A. (2008). Fossil Ecosystems of North America: A Guide to the Sites and Their Extraordinary Biotas. Chicago: University of Chicago Press.

Rothwell, G.W. (2002). Coal Balls: Remarkable Evidence of Palaeoxoic Plants and the Communities in Which They Grew. . In Dernbach, U. & Tidwell, W.D. Secrets of Petrified Plants: Fascination from Millions of Years (pp. 39-47). Germany: D’ORO Publishers.

Stein, W.E., Mannonlini, F, VanAller Hernick, L., Landing, E. & Berry, C.M. (2007). Giant cladoxylpsid trees resolve the enigma of the Earth's earliest forest stumps at Gilboa. Nature, vol 446: pp. 904-907.

Willis, K.J. & McElwain, J.C. (2002). The Evolution of Plants. New York: Oxford University Press.