Sunday, November 21, 2010

Dryopteris: A Fossil Fern from Florissant

Ferns are among the fossil plants found in the Florissant Fossil Beds of Colorado. Fossil plants like these ferns are records of prehistoric life—providing information about when an organism lived, where it lived, and how it lived. Fossils are vital in helping paleontologists reconstruct ancient environments and establishing the geologic history of the Earth.

Dryopteris guyotti was abundant in past geologic ages. 
Broad, flat leaves helped the fern catch more sunlight.
Florissant Fossil Beds National Monument specimen
number 3135a.  Photo by R. Wood.
The origins of ferns are not well understood, but continued study of fossil ferns may reveal more about their beginnings. The earliest recognizable ferns come from the Carboniferous (359-299 million years ago).

Ferns have large complex fronds (leaves) and are spore bearers. Some ferns are non-woody, but other ferns are woody and are called tree ferns. Ferns were common late Paleozoic plants and widespread in the Mesozoic. Today ferns are the most common and diverse spore-bearing land plants with over 10,000 species. They generally live in moist, shady areas of the forest understory.

Dryopteris, from the Greek, drus (oak) and pteris (fern) occurs in Florissant’s Eocene fossil flora as well as other Tertiary floras. Its common name—wood fern—is from the preferred woodland habitat of most Dryopteris species. Other common names include shield fern, Goldie’s fern, male fern and buckler ferns.

Today Dryopteris is a genus of about 250 species of ferns growing in the temperate Northern Hemisphere and in eastern Asia. Fronds are bipinnate (branching of leaflets at right angles to the central axis). The leaflets, or pinnules, are lobed. Fertile pinnules have round sori, which are fruit dots or reproductive bodies (Tidwell, 1998). Many of the species have solid rootstocks forming a crown with a ring of fronds.

Line drawing of Dryopteris filix.
Note round sori near top.
 Britton, N.L., and A. Brown. 1913.
An illustrated flora of the northern United States,
Canada and the British Possessions.
Off all the fossil ferns at Florissant, Dryopteris guyottii is the only species of fern described from a frond. Other ferns are known only from fossil spores. It appears that Dryopteris guyotti grew in the understory of the Eocene forest at Florissant (preferring damp and shaded environments like its modern relatives) or near ancient Lake Florissant.

1 comment:

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