July 22. Ferns

Earlier this month we talked about the tree-sized plants that dominated the swamps and forests of the Pennsylvanian – at least the tropical Pennsylvanian, which spanned much of North America and Europe. Some of the common trees such as sigillaria and lepidodendron are sometimes called “tree ferns” but they were really more closely related to club mosses and rushes.

True ferns evolved by the very late Devonian or Mississippian Period and certainly contributed to the biomass that produced the Pennsylvanian coal. Some grew to tree-like proportions, including a group historically named Megaphyton or Aristophyton that was probably as much as 50 feet tall.

Ferns have neither seeds nor flowers, and they reproduce by spores. The dense forests of the Pennsylvanian were completely without flowers – flowering plants didn’t evolve until the Cretaceous Period, about 175 million years after the end of the Pennsylvanian. Ferns apparently evolved leaves, or fronds, independently from other groups of plants, possibly by developing photosynthetic webbing between branches.

A group separate from true ferns, called seed ferns, also flourished during the Pennsylvanian, and their leaves form some of the most common fossils in coal beds and concretions. Seed ferns are not really very closely related to true ferns, so the name is a misnomer. The were probably a little more close to cycads and ginkoes than to ferns, but the name “seed fern” is still commonly used. Their fronds look an awful lot like ferns, and some of the fronds were as much as 6 feet long. Two types that are common in Pennsylvanian coal beds and concretions in the U.S. are called Neuropteris and Alethopteris. Neuropteris means “nerve-wing” for the complex vein system in its leaves. They are all extinct, with most of them disappearing at the end of the Permian but some may have survived as late as the Eocene, about 55 million years ago. It’s not completely agreed that the possible survivors should be classified as seed ferns or not.

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Today’s birthday is Kirk Bryan’s, born July 22, 1888, in Albuquerque, New Mexico. Bryan was a geomorphologist, a student of landforms, especially those in arid lands, as well as glacial geology. The Geological Society of America gives the Kirk Bryan award annually for distinctive work in the areas of science that he pioneered.

—Richard I. Gibson

Tree fern drawing by Dawson (from an old textbook; public domain)