July 4. Pennsylvanian calamites

The plants that became Pennsylvanian coal came in a wide variety of sizes and shapes. One that Mark Twain mentioned in the quote yesterday, calamites, was a common swamp plant similar to modern horsetail rushes – but much larger. Modern rushes are maybe a foot tall, but Pennsylvanian calamites grew to 30 meters high, or more than 100 feet. They were definitely trees, at least in terms of size.


Calamites trunks were thick segmented stems with ribbed surfaces, sort of like bamboo, and the stems were hollow, like bamboo, so when they died and fell, often they would split open and some of the most common fossils are internal casts of the hollow stem that had filled with sediment after death.  

Annularia (Calamites leaves)

Calamites were spore-bearing plants, like ferns, and they carried the spores in elongate fruits somewhat like cones on pine trees. They also spread asexually by underground rhizomes, essentially creating a clone of the original plant. Aspens spread like that today.   Their leaves were needle-like but arranged in whorls that radiated from a central point, and sequences of whorls were mounted on stems that extended from the trunks.

As a group, calamites began during the Mississippian but proliferated during the Pennsylvanian at least in the equatorial regions, which included much of North America and Eurasia, which were attached to each other. They declined and became extinct fairly soon after the end of the Pennsylvanian, during the early Permian Period.

—Richard I. Gibson

Annularia photo by Woudloper, Collection of the Universiteit Utrecht, public domain

Drawings from old texts, public domain