The Calamitaceae include the great "calamite trees" of the Carboniferous Period. Most paleobotanists agree that they are closely related to the living horsetails in the Equisetaceae. Mature stems of members of the Calamitaceae, like those of their living counterparts, were hollow, and frequently filled up with sediment after the death of the plant. When the sediment compacted and hardened, an internal mold, like the fossil pictured here, was formed; such molds are fairly common fossils at some localities.
The Illinois State Museum exhibits a number of fossil plants, including sphenopsids, from the Pennsylvanian-age Mazon Creek locality in Illinois. Also, fossil collector Prem Subrahmanyam has placed on-line images of fossils in his collection, including a number of calamites from Alabama.