Sphenophyta: Fossil Record
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The first fossils that can be unambiguously placed in the Sphenophyta are Late Devonian in age. The Sphenophyta appear to have evolved from ancestors in the Trimerophytophyta, a paraphyletic grouping of Devonian plants that appear close to the common ancestry of sphenophytes, ferns, and higher plants. In particular, a group of Late Devonian plants classified in the order Hyeniales seems to be close to the ancestry of the Sphenophyta; the best-known genus, Hyenia, had massive rhizomes and recurved sporangiophores like sphenophytes, but lacked the nodes and internodes typical of sphenophytes. However, the Hyeniales also show similarities with the earliest fossil ferns, and may be close to the common ancestry of ferns and sphenophytes.
During the Carboniferous, sphenophytes diversified into prominent members of lowland plant communities, and reached maximum diversity of forms. They included the woody, shrub-like Sphenophyllum, small herbaceous forms such as Equisetites and Phyllotheca, and trees up to thirty meters tall, such as Calamites. Annularia, shown at the top left of this page, is a Late Pennsylvanian sphenophyte from the Mazon Creek region of Illinois, showing the typical whorls of leaves arranged regularly along the stem. Calamites, shown at the right, is a fairly common Pennsylvanian fossil, formed when sediment filled the interior of the plant's hollow stem and made an internal mold of the stem.
The Early Permian saw the extinction of the great calamite trees and shrubby Sphenophyllum, and the origin of the family Equisetaceae, to which the only living sphenophyte genus Equisetum belongs. The Equisetaceae underwent a minor renaissance in the Triassic, with the evolution of several new forms. Some grew quite large -- stems of up to 14 cm in diameter have been found -- although none were as large as the extinct calamite trees. By the close of the Jurassic, however, sphenophyte diversity had declined again. Cenozoic sphenopsid fossils are small forms indistinguishable from living Equisetum.
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 fossil sphenophytes from Alabama.
