Our modern diminutive " horsetails " with scaly leaves were represented in the Carboniferous period by gigantic calamites, often with a diameter of I to 2 ft.
Sphenophyllum was a slender climbing plant with whorls of leaves, which was probably related both to the calamites and the lycopods.
Fossils are extremely rare in these beds; Buthotrephis has long been known, and doubtful traces of Calamites and ferns have been found, but it was not until 1897 that undoubted Palaeozoic fossils were obtained.
In the primary structure of the stem the Calamites present many points of resemblance to Equisetum, but secondary thickening went on in both stem and root.
Some Calamites were heterosporous, sporangia with microspores and megaspores being found in the same cone.
Our knowledge of the extinct Equisetales, full as it is with respect to certain types, does not suffice for a strictly phylogenetic classification of the group. The usual subdivision is into Equisetaceae including Equisetum and Equisetites (with which Phyllotheca and Schizoneura may be provisionally associated), and Calamariaceae, including Calamites and Archaeocalamites.