Sphenopsida (horsetails) Class of the Pteridophytina that first appeared in the Devonian and reached the peak of its abundance and diversity during the Carboniferous, forming a major component of the coal-swamp vegetation. Sphenopsids are characterized by jointed stems with whorls of leaves and branches borne at the joints (or nodes). The internodal part of the stem is vertically ridged and spores are produced in rings of sporangia arranged in cones, usually at the tips of the fertile shoots. The only living genus, Equisetum, is a comparatively small plant (different species ranging between 4 or 5 cm and 12 m), but one of the best-known fossil genera, Calamites, included tree-like forms that grew up to 30m in height. Another common fossil sphenopsid, Sphenophyllum, was a slender plant with a ribbed stem only 1–7 mm in diameter but up to several metres in length, that probably scrambled over other vegetation. See also ARCHAEO-CALAMITES RADIATUS; CALAMITES CISTIIFORMES; and EQUISITITES HEMINGWAYI.

Sphenopsida A subdivision of the Pteridophyta, represented today by 1 genus only, Equisetum. Sphenopsids flourished in the Carboniferous coal swamps and one of the fossil types of this period, Calamites, included tree-like forms that grew to 30 m.