Living horsetails have jointed stems and whorls of thin leaves. Triassic horsetails, called Neocalamites (KNEE • oh • cal • uh • MIGHT • eez), were similar but generally larger. All horsetails are sphenophytes, a group of plants related to ferns. The stems of the extinct horsetail formed branches that extended out and up from the central trunk. Long, thin oval-shaped leaves were attached to the stems, typically forming round whorls of many leaves.