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Iridescent ammonite 20-30mm
Today this group is composed of about 20-30 species within the genus Equisetum. These plants , also called horsetails. They have hollow, jointed stems and needle-like leaves and have a characteristic appearance. With few exceptions (the largest modern horsetail, a tropical species from Mexico, can grow upo to almost 3 metrs high), most current forms are herbaceous and relatively small in size.
Although Sphenophytes have never been as diverse as the other spore plants, such as ferns and Lycophytes, they have left behind an interesting fossil record. Besides the Equisetales (which include all current representatives) the group consists of two extinct orders, namely Pseudoborniales and Sphenophyllales.
The oldest member of the Sphenophyta belongs to the Pseudoborniales and comes from the Late Devonian of Alaska . This plant, Pseudobornia, had a trunk diameter up to 60 cm and could reach a height of 20 meters. One section of the stem bore only one or two branches, the secondary branches were full of leaves. The spores were in the cones at the ends of the primary branches. This early form had many horsetail characteristics, ribbed, jointed stems, leaf wreaths and cones. The complexity of the leaves however they have in common with the Pseudobornia cladoxylales, a group of early fern plants.
The Sphenophyllales appeared in the Lower Carboniferous and did well in the tropical paleo - equatorial forests of the Late Carboniferous. Sphenophyllum and the other genera in this group are characterized by grouped, but more complex leaf shapes, and the triangular cross-section of their articulated stems. They were herbaceous, climbing plants. The Sphenophyllales probably became extinct at the end of the Permian, although some questionable Triassic records are known.
The Equisetales history goes back far. Horsetail trees from Calamitaceae family were not very different in appearance than the current Equisetum, but could reach heights of at least 10 meters (thanks secondary thickening wood). They grew in the swampy forests of the Upper Carboniferous. The hollow stems of these plants often became filled with sediment during deposition. Although the plant itself generally was not preserved, fossil internal molds of the inside of the stem of the specimens are general fossils from the Carboniferous. Such finds are classified in the form-genus Calamites. Like the modern Equisetales they spread through roots called rhizome. The leaves of Calamites are known as Asterophyllites (side view) and Annularia ( wreaths). Also in the Mesozoic such stems occur, which are named Neocalamites.
Modern Equisetaceae family shows no secondary growth, and has whorls that are grown together with the base. The oldest certain Equisetaceae come from the Paleogene. There are also older discoveries which are very similar with Equisetum, but due to small differences or insufficient information, they are usually considered to be Equisetites.
Photos of locations of Sphenophyta at this site
Thanks to Tim Wolterbeek for writing this text. English translation by Herman Zevenberg.
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