Equisetaceae, sometimes called the horsetail family, is the only

extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, ...

family of the order

Equisetales Equisetales is an order of subclass Equisetidae with only one living family, Equisetaceae, containing the genus ''Equisetum'' (horsetails). Classification In the molecular phylogenetic classification of Smith et al. in 2006, Equisetales, in its ...

, with one surviving genus, ''

Equisetum ''Equisetum'' (; horsetail, snake grass, puzzlegrass) is the only living genus in Equisetaceae, a family of ferns, which reproduce by spores rather than seeds. ''Equisetum'' is a "living fossil", the only living genus of the entire subclass Eq ...

'', which comprises about twenty species.

Evolution and systematics

Equisetaceae is the only surviving family of the

, a group with many fossils of large tree-like plants that possessed ribbed stems similar to modern horsetails. '' Pseudobornia'' is the oldest known relative of ''Equisetum''; it grew in the late

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

, about 375

million years ago The abbreviation Myr, "million years", is a unit of a quantity of (i.e. ) years, or 31.556926 teraseconds. Usage Myr (million years) is in common use in fields such as Earth science and cosmology. Myr is also used with Mya (million years ago). ...

and is assigned to its own order. All living horsetails are placed in the

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

''. But there are some

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

that are not assignable to the modern genus. ''Equisetites'' is a "

wastebin taxon Wastebasket taxon (also called a wastebin taxon, dustbin taxon or catch-all taxon) is a term used by some taxonomists to refer to a taxon that has the sole purpose of classifying organisms that do not fit anywhere else. They are typically defined ...

" uniting all sorts of large horsetails from the

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceo ...

; it is almost certainly

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

and would probably warrant being subsumed in ''Equisetum''. But while some of the species placed there are likely to be ancestral to the modern horsetails, there have been reports of

secondary growth In botany, secondary growth is the growth that results from cell division in the cambia or lateral meristems and that causes the stems and roots to thicken, while primary growth is growth that occurs as a result of cell division at the tips of ...

in other ''Equisetites'', and these probably represent a distinct and now-extinct horsetail lineage. ''Equicalastrobus'' is the name given to fossil horsetail strobili, which probably mostly or completely belong to the (sterile) plants placed in ''Equisetites''. (2005): ''Equisetites aequecaliginosus'' sp. nov., ein Riesenschachtelhalm aus der spättriassischen Formation Santa Clara, Sonora, Mexiko 'Equisetites aequecaliginosus'' sp. nov., a tall horsetail from the Late Triassic Santa Clara Formation, Sonora, Mexico ''Revue de Paléobiologie'' 24(1): 331-364 erman with English abstractbr>PDf fulltext
/ref>

References

External links

{{Taxonbar, from=Q517643 Fern families Mesozoic first appearances