''Equisetum dimorphum'' is an extinct horsetail species of the family

Equisetaceae Equisetaceae, also known as the horsetail family, is a family of ferns and the only surviving family of the order Equisetales, with one surviving genus, ''Equisetum'', comprising about twenty species. Evolution and systematics Equisetaceae is ...

, and one of the oldest records of the genus ''

Equisetum ''Equisetum'' (; horsetail) is the only living genus in Equisetaceae, a family of vascular plants that reproduce by spores rather than seeds. ''Equisetum'' is a "living fossil", the only living genus of the entire subclass Equisetidae, which ...

''. It was found in rocks from the Lower Jurassic of Chubut,

Argentina Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...

, among other plants as

ferns The ferns (Polypodiopsida or Polypodiophyta) are a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular, i.e., having specialized tissue ...

conifers Conifers () are a group of cone-bearing seed plants, a subset of gymnosperms. Scientifically, they make up the division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single extant class, Pinopsida. All e ...

and pteridosperms. Their remains consist of stems, leaves, strobili, and pagoda structures, which are preserved as impressions and casts. The combination of fine grained sediment, and the probable silica deposits in the epidermis of the plant, have managed to conserve not only its gross morphology, but also epidermal details not often present in this kind of preservation. This species was described in 2015 in Ameghiniana by a team led by Andres Elgorriaga, that included investigators of the Museum of Paleontology Egidio Feruglio, the

Swedish Museum of Natural History The Swedish Museum of Natural History (), in Stockholm, is one of two major museums of natural history in Sweden, the other one being located in Gothenburg. The museum was founded in 1819 by the Royal Swedish Academy of Sciences, but goes bac ...

, and the Buenos Aires University.

Description

The stems are dimorphic, unbranched and present no sign of superficial ribs and valleys. They are hollow except at the nodal regions, where complex nodal diaphragms occur. The diaphragms are flat, with a pitted surface and a cart-wheel internal structure. Each node is covered by a leaf sheath, bearing up to 42 lanceolate leaves. The sheaths are fused nearly 75% of the leaf length, having a distinct commisural furrows dividing the individual leaves. The stem apices are usually topped with a pagoda-like structure formed by detached leaves from the nodes below. The strobili occur singly at the apex of reproductive stems, they are elliptic to oblong, with a rounded apex, and bear numerous whorls of hexagonal sporangiophores. The leaves present on the reproductive stems are far longer than normal ones. Stomata occur in broad bands, both superficially and sunken.

Morphological curiosities

One of the most notable features of this species is the presence of pagoda-like structures at the tip of its stems. This kind of structures is also present in one living species, '' Equisetum hyemale'', and is formed thanks to the unique type of growth that this group processes, with intercalary meristems at the base of each node. Another feature of this species is the presence of twin stems, result of a dichotomy of the main stems. This kind of feature was also noted in ''E. hyemale''.

Subgeneric placement

This species possess characters from both present day subgenus. It shares with the ''Equisetum'' genus the rounded strobilar apex and the dimorphic habit. It shares with the ''Hippochaete'' subgenus its dimorphic habit, the absence of branches, and the presence of pagoda structures. This species also shares characters with both subgenus at the same time, such as the position of stomata and the absence and presence of leaf tips. As '' Equisetum thermale'', another Jurassic fossil ''Equisetum'' from Argentina, ''E. dimorphum'' shows a combination of characters from both living subgenera, ''Equsietum'' and ''Hippochaete''. There are two other ''Equisetum'' species, very similar to ''E. dimorphum'', that also possess this mosaic of characters, and perhaps the three species belong to a previously unidentified fossil subgenus.

References

{{Taxonbar, from=Q30591068 dimorphum Jurassic plants Prehistoric plants of South America Jurassic Argentina Flora of Argentina Fossils of Argentina Fossil taxa described in 2015 Plants described in 2015