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

Equisetaceae Equisetaceae, sometimes called the horsetail family, is the only extant family of the order Equisetales, with one surviving genus, ''Equisetum'', which comprises about twenty species. Evolution and systematics Equisetaceae is the only survivin ...

, and one of the oldest records of the 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 ...

''. It was found in rocks from the

Lower Jurassic The Early Jurassic Epoch (in chronostratigraphy corresponding to the Lower Jurassic Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-Jurassic extinction event, 201.3 Ma&nb ...

of Chubut,

Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, th ...

, among other plants as

ferns A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes except th ...

, conifers and

pteridosperms The term Pteridospermatophyta (or "seed ferns" or "Pteridospermatopsida") is a polyphyletic group of extinct seed-bearing plants (spermatophytes). The earliest fossil evidence for plants of this type is the genus ''Elkinsia'' of the late Devonian ...

. 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 ''Ameghiniana'' is a peer-reviewed scientific journal covering palaeontology published by the Asociación Paleontológica Argentina. It is named after the 19th century Italian Argentine palaeontologist Florentino Ameghino. The discovery of many ...

by a team led by Andres Elgorriaga, that included investigators of the

Museum of Paleontology Egidio Feruglio The Museum of Paleontology Egidio Feruglio (MEF, in es, Museo Paleontológico Egidio Feruglio) is a science research and exhibition center in Trelew city, Patagonia, Argentina. Its permanent and travelling exhibitions focus on the fossil remains ...

, the Swedish Museum of Natural History, and the

Buenos Aires University The University of Buenos Aires ( es, Universidad de Buenos Aires, UBA) is a public research university in Buenos Aires, Argentina. Established in 1821, it is the premier institution of higher learning in the country and one of the most prestigi ...

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 are also present in one living species,

Equisetum hyemale ''Equisetum hyemale'' (commonly known as rough horsetail, scouring rush, scouringrush horsetail and, in South Africa, as snake grass) is a perennial herbaceous vascular plant in the horsetail family Equisetaceae. It is a native plant throughout ...

, and are 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 ''Equisetum thermale'' is an extinct horsetail species in the family Equisetaceae described from a group of whole plant fossils including rhizomes, stems, and leaves. The species is known from Middle to Late Jurassic sediments exposed in the p ...

, 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