The first fossil record of a giant horsetail (Equisetum, Equisetaceae) is from the Miocene of Patagonia, Argentina

Marcos Asensio,Carlos Zavala,Daniela Olivera,Maria C Zamaloa,Elina Cornou,Marcelo Martínez,Mirta Quattrocchio

doi:10.5027/andgeov49n2-3417

Marcos Asensio, Carlos Zavala + Show 5 more

Open Access

https://doi.org/10.5027/andgeov49n2-3417

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Abstract

A Miocene macroflora recovered from Ñirihuau Formation sediments exposed at Quebrada Vera site in northwesterern Patagonia, Argentina, is recorded and described for the first time. The assemblage is composed exclusively of free-sporing plants, mainly by Equisetum remains with subordinate occurrences of four fern morphotypes (represented by bi-, tri- and pinnate fronds). Equisetum imprints and molds include distinctive jointed stems with whorls of linear and basally fused leaves, numerous scars of lateral branches arranged in a radially symmetrical pattern situated at the nodes, and nodal diaphragms of up to 4 cm in diameter. The large size and regular branching of the stems link the fossils to the South American giant members of the genus and they probably represent a new fossil species. This is the first conclusive fossil record of a giant Equisetum worldwide, and consequently, it is biogeographically and evolutionarily relevant. The new findings attest that members of the giant horsetail clade were components of the Patagonian vegetation in the Miocene, implying that the age of the clade must predate that estimated from morphological and/or molecular data. The plant fossil assemblage represents part of a wetland community probably growing close to a riverside or lakeshore in coincidence with previous sedimentological estimates.

Highlights

A Miocene megaflora recovered from outcrops of the Ñirihuau Formation exposed at Quebrada Vera site, northwest Patagonia, Argentina, is reported and described for the first time
A Miocene flora collected at the Ñirihuau Formation sediments exposed at the Quebrada Vera locality is recorded and described for the first time
By comparison with the ecological requirements of the living analogous of the paleoflora, it is estimated that the assemblage represents part of a littoral plant community growing in a wet environment, probably close to a riverside or lake margin, and that it was composed of a population of tall Equisetum sp. plants that occupied the upper stratum while several types of ferns developed in the lower community levels

Summary

Introduction

A Miocene megaflora recovered from outcrops of the Ñirihuau Formation exposed at Quebrada Vera site, northwest Patagonia, Argentina, is reported and described for the first time. Representatives of the genus Equisetum are the only living members of the once abundant, diverse, and worldwide distributed group of vascular plants, the Equisetales. These ancient and morphologically distinctive free-sporing plants have a long geologic history that can be traced from the Devonian to the recent (Taylor et al, 2009). Living Equisetum species, commonly known as horsetails, grow naturally throughout much of the world; they are absent in Australia, New Zealand, Antarctica, and islands of the Central Pacific, Indian, and South Atlantic oceans (Schaffner, 1930). Most of the 18 living species are distributed in the Northern Hemisphere and four species grow naturally in South America (Christenhusz et al, 2019)

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