Newly Discovered Plant Fossil Reveals More Than Age

The horsetail plant developed tools for extreme environments and maintained them for millions of years.

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Over 100 million years ago, the understory of late Mesozoic forests was dominated by a diverse group of plants of the class Equisetopsida. Today, only one genus from this group, Equisetum (also known as horsetail or scouring rush), exists—and it is a prime candidate for being the oldest extant genus of land plant.

There is some debate as to the evolutionary beginnings of the genus Equisetum. Molecular dating places the divergence of the 15 extant species of the genus around 65 million years ago (mya), yet the fossil record suggests that it occurred earlier than that, perhaps around 136 mya. A discovery of a new fossil Equisetum species now places this genus at 150 mya, living in an environment where it still can be found today—hot springs.

Hot spring ecosystems and how they become fossilized are the main areas of research interest for Alan Channing, a geobiologist at Cardiff University, Wales. When he and colleagues from Wales and Argentina discovered the abundant fossilized remains of a species of Equisetum in southern Patagonia, they realized that not only could these fossils shed light on the phylogenetic age of this group of ancient plants, but they could also tell us something about how these plants lived, what types of environments they were adapted to, and their ecology and physiology. Moreover, they were intrigued to find out how closely these aspects might match extant species of Equisetum, especially those living in similar environments today. Their findings are published in the April issue of the American Journal of Botany (http://www.amjbot.org/cgi/reprint/98/4/680).

When Channing and his colleagues first began working in the remote Deseado Massif of Santa Cruz Province, Argentina—an area of ancient volcanic rocks—they did not know if they would find plant fossils. But the discovery of a near-intact and extremely well-exposed fossil hot spring deposit of Jurassic age at San Agustin Farm led them to some amazing discoveries—not the least of which was the hot spring itself. Hot spring deposits are extremely rare in the fossil record, and examples that are older than the Miocene are even rarer.

One of their interesting finds was the abundant fossilized remains of a species of Equisetum. Not only were dense stands of aerial stems discovered, but also a wide variety of organs—including leaf sheaths, roots and rhizomes, branches, apices, and strobili (reproductive parts)—were found intricately preserved in blocks of chert.

"Because these plants were located in a hot spring," notes Channing, "the normal preservation processes were replaced by a process of cellular permineralization—plant tissues and cells were permeated by water containing dissolved silica, which was precipitated prior to plant decay and resulted in magnificent three-dimensional preservation of complete plants."

By cutting, polishing, and thinly sectioning blocks of chert (layers of crystallized silica formed by the ancient hot springs) from the deposit and then examining the preserved fossils with high-powered microscopes, the authors were able to describe in intricate detail the anatomy and morphology of a Jurassic Equisetum for the first time.

The authors discovered that in many ways the morphology and anatomy of this fossilized Equisetum is indistinguishable from those of species living today in two subgenera, Equisetum and Hippochaete. For example, it was evergreen, grew upright in a single straight stem, and had a double endodermis. Yet, there were some features that did not fit with any extant or fossil species of Equisetum—thus justifying the erection of a new species: Equisetum thermale.

"Equisetum thermale appears to be the oldest record of the genus Equisetum and at the very least, records that anatomically, essentially modern Equisetum–like horsetails have a history extending back to the Late Jurassic," said Channing. Indeed, these findings support the idea that Equisetum is an extremely ancient genus that has undergone little evolutionary innovation over the last 150 million years.