Yet this species' distinguished phylogenetic placement is not the only exciting and unique aspect of this fossil discovery. Because E. thermale was preserved in life position where they grew in geothermally influenced habitats, Channing and his colleagues were also able to deduce many aspects about its habitat, the stresses it endured, and its potential mechanisms of stress tolerance. They could also compare its morphological features to extant species of Equisetum today—which can also be found in mineral- and geothermal-spring environments—and make inferences about the fossil species' ecology.
Indeed, the authors found that E. thermale had anatomy that suggests it was adapted for both wetland and dry settings. For example, E. thermale had an extensive network of air spaces in its stems and rhizomes that provided aeration for its water-flooded rooting system.
"Hot spring waters, of course, can cause heat stress," explains Channing. "But the water also has high pH and alkalinity and contains dissolved salt and heavy metals that may be toxic to plants. These stresses mean local plants suffer physiological drought—because taking up water also means an increase in the uptake of, for example, salt—and typically have anatomical features that help reduce water loss through evapotranspiration."
Channing points out that E. thermale also exhibited a number of features that would reduce water loss. Its epidermis had thick outer walls, a well-developed cuticle and silica deposits, and its stomata were situated well below the stem surface and were protected by cover-cells and silica deposits. "The silica deposits of E. thermale hint at a physiological mechanism of stress tolerance," adds Channing, "as silicon uptake has been demonstrated to ameliorate salt, heat, and heavy metal stresses in living crop plants."
"These adaptations exist in the horsetails to this day," Channing said, "illustrating that the genus developed a successful set of tools for life in extreme environments and has maintained them for millions of years."
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