Growth Architecture Diversity Among Permian Calamitaleans in Brazil

Sizeable stems of exceptional three-dimensional anatomical preservation add an important aspect to the growth form diversity of calamitaleans found in the Permian fossil sites of the Parnaíba Basin, central-north Brazil. A thorough study of new material provided a novel account to arborescent horsetail relatives and made us aware of a much more diverse picture that these extinct giants deserve. As part of the riparian vegetation of rivers and lakes, calamitalean trees inhabited sandy substrates. Thus, they were found parautochthonously in well-sorted mature medium-grained sandstones of fluvial plain settings. After burial, the tree trunks were petrified by silica and staining constituents. Being among the youngest calamitaleans of the geological past, the finds paved the way for a more sophisticated view on this group of ancient spore-producing plants. The woody horsetail trees from Brazil suggest a considerable adaptive potential, which is revealed by diversity in both branching and anatomy. Accordingly, variable growth architectures resulted. They are reflected in several new reconstructions established to illustrate how these trees may have looked like. In addition, finds from the Parnaíba Basin provided the opportunity to study calamitalean root systems and challenge the traditional interpretation of exclusively rhizomatous growth. As a result, till they became extinct at the end of the Paleozoic, they survived in increasingly dynamic environments of Permian equatorial Pangaea, which were characterized by strong seasonality and multiple disturbances, which were meticulously recorded in the wood.