Deep-Time Evolution of Tubers in <i>Equisetum</i> and the Broader Sphenophytes

Tubers are locally enlarged stem or root organs that primarily function in carbohydrate storage. Compared to angiosperms, much less taxonomic and morphological diversity of tubers is known among the vascular spore plants. Belowground lateral stem tubers are known in Equisetum, the sole living genus of the once-diverse sphenophytes, and this review addresses the evolution of tubers in Equisetum and the broader sphenophytes in deep time. The anatomy of Equisetum tubers shows increased storage parenchyma and reduced structures for water transport and aeration, features distinct from the rhizomes. Tubers are morphologically variable in shapes, arrangements, and numbers; however, the variation reflects the developmental plasticity and bears little phylogenetic significance among living taxa. Therefore, tuber morphology may be ill fitted as a taxonomically diagnostic character of fossil taxa. Tuberous Equisetum fossils are exclusively known from the northern hemisphere, predominantly from the Cretaceous and Cenozoic deposits. They are mostly from the mid- to high paleolatitudes, which may relate to the role of photoperiod in tuberization. Tubers have been exclusively described from members of the subgenus Equisetum among extant taxa, and the spatiotemporal distribution of the fossil tubers largely concurs with the reconstructed evolutionary history of this subgenus. However, the oldest tuberous Equisetites from the latest Triassic–earliest Jurassic, separated by a 50-million-year gap from the other records and far preceding the estimated divergence age between the tuberous subgenus Equisetum and the nontuberous Hippochaete, suggests that the lateral stem tubers may be plesiomorphic or may have evolved more than once within the genus. Among the broader sphenophytes, disparate and nonhomologous types of stem tubers, whose tuberization occurs along the main plant axis via secondary growth, have been described in Archaeocalamitaceae and Calamitaceae. Such occurrences demonstrate that a greater diversity of stem tubers existed in the sphenophytes in the geologic past, expanding our understanding of the evolution of belowground storage organs in deep time.