Exploring the origins of secondary growth – two Lower Devonian (Emsian) euphyllophytes from Gaspé, Canada, and their implications for the evolution of secondary growth
Secondary growth from a vascular cambium has a deep fossil record among euphyllophytes, with the earliest occurrence dated to 407 Ma. To date, Armoricaphyton and Franhueberia represent the only instances of secondary growth formally documented in the Early Devonian. Secondary growth diversified rapidly and was present in all major euphyllophyte lineages by the Middle Devonian. Here, I describe two new Early Devonian euphyllophytes exhibiting secondary growth, from the Emsian (c. 400-395 Ma) Battery Point Formation (Québec, Canada): Gmujij tetraxylopteroides gen. et sp. nov. and Perplexa praestigians gen et sp. nov. Both these plants possess mesarch actinosteles with Psilophyton-type tracheid thickenings, and each represents a new type of anatomical organization among Early Devonian wood-producing euphyllophytes. These new Early Devonian plants reveal an unexpected level of diversity in the early stages of the first major tracheophyte radiation. Importantly, Gmujij and Perplexa, together with the other Early Devonian euphyllophytes exhibiting secondary growth, reinforce existing hypotheses about the modular regulation of vascular cambial growth and demonstrate that vascular cambial growth is an assemblage of regulatory modules whose deployment followed a mosaic pattern in the evolution of tracheophytes
