Plant evolution and terrestrialization during Palaeozoic times—The phylogenetic context

• The phylogenetic context of the terrestrialization of the Streptophyta is reviewed.
• The relationships within streptophytes are compiled in two informal consensus trees.
• The definition and the biological affinities of the cryptospores are discussed.
• All clades of Palaeozoic embryophytes are described and discussed.

Terrestrialization probably began more than one billion years ago and irreversibly altered biogeochemical processes at planetary scale. In this paper, we focus on the terrestrialization process of the Streptophyta, the division that includes charophytes and land plants (embryophytes) and whose members are today ecologically dominant in all terrestrial environments. The timing and the phylogenetic context of the early evolution of land plants are reviewed. The available information on the relationships within embryophytes and related organisms is compiled in two informal consensus trees based either on morphological/anatomical or on molecular data. We also consider the algal/embryophyte transition through the analysis of the evidence provided by microfossils (cryptospores and spores). The ongoing debate about the definition of the term cryptospores, but more importantly about the biological affinities of these microfossils that are possibly derived from early land plants, is discussed. All important clades of embryophytes, with a focus on their Palaeozoic representatives, are described; the significance of several embryophyte key characters is evaluated. The terrestrialization of land plants evolved in different steps. The new term “proembryophytic phase” is introduced to define the very long period of time during which the green algae ancestor of land plants acquired all the evolutionary characters that ultimately allowed their terrestrialization since the late Precambrian. An “eoembryophytic phase”, spanning the Middle-Upper Ordovician, is defined based on the occurrence of the earliest evidence of liverwort-like plants. The inception of the trilete spores in Late Ordovician times is then taken to define the start of the eo/eutracheophytic phase, which lasts until the first occurrence of vascular plant macrofossils in the Silurian.