An alternative model for the earliest evolution of vascular plants

Abstract

Land plants comprise the bryophytes and the polysporangiophytes. All extant polysporangiophytes are vascular plants (tracheophytes), but to date, some basalmost polysporangiophytes (also called protracheophytes) are considered non‐vascular. Protracheophytes include the Horneophytopsida and Aglaophyton/Teruelia. They are most generally considered phylogenetically intermediate between bryophytes and vascular plants and are therefore essential to elucidate the origins of current vascular floras. Here, we propose an alternative evolutionary framework for the earliest tracheophytes. The supporting evidence comes from the study of the Rhynie chert historical slides from the Natural History Museum of Lille (France). From this, we emphasize that Horneophyton has a particular type of tracheid characterized by narrow, irregular, annular and/or, possibly spiral wall thickenings of putative secondary origin, and hence that it cannot be considered non‐vascular anymore. Accordingly, our phylogenetic analysis resolves Horneophyton and allies (i.e. Horneophytopsida) within tracheophytes, but as sister to eutracheophytes (i.e. extant vascular plants). Together, horneophytes and eutracheophytes form a new clade called herein supereutracheophytes. The thin, irregular, annular to helical thickenings of Horneophyton clearly point to a sequential acquisition of the characters of water‐conducting cells. Because of their simple conducting cells and morphology, the horneophytophytes may be seen as the precursors of all extant vascular plant biodiversity.

The Stele - A Developmental Perspective on the Diversity and Evolution of Primary Vascular Architecture

“…The sharp transition that is obvious in these fossil plants between tissues with very different identities -those of the stele and those of the surrounding cortex -indicates the presence of well-defined boundary conditions, thus, it is likely that a mechanism for boundary layer specification existed in the early protostelic tracheophytes. In fact, the protostele-type radial patterning of tissues pre-dates the tracheophytes, as it is seen in plants that lack tracheids and sieve cells -protracheophytes (Boyce 2008;Cascales-Miñana et al 2019) and even bryophytes, both in the gametophyte and sporophyte phases (Hébant 1977). In all these plants, the conducting tissues form a central cylinder, irrespective of the identity and degree of specialization of these cells, thus, a protostele sensu lato is the plesiomorphic condition of all axial plant organs.…”

Revisiting the Great Ordovician Diversification of land plants: Recent data and perspectives

“…A number of recent papers discuss the evolutionary context of land plants. As the focus of the present paper is not plant phylogenetics, the reader is referred to these latter papers (e.g., Wickett et al, 2014;Edwards and Kenrick, 2015;Delwiche and Cooper, 2016;Harholt et al, 2016;Gerrienne et al, 2016;Puttick et al, 2018;Cascales-Miñana et al, 2019).…”

What can lycophytes teach us about plant evolution and development? Modern perspectives on an ancient lineage

“…An ancient evolutionary divergence gave rise to today's dominant vascular plant flora and the bryophyte sister lineage of vascular plants (Puttick et al, 2018). Provascular fossil intermediaries have vastly different forms from either of these groups, showing incremental acquisition of sporophyte branching, differentiated vasculature, and indeterminate growth from a shoot tip (Cascales‐Miñana, Steemans, Servais, Lepot, & Gerrienne, 2019; Edwards, 1986; Edwards and Kenrick, 2015; Gensel and Berry, 2001; Harrison, 2017b; Lang, 1937). These traits contributed to the rise of vascular plants and set a platform for all subsequent plant radiations (Figure 1a).…”