Abstract
The Early Devonian Rhynie chert preserves the earliest terrestrial ecosystem and informs our understanding of early life on land. However, our knowledge of the 3D structure, and development of these plants is still rudimentary. Here we used digital 3D reconstruction techniques to produce the first well-evidenced reconstruction of the structure and development of the rooting system of the lycopsid Asteroxylon mackiei, the most complex plant in the Rhynie chert. The reconstruction reveals the organisation of the three distinct axis types - leafy shoot axes, root-bearing axes, and rooting axes - in the body plan. Combining this reconstruction with developmental data from fossilised meristems, we demonstrate that the A. mackiei rooting axis - a transitional lycophyte organ between the rootless ancestral state and true roots - developed from root-bearing axes by anisotomous dichotomy. Our discovery demonstrates how this unique organ developed and highlights the value of evidence-based reconstructions for understanding the development and evolution of the first complex vascular plants on Earth.
Highlights
The Silurian–Devonian terrestrial revolution saw the evolution of vascular plants with complex bodies comprising distinct roots, root-bearing organs, shoots, and leaves from morphologically simpler ancestors characterised by networks of undifferentiated axes (Bateman et al, 1998; Gensel and Edwards, 2001; Kenrick and Crane, 1997; Stewart and Rothwell, 1993; Xue et al, 2018)
To discover the structure and infer the development of the lycopsid A. mackiei, we produced a series of 31 consecutive thick sections through a block of Rhynie chert that preserved a branched network of connected A. mackiei axes in situ (Figure 1—figure supplement 1, Figure 1—figure supplement 2)
Four leafy shoot axes with similar anatomy attached to the main axis at anisotomous branch points; an anisotomous branch point is a description of morphology and means that the diameters of the two axes connected at a branch point are different (Gola, 2014; Imaichi, 2008; Ollgaard, 1979; Yin and Meicenheimer, 2017)
Summary
The Silurian–Devonian terrestrial revolution saw the evolution of vascular plants with complex bodies comprising distinct roots, root-bearing organs, shoots, and leaves from morphologically simpler ancestors characterised by networks of undifferentiated axes (Bateman et al, 1998; Gensel and Edwards, 2001; Kenrick and Crane, 1997; Stewart and Rothwell, 1993; Xue et al, 2018). While the cellular detail that can be observed in these sections allows high-resolution reconstruction of tissue systems, the three-dimensional relationship between the cells, tissue, and organs is obscured. This makes generating accurate reconstructions of body plans difficult (Edwards, 2003; Kidston and Lang, 1921). Reconstructions that have been published are based on combining material from thin sections from multiple individuals (Kidston and Lang, 1921).
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