An evidence-based 3D reconstruction of Asteroxylon mackiei, the most complex plant preserved from the Rhynie chert

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.

An evidence-based 3D reconstruction of Asteroxylon mackiei the most complex plant preserved from the Rhynie chert

The 407-million-year-old 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 complete reconstruction of the lycopsid Asteroxylon mackiei, the most complex plant in the Rhynie chert. The reconstruction reveals the organisation of the three distinct axes 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 plants on Earth.

On the problematic placement of the fossil arthropod Devonopilio hutchinsoni in Opiliones (Arachnida)

Harvestmen (Arachnida: Opiliones) were among the earliest terrestrial arthropods but their unmineralized exoskeletons are scarce and often fragmentary as fossils (Palencia et al. 2019). Consequently, the discovery and interpretations of fossil harvestmen from the early Palaeozoic can have disproportionate effects on the understanding of evolution in Opiliones. Recently, Devonopilio hutchinsoni Tihelka, Tian & Cai, 2020, was described as a new fossil harvestman from the well-known Rhynie chert deposits of Scotland, an important source of information on freshwater and terrestrial ecosystems of the early Devonian (Tihelka et al. 2020). This species would be one of the earliest records of harvestmen. The description was based on a single slide showing fragments of arthropod cuticle, which Tihelka et al. interpreted as a harvestmen penis and unspecified associated body parts. As specialists on harvestman morphology and systematics, we were intrigued by these conclusions. However, based on the material presented by Tihelka et al. we find no compelling evidence supporting the proposal that the specimen is a harvestman.