The earliest wood and its hydraulic properties documented inc. 407-million-year-old fossils using synchrotron microtomography

Abstract

We document xylem structure and hydraulic properties in the earliest woody plant Armoricaphyton chateaupannense gen. nov. & sp. nov. based on c. 407-million-year-old fossils from the Armorican Massif, western France. The plant was small, and the woody axes were narrow and permineralized in pyrite (FeS2). We used standard palaeobotanical methods and employed propagation phase contrast X-ray synchrotron microtomography (PPC-SRμCT) to create three-dimensional images of the wood and to evaluate its properties. The xylem comprised tracheids and rays, which developed from a cambium. Tracheids possessed an early extinct type of scalariform bordered pitting known as P-type. Our observations indicate that wood evolved initially in plants of small stature that were members of Euphyllophytina, a clade that includes living seed plants, horsetails and ferns. Hydraulic properties were calculated from measurements taken from the PPC-SRμCT images. The specific hydraulic conductivity of the xylem area was calculated as 8.7 kg m −1 s −1 and the mean cell thickness-to-span ratio (t/b) 2 of tracheids was 0.0372. The results show that the wood was suited to high conductive performance with low mechanical resistance to hydraulic tension. We argue that axis rigidity in the earliest woody plants initially evolved through the development of low-density woods.

Mineralogy of Non-Silicified Fossil Wood

“…Other possibilities exist. For example, the oldest known woody plant Armoricaphyton chateaupannense from Lower Devonian of France is preserved with carbonized cell walls and microcrystalline pyrite in cell lumen and as an accessory material in the walls [36]. At Macon, Georgia, USA, Holocene wood preserved in an anaerobic aquifer environment has been found to be mineralized with both silica and pyrite, with some adjacent silicate mineral grains in the matrix having been replaced by pyrite.…”

X-ray micro-computed tomography (micro-CT) of pyrite-permineralized fruits and seeds from the London Clay Formation (Ypresian) conserved in silicone oil: a critical evaluation

“…2 mm cube) within a 10 mm × 8 mm × 3 mm piece of iron pyrite permineralized woody axis from an early Devonian plant using propagation phase contrast X-ray synchrotron microtomography (PPC-SRCT). Strullu-Derrien et al (2014) succeeded in digitally dissecting out the iron pyrite in some areas to reveal the organic tracheid walls and hence reconstruct the hydraulic properties of the wood. DeVore et al (2006) studied one iron pyrite permineralized fruit of Palaeorhodomyrtus subangulata (Bowerbank) Reid and Chandler from the London Clay Formation by high resolution X-ray computed tomography (HRXCT).…”

“…Perhaps the silicone oil will be digitally separable from the specimens, despite its similar X-ray attenuation with that of the organic material. The iron pyrite could perhaps be readily separated from the organic material, in a similar manner to the work by Strullu-Derrien et al (2014), although in some cases, iron pyrite dispersed through the organic material could complicate this approach and it may be more difficult with silicone oil. Smaller voxel size and thus higher resolution scans might reveal the fine cellular detail that is currently lacking in micro-CT images.…”

Forbidden phenotypes and the limits of evolution

“…Low ion concentrations could explain some of these absences, but lakes and rivers in limestone-rich areas support relatively thick shells and could be favourable to reef development. Woody vegetation originating in the Early Devonian [57] builds rigid frameworks on land, in mangrove habitats and in freshwater swamps, but the trees are always emergent.…”