The earliest fossil wood and its hydraulic properties documented in a 407 million year old plant from France

Understanding the early evolution of wood requires a detailed documentation of early fossil materials. Here we describe xylem structure and hydraulic properties in the earliest woody plant, Armoricaphyton chateaupannense gen nov., sp nov., based on ca 407 million year old fossils from the Armorican Massif, western France. We used standard palaeobotanical methods and a new approach: propagation phase contrast X-ray synchrotron microtomography (PPC-SRμCT) for fossil plant infilled with metallic sulphides and oxides, which enabled a three-dimensional analysis of the fossil wood in virtual histology. We show that the xylem was composed of P-type tracheids (an extinct wall structure of scalariform type), contained rays, and showed evidence of both periclinal and anticlinal cell divisions. A transverse section of xylem cell lumens extracted from the PPC-SRμCT volume was used to map cell dimensions. The specific hydraulic conductivity Ks was calculated as 17.4 kg m-1 s-1 MPa-1, and the mean cell thickness-to-span ratio (t/b)2 was 0,0372. These measurements indicate that the wood was suited to high conductive performance with low hydraulic safety. Our analysis indicates that wood initially evolved in small stature plants that were basal members of the Euphyllophytina, a clade that includes living seed plants, horsetails and ferns. We argue that stem rigidity in the earliest woody plants evolved through the development of low-density porous wood