A new species of the morphogenus Rhysocaryoxylon Dupéron (Rhysocaryoxylon madsenii Sakala and Gryc sp. nov.) is described from the Lower Eocene of the Fur Formation, Mors island, Denmark. This permineralized fossil angiosperm wood is semi-ring-porous with distinct growth ring boundaries, vessels solitary or in radial multiples of 2-5, perforation plates exclusively simple, and tyloses abundant.
Rays are 1-5-seriate and heterocellular with a body composed of procumbent cells and 1-4 rows of upright marginal cells. Axial parenchyma is reticulate with numerous prismatic crystals both in chambered cells and idioblasts, forming long chains up to 12 cells high.
Its equivocal botanical affinities within the family Juglandaceae are discussed.