Charcoalified Agathoxylon-type wood with preserved secondary phloem from the lower Permian of the Brazilian Parana Basin

For the first time, secondary phloem tissue of Agathoxylon Hartig (sensu Rößler et al. 2014) is described from Permian charcoalified wood remains. Large (up to 13.4 × 21.8 cm), highly compressed, charred logs and branches were collected from the lower and upper boundaries of a tonstein bed (U/Pb age 291 ± 1.3 Ma) interlayered in a Sakmarian coal seam in the Faxinal Coalfield, Rio Grande do Sul, Brazil (Rio Bonito Formation). Small pieces were sampled with a dissecting knife and needles, mounted on standard stubs, gold-coated, and photographed under a scanning electron microscope. The phloem tissue is composed of thin-walled sieve cells, thick-walled fibers, and axial and radial uniseriate parenchyma. The extraordinary preservation shows sieve cells with pores grouped in conspicuous sieve areas occurring on lateral cell walls. Sieve cells are inferred to be arranged in tangential layers alternating with mixed rows of fibers and scarce parenchyma. Conclusions are drawn about preservation conditions involving cyclic environmental dryness and possible affinities with pteridosperms and cordaitaleans.