ИСТИНА

Permineralised wood (up to 24.5 cm long and 19.5 cm wide) was found in the Upper Devonian deposits of Middle Timan (north-west of European Russia) for the first time. The specimen is preserved as a flattened piritised petrifaction and was identified as a trunk of Archaeopteris/Callixylon by the structural features of its secondary xylem. About 30 small wood fragments from the different portions of the trunk has been studied with a scanning electron microscopy (SEM CAMSCAN). The secondary xylem and some elements of mesarch primary xylem are well-preserved in the wood fragments. Metaxylem in secondary branches is characterized by polygonal in shape tracheids (12-25 μm in diameter) with scalariform bordered pits both on radial and tangential walls of tracheids. Secondary xylem is of pycnoxylic type of the wood. The tracheids of secondary xylem are narrow (19-45 μm in diameter) and very long (about 1 mm long). The tracheids on their radial walls show distinct groups of bordered pits horizontally aligned in various tracheids. Accoding to N.S. Snigirevskaya (2000), such type of radial pitting is designated as cohortoid pitting typical for Callixylon. There are 2-3 rows of the pits in each individual group. The pits are hexagonal in shape, 8 - 11 μm in diameter with inclined split-like apertures. Xylem rays are numerous, low, mostly uniseriate, only some partially biseriate. Ray cells are various in their sizes, and rectangular in shape. Cross-fields show 6-14 cupressoid pits with inclined split-like apertures. Early Archaeopteris/Callixylon wood was found in five localities of north-west of European Russia. This new finding of Callixylon in the Late Devonian flora of Russia extends our knowledge about distribution of Archaeopteris/Callixylon forest formations.