WUCHIAPINGIAN ARAUCARIA-LIKE WOOD FROM THE WUTONGGOU FORMATION IN NORTHERN BOGDA MOUNTAINS, NORTHWESTERN CHINA

Systematic studies on fossil woods, the main organs of terrestrial plants, improve our knowledge of the composition of past vegetation. Here we describe a kind of silicified fossil wood from the lowermost Wutonggou Formation (Wuchiapingian) in the Dalongkou section, northern Bogda Mountains, Xinjiang. It has a homoxyleous secondary xylem, and is pycnoxylic, composed of tracheids and parenchymatous rays. Growth rings are distinct. False rings are common. Axial parenchyma and resin ducts are absent. The tracheids are circular, elliptical, or rectangular with round corners in transverse section, 18× 20 to 57× 68 μm in size. Bordered pits on tracheidal walls are circular, elliptical or hexagonal in outline, 13-18 μm in diameter with inclined elliptical apertures. Pits are uni- to pentaseriate, commonly bi- to triseriate. When uniseriate, the radial bordered pits of tracheids are contiguous; when multiseriate, pits are alternately arranged. Rays are homogeneous, uniseriate, 2–10 cells high, and are composed of procumbent rectangular parenchymatous cells, ranging from 88-194 μm long and 17-28 μm high radially. Cross-field pitting is of the araucarioid. The pits in the cross-field are bordered, circular, elliptical or hexagonal, with inclined elliptical apertures, arranged in 3–6 rows. There are 8–22 pits in each cross-field unit. The anatomical features indicate current fossil wood is consistent with Araucarioxylon kuznetzense Lepekhina.

Araucaria-like fossil wood (i.e., isolated pieces of secondary xylem with araucarian pitting on the radial walls of tracheids and araucarioid cross-field pitting, together with mostly uniseriate rays), unless it bears special features (e.g., resin canals, inflated axial parenchyma, or ray cell wall thickenings), has been given various names. There have been long-lasting controversies concerning the nomenclature of Araucaria-like fossil woods. However, it has been proved that Agathoxylon Hartig is the only legitimate name based on ICBN rules, regularly used and was circumscribed by a diagnosis that permits inclusion of fossil species with Araucaria-like anatomy. Therefore, according to the anatomical characteristics, we assign present material to Agathoxylon Hartig and transfer Araucarioxylon kuznetzense Lepekhina to Agathoxylon kuznetzkense comb. nov..