A new species of the sphenopsid stem Arthropitys from Late Permian volcaniclastic sediments of China

Abstract

Anatomically preserved calamitean stems are described from a previously unknown Late Permian locality in Sichuan Province, China, where they are preserved in volcaniclastic tuffs. Stems possess a large pith with secretory cells and pith cavity, and a narrow perimedullary zone. Primary xylem strands have poorly developed carinal canals whereas the secondary xylem is divided into parenchymatous interfascicular zones and fascicular wedges of tracheids and secondary rays, with the tracheids of the inner part of the secondary xylem having nearly the same size and shape with the interfascicular rays. Interfascicular rays taper abruptly centrifugally. Tracheids of the secondary xylem have scalariform thickenings and cross-fields possess oval–elongate scalariform pits. Secondary rays are parenchymatous and narrow, usually uni- to biseriate and less frequently organized in three to four seriates. The cortex is divided into an inner and outer zone, and primary xylem strands alternate at the nodal region. These stems conform to the features of the genus Arthropitys Geoppert and are most similar to Arthropitys lineata from the Early Permian of Europe; they also are similar to Arthropitys communis and Arthropitys hirmeri var. intermedia from Late Carboniferous coal balls in Europe and North America. However, the Chinese specimens are distinct in a number of ways including characters of the cells surrounding the carinal canals, the carinal canals themselves, features of the secondary xylem and also in their tracheid morphology, leading to the creation of a new species, Arthropitys junlianensis sp. nov. Implications of A. junlianensis on sphenopsid taxonomy, phylogeny and distribution are discussed, and the nearly identical morphology of this new sphenopsid to those of the Late Paleozoic coal swamps of Europe and North America is considered.