THE EARLIEST VASCULAR PLANT OR A LATER ROOTING SYSTEM? PINNATIRAMOSUS QIANENSIS FROM THE MARINE LOWER SILURIAN XIUSHAN FORMATION, GUIZHOU PROVINCE, CHINA

摘要

Pinnatiramosus qianensis Geng, a plant with a complex, extensive, pinnate branching system and pitted tracheids from marine Llandovery (lower Silurian) rocks in Guizhou Province, challenges long-held theories on the origin and early evolution of vascular plants in the Silurian and Devonian periods. This has led to the hypothesis that the fossils are not syngenetic with the entombing rock but represent the rooting systems of much younger plants, possibly of Permian age. Permian strata overly the Llandovery rocks unconformably in the succession. Leafless axes with less ordered branching occur closer to the boundary and may have had a similar source. Existing and new material of Pinnatiramosus has been subjected to detailed analyses in an attempt to resolve the problem. This has involved examination of the branching systems with respect to the surrounding matrix, comparative morphological descriptions of the systems, and anatomical investigations based on compression and permineralized fossils. The latter in particular indicate an endogenous origin of the lateral branches, typical of roots. Collectively such evidence is highly suggestive of roots of younger plants growing through lithified sediments—plants whose affinity and age remain to be determined but which show an amazing capacity to fill two-dimensional space efficiently, accompanied by an avoidance strategy that is also seen in recent angiosperms.