Bitelaria, an Early Devonian vascular plant, and its significance relative to the origin and radiation of early land plants.

摘要

A new occurrence of Bitelaria dubjanskii Istchenko & Istchenko 1979 emend. nov. from Emsian strata in New Brunswick, Canada, is investigated in the context of its previously proposed algal affinity and land plant features. Bitelaria is determined to be a tracheophyte (incertae sedis) on the basis of its central strand of scalariform-pitted tracheids with tapered end-walls. Its putative algal reproductive organs are reinterpreted as vegetative structures: "Conceptual-like structures" and "nemathecia-like structures" are reinterpreted, respectively, as lenticel-like structures and as arrested apices. Actual reproductive structures have not been found. The cuticular structure of Bitelaria did not, at first, appear to reflect a cellular organization consistent with vascular plant origin. The structure is reinterpreted as a cuticular epithelium, an adaptation allowing for tangential expansion of the surface of a primary tracheophyte axis. Variations in cuticular structure are used to propose an ontogenetic sequence for deviation of the three morphological types of axes present in Bitelaria. Knowledge of morphological variation within the species is broadened significantly, and the species concept is broadened to include Donotela fistulata Istchenko & Istchenko 1981.; Bitelaria is compared with seven genera of Silurian and Devonian "enigmatic" plants (plant-like organisms associated with terrestrial sedimentary deposits or having other features associated exclusively with land plants, but which, like Bitelaria, have previously been thought not to be embryophytes): Prototaxites, Nematothallus, Pachytheca, Parka, Protosalvinia, Spongiophyton, and Orestovia. Orestovia is most comparable to Bitelaria. The other "enigmata" are different; however, it is concluded that, like Bitelaria, some previous interpretations of their phylogenetic affinities need to be reassessed.; A cladogram representing hypothetical genealogical branching patterns of modern land plants and green algae is used to assess some possible relationships of the "enigmata." It is suggested that those "enigmata" with waxy plant-like cuticles or desiccation-resistant meiotic spores, or both, are probably members of the Streptophyta, either near or within the embryophyte clade (Parka, Protosalvinia, Spongiophyton), and that Orestovia is probably a tracheophyte. It is also suggested that those with filamentous anatomy, but no plant-like cuticles or spores (nematophytes, Pachytheca), cannot be assigned with certainty to any higher-ranked taxon.