Sporangial morphology of the early devonian zosterophyll sawdonia ornata from the type locality (gaspé).

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Gensel, Patricia ; Berry, Christopher Mark (2016)
  • Publisher: University of Chicago Press

Premise of research. Despite being an iconic and recognizable zosterophyll from the Early Devonian, sporangia of Sawdonia ornata are not known in any detail from the type locality, being represented by a drawing and a few illustrations from other localities. This has hampered recognition and/or comparison of other zosterophylls or newly discovered plants to that taxon. A detailed study of sporangia obtained from axes of S. ornata from the type locality was conducted to investigate sporangial morphology and range of vegetative features.\ud \ud Methodology. Fertile specimens were prepared by dégagement and maceration to expose both vegetative and reproductive features more fully than previously. Maceration produced axes and sporangia that were examined by LM and SEM.\ud \ud Pivotal results. Sporangia of S. ornata from the type locality consist of unequal valves: a larger and more rounded outer/abaxial valve—in which the stalk merges imperceptibly with the back of the valve and both are covered with spines—and a smaller, thinner, and flat inner/adaxial valve that lacks spines. Each valve has a narrow flat rim not readily visible unless the sporangium is open. The sporangia are located near and well below the apex of aerial stems and may be located on one or two sides of the stems, with the sporangia being curved in toward the stem and slightly around it, mostly at a slightly acute angle to the stem. Variation in density and angle of lateral branching within a pseudomonopodial system is also demonstrated. A new reconstruction of the sporangia is provided.\ud \ud Conclusions. The new data on sporangial morphology from type locality specimens provide a clear set of characters delimiting S. ornata, type for the genus, and require that the taxonomic circumscription of S. ornata be emended. This in turn affects identification of closely similar taxa, such that some are excluded from the genus, others are questionably attributed to Sawdonia as cf. Sawdonia sp. These differences are considered significant enough to recommend placing Sawdonia acanthotheca Gensel, Andrews, and Forbes in another genus and to rename Ensivalia deblondii Gerrienne as a new combination, Sawdonia deblondii (Gerrienne) Gensel et Berry. Sawdonia is the third zosterophyll taxon to demonstrate sporangia with unequally sized valves and, more intriguingly, little differentiation between stalk and distal valve. These results will allow more accurate comparison of other zosterophylls to this genus in terms of reproductive characters and assessment of diversity in zosterophylls than previously. It is less clear whether a certain set of vegetative—particularly cuticular—features continue to allow one to recognize this taxon from fragments.
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