An association of Alethopteris foliage, Trigonocarpus ovules and Bernaultia-like pollen organs from the Middle Pennsylvanian of Nova Scotia, Canada

摘要

The early Cantabrian roof shale of the Lloyd Cove Seam in the Sydney Coalfield, Nova Scotia, has yielded a parautochthonous fossil assemblage of medullosalean remains dominated by Alethopteris pseudograndinioides var. subzeilleri foliage. In association are large ovules of the morphospecies Trigonocarpus grandis, which is probably an adpression analogue of the anatomically-preserved ovules that have been widely but incorrectly named Pachytesta gigantea, and pollen organs that are similar in morphology to Bernaultia.Although direct organic attachment of the ovules and foliage to the same parent plant could not be confirmed, FTIR spectral analysis of their cuticles yielded similar results (notably, similar Al/ox ratios). No evidence was found that the ovules were attached to normal vegetative fronds. Rather, a number of ovules were found immediately adjacent to striate, presumed medullosalean axes, suggesting that the ovules may have originally been attached to specialised fertile fronds that lacked lamina. Maceration of the ovules yielded a thin megaspore membrane, and cuticles probably from the outer surface of the nucellus and the inner surface of the integument;the cuticle from the outer surface of the integument was probably lost during fossilisation. Similarities can be drawn with the stratigraphically younger association of Alethopteris zeilleri foliage, Pachytesta incrassata ovules and Bernaultia pollen organs; also with the stratigraphically older association of Alethopteris sullivantii foliage, Pachytesta noei ovules and Sullitheca pollen organs. This suggests that through Stephanian times this group of medullosaleans showed a progressive increase in size and complexity of the reproductive organs, accompanied by an increase in venation density. These results suggest that, although robust whole-plant reconstrucdons are difficult to achieve with these relatively large medullosalean trees, it is possible to determine broad patterns of phylogenetic change by the independent study of different plant organs.