Phytochemistry of the fossilized-cuticle frond Macroneuropteris macrophylla (Pennsylvanian seed fern, Canada)

In Canada's Sydney Coalfield, specimens of the extinct Carboniferous seed fern Macroneuropteris macrophylla (Brongniart) invariably show preservation stages intermediate between compression and fossilized-cuticle, even concerning a single pinnule. In this interdisciplinary approach, we study a ca. 300 to 350 mm long fossilized-cuticle-preserved frond section of M. macrophylla (Brongniart) that represents about one third of the length of a frond that was originally 1 m long. Size and preservation allow us to study the phytochemistry of the cuticle biomacropolymers over the length of the frond to assess what impact, if any, results would have on Carboniferous palaeophytochemotaxonomy. For comparison, the phytochemistry of compressions with their extracted cuticles from the same species and the same sample locality is also investigated. We use solid- and liquid-state, semi-quantitative Fourier transform infrared spectroscopy (FTIR) for the chemical characterization of the frond.Based on our results, we infer an essentially uniform phytochemistry over the fossilized-cuticle frond, suggesting that only a single pinnule needs to be analyzed to get an overall phytochemical picture of the frond, which has been our long-time working hypothesis. We distinguish between phytochemistry and cutinization. The latter is much less pronounced above than below the frond dichotomy, and we suggest a palaeoecological cause, rather than differing pathways of organic matter transformation. Moreover, cuticles below and above the frond dichotomy have essentially the same epidermal pattern, but those from below have features that may have been an adaptation to prevent stomatal flooding during the tropical, rainy season.This study suggests that chemically the fossilized-cuticle is more similar to the compression than to the cuticle obtained from that compression of the same species which invites reevaluation of the classical compression concept.

Research Highlights
► First-time morphophytochemical study of a Carboniferous seed fern frond.
► The frond segment Macroneuropteris macrophylla is preserved as fossilized-cuticle.
► Modern solid- and liquid-state FTIR methods provide semi-quantitative phytochemistry.
► Main phytochemical parameters are virtually invariant over the frond.
► Analysis of a single leaf reflects overall phytochemistry of the frond.