Secretory organs: Implications for lipoid taxonomy and kerogen formation (Seed ferns, Pennsylvanian, Canada)

Abstract

Secretory organs likely evolved with land plants in Silurian-Devonian time, but it is questionable if they were passed on to living cycadaleans upon the extinction of seed ferns (Triassic-Jurassic?). They are defined as ducts of schizo-, lysig- or rhexigenous origin that exuded a heterogeneous lipoid mixture that fossilized as secretory products (droplets). In this study we detailed (1) the physical properties and distributions based on the compression-preserved Neuropteris ovata var. simonii and Laveineopteris rarinervis frond sections (Late Pennsylvanian Sydney Coalfield, Canada). Examined were also 1300 cuticular slides representing a number of plant groups, complemented by published data to infer biomass accumulation as potential kerogen in the geological column. In addition, (2) from select pinnules of the two species mentioned, nine secretory products and four surrounding cuticles were analyzed by micro-FTIR to evaluate statistically (PCA) the chemotaxonomic potential, and the kerogen chemistry. Further studies in support of (1) or (2) included methods of NICI, SEM, TEM and AFM, and by EDS. Results indicate that the secretory organs occurred in the entire N. ovata frond, its associate petiole and trunk, and in great abundance on the pinnules of both species in a random fashion. Secretory products show layering effects, surfaces can be intact, convoluted, folded, or damaged. At 100,000 magnification, microstructures are not observable. The exact chemical composition is unknown because of insolubility. IR spectra show peaks of functional groups frequently found in isocyanates, disubstituted alkynes, nitriles, polyynes, thiocyanates, and allenes, in part underpinned by EDS results. These compounds were presumably derived from the diagenetic alteration of resin-like terpenoid- or phenolic-related structures. That is particularly the case for the polyynes (i.e. ‘polyacetylenes’), synthesized by living plants with a variety of biological functions, including pigments and flavorings, toxins, and chemical repellents. The chemical information of the lipoid exudate, together with their morphological information and distribution will contribute to (i) the progress of chemotaxonomy and systematics of plant fossils, and (ii) a better understanding of the genesis of kerogens from plant-derived materials, particularly resinous remains.