Spectrochemical study of coalified Trigonocarpus grandis (Pennsylvanian tree-fern ovule, Canada): Implications for fossil-organ linkage

Abstract

Five coalified ovules of the type Trigonocarpus grandis are investigated four of which co-occur with tree-fern foliage Alethopteris pseudograndinioides in the medullosalean forest (basal Cantabrian), and the fifth occurs in top Asturian D (Sydney Coalfield, Canada). Addressed are questions of variability (what is a coalified ovule?), comparison with petrified ovules, pyrolysates and the original make-up of the grandis seeds, and can similar chemistry proxy for organic connection between ovule and foliage? The results demonstrate variable preservation quality despite similar thermal-maturity levels in the geological interval in which the ovules were collected. Nevertheless, the proposed T. grandis model is based on evidence from epidermises associated with inner and outer integuments, and a two-layered nucellus with granulose exine that is covered by a diaphanous layer (tectum?) and nucellar cuticle. The latter separates the inner cuticle of the inner integumentary surface from the megaspore membrane. Parenchymatous and sclerenchymatous cell structures are rare, whereas evidence for integuments, vasculature, and sclerotesta is equivocal. Overall, these features compare with petrified seeds. 13C nuclear-magnetic resonance analysis suggests that the A. pseudograndinioides tree fern bore T. grandis seeds. Pyrolysates from low and high molecular weights can almost exclusively be grouped with alkenes and aromatics; phenolics, furan and branched alkenes; and with n-alkene/n-alkane homologous series (~ 3 to 1) for cuticles from the inner integumentary surface which suggests a cutin-based, aliphatic-rich biomacromolecule. More generally, preservation is presumed correlative with aliphatic content, but not exclusively, and organ–organ linkage by spectrochemical means certainly has potential as a new research vector in palaeobotany.