Cuticle chemistry of the Cheirolepidiaceae Frenelopsis teixeirae from the Lower Cretaceous of Portugal. A case of study using ATR-FTIR spectroscopy

Abstract

A Cheirolepidiaceae Frenelopsis teixeirae from the Lower Cretaceous of the Santa Susana Formation, near Torres Vedras locality (Estremadura region, western Portugal), is analyzed by means of Fourier Transform Infrared (FTIR) spectroscopy in Attenuated Total Reflection (ATR) mode at both micro and macro scales. This analytical technique (ATR-FTIR spectroscopy), which requires minimal sample preparation and a small amount of material, is used to evaluate the chemical composition of the F. teixeirae cuticles. For this, functional groups (i.e., specific groupings of atoms within molecules), that compose the complex chemical structure making up the F. teixeirae cuticles, are detected, analyzed, and interpreted from the resulting spectra. The Frenelopsis material comprises compressions of eighteen leafy axes with very well-preserved cuticles. Previously reported chemical information derived from transmission FTIR spectra of the foliar cuticle of the podocarp Squamastrobus tigrensis from the Lower Cretaceous of Argentina is used for comparison in order to assess the existence of a distinctive spectroscopic pattern for this Portuguese taxon. Semi-quantitative data derived from FTIR spectra of both fossil taxa were statistically analyzed using principal component analysis (PCA). The F. teixeirae cuticles have a lower contribution of C=O stretch and a lower degree of biopolymeric matrix cross-linking (higher CHal/C=O ratio) than those of S. tigrensis. Despite the existing differences between both conifers, spectroscopic patterns characterizing each fossil taxon are not fully distinctive from a chemotaxonomic perspective. This is mainly related to the relatively variable morphological cuticular features (e.g., trichomes) and to the biomacropolymeric chemistry among the leaves of each fossil taxon.