Factors influencing the preservation of plant cuticles: a comparison of morphology and chemical composition of modern and fossil examples

Abstract

Samples of Recent Ginkgo biloba, two Cretaceous Ginkgo and two Cretaceous conifer cuticles from different enclosing lithologies but with similar thermal maturity of the fossils, have been analysed by scanning and transmission electron microscopy (SEM, TEM), Fourier transform–infrared spectroscopy (FT–IR), and pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS). Recent and fossil Ginkgo cuticles under SEM reveal sheets, similar in appearance, varying in the abundance and texture of the cuticular papillae. TEM of the Recent Ginkgo shows an outer amorphous cuticle layer, a structured middle layer and an inner laminated layer of cell wall. The Cretaceous Ginkgo cuticles retain the amorphous layer and a modified structured layer. SEM of Cretaceous Abietites and Frenelopsis also shows preservation of cuticle sheets but each has distinctive morphology. These conifer cuticles are very thick (TEM), Frenelopsis cuticle has remarkable multilaminar ultrastructure whilst Abietites is amorphous. G. biloba cuticle consists mainly of the natural polyester, cutin, as revealed by FT–IR and pyrolysis, indicated by an abundance of saturated, unsaturated and hydroxy fatty acids. IR spectra of fossil cuticles, like modern cuticles, show aliphatic C–H, hydroxyl and carbonyl functions. However, in fossils, the carbonyl ester is transformed to carboxylic acid or ketone groups. Pyrolysates of fossils show phenolic constituents like modern cuticles but loss of cutin fatty acid monomers and an increased prominence of an homologous series of n-alkene and n-alkane fragments up to n-C 30. Since most Recent cuticles, including those of conifers and Ginkgo biloba which we have studied, do not yield a non-saponifiable highly resistant residue it is proposed that organic preservation of fossil species investigated involves the diagenetic stabilisation of chemically-labile aliphatic cutin constituents along with incorporation of waxes. These general chemical modifications characterise all fossil Ginkgo and conifer cuticles, irrespective of their enclosing lithology, systematic affinity, external morphology or internal ultrastructural preservation. However there are also clear chemical differences between the fossil samples which may relate to their systematic affinity (ginkgos vs Abietites and Frenelopsis).