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

Samples of Recent Ginkgo biloba, two Cretaceous Ginkgo and two Cretaceous c onifer cuticles from different enclosing lithologies but with similar therm al maturity of the fossils, have been analysed by scanning and transmission electron microscopy (SEM, TEM), Fourier transform-infrared spectroscopy (F T-IR), and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Recen t and Fossil Ginkgo cuticles under SEM reveal sheets, similar in appearance , varying in the abundance and texture of the cuticular papillae. TEM of th e Recent Ginkgo shows an outer amorphous cuticle layer, a structured middle layer and an inner laminated layer of cell wall. The Cretaceous Ginkgo cut icles retain the amorphous layer and a modified structured layer. SEM of Cr etaceous Abletites and Frenelopsis also shows preservation of cuticle sheet s but each has distinctive morphology. These conifer cuticles are very thic k (TEM), Frenelopsis cuticle has remarkable multilaminar ultrastructure whi lst Abietites amorphous. G. biloba cuticle consists mainly of the natural p olyester, cutin, as revealed by FT-IR and pyrolysis, indicated by an abunda nce 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 carbox ylic acid or ketone groups. Pyrolysates of fossils show phenolic constituen ts like modern cuticles but loss of cutin fatty acid monomers and an increa sed prominence of an homologous series of n-alkene and n-alkane fragments u p to n-C-30 Since most Recent cuticles, including those of conifers and Gin kgo biloba which we have studied, do not yield a non-saponifiable highly re sistant residue it is proposed that organic preservation of fossil species investigated involves the diagenetic stabilisation of chemically-labile ali phatic 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 m orphology or internal ultrastructural preservation. However there are also clear chemical differences between the fossil samples which may relate to t heir systematic affinity (ginkgos vs Abietites and Frenelopsis). (C) 1998 E lsevier Science Ltd. All rights reserved.