Compounds released by sequential chemolysis from cuticular remains of the Cretaceous Gymnosperm Squamastrobus tigrensis (Patagonia, the Argentine)

Abstract

The chemical composition of morphologically well-preserved cuticular remains of the Cretaceous Gymnosperm Squamastrobus tigrensis found in the Bajo Tigre locality (Patagonia) was analysed by a progressive chemical degradation sequence involving: (i) lipid extraction after ultrasonic treatments; (ii) further BF3-MeOH transesterification; and (iii) repeated mild NaBO2-H2O2 degradation of the final residue. The compound assemblages released after the successive treatments were considered to reflect lipid speciation patterns in the different levels of the cuticular structure. In particular, after all the treatments, the dominance of a homologous series of α,ω-alkanedioic acids with maxima at C9 and C10 indicates a polymethylene network based on macromolecules; the units of this network contain in-chain unsaturations or substitutions in positions similar to those in precursor unsaturated C16 and C18 acids. The low yields of ω-hydroxyacids typical of cutins and suberins of extant plants are interpreted as an intense microbial alteration and/or diagenetic transformation of the fossil cuticle. In conjunction with data from infrared spectroscopy and analytical pyrolysis, this suggests that the Squamastrobus remains consist of a macromolecular alkyl mixture in which the original C backbone has been defunctionalized, reoxidized, and subjected to additional condensation by the probable contribution of newly-formed, nonhydrolyzable bridges. It appears that this cellulose-lacking material includes a small moiety of altered lignin, and has been subjected to extensive oxidation, but not to further thermal alteration leading to decarboxylation. A possible result of this diagenetic alteration is that a portion of the resin constituents were transformed into lower molecular weight alkylbenzenes and alkylnaphthalenes, whereas another portion condensed into a very stable, nonhydrolyzable residue. The chemical stability of this residue is not due to aromatization, but to secondary condensation of altered aliphatic biomacromolecules.