No abstract

While oxidation products of unsaturated fatty acids, for example dicarboxylic acids (hereafter diacids), must form during the use of unglazed ceramic vessels for the processing of animal and plant products, such components have never been observed during studies of absorbed lipids. Their absence from the extractable lipid fraction is presumed to be the result of their loss from potsherds through groundwater leaching. Lipid oxidation products, including short-chain dicarboxylic acids, 3-hydroxy acids and longerchain hydroxy and dihydroxy acids, have now been observed as components probably covalently bound into solvent insoluble residues of potsherds recovered from waterlogged deposits. These components were only revealed following alkaline treatment of the insoluble residues. A similar mixture of diacids was observed in high abundance in the free lipid fraction of vessels recovered from an exceptionally arid deposit where groundwater leaching would never have occurred. These results con¢rm the formation of oxidation and probable polymerization products of unsaturated fatty acids during vessel use and burial.

Studies performed during the last two decades have shown that lipids are preserved in association with a wide range of artefacts and ecofacts recovered from archaeological sites, e.g. pottery vessels and skeletal remains. The majority of work in this area has focused on the use of molecular structures (`biomarkers') and distributions (`¢ngerprints') to assess the nature and origin of commodities associated with past cultural, economic and agricultural practices. However, since lipids, like all other classes of biomolecule, are a¡ected by degradation (both pre-and post-burial), emphasis is now being placed on the complementary use of diagenetically robust, compound-speci¢c stable isotope measurements to enhance the scope and reliability of archaeological interpretations. A feature of the 13 C values of individual lipids, rather than bulk measurements of biochemically more heterogeneous materials, lies in their capacity to re£ect di¡erences in both the isotopic composition of the carbon sources used in their biosynthesis and the routing of dietary lipids and their metabolites in consumer organisms. This isotopic information, accessible by gas chromatography^combustion-isotope ratio mass spectrometry, has opened up new avenues of investigation concerning human activity in prehistory.

A review of our current understanding of resistant biomacromolecules derived from present and past algae and higher plants is presented. Insight in the nature of recent and fossil macromolecules is strongly hampered by the difficulties in obtaining the material in pure and unaltered form. For the extant material, avoiding artificial condensation and structural alteration as a result of chemical isolation and purification of biomacromolecules requires constant attention. To date, several types of sporopollenin seem to occur. One type is characterised by oxygenated aromatic building blocks, in particular p-coumaric acid and ferrulic acid. The other type is thought to consist predominantly of an aliphatic biopolymer. In this review it is concluded that extant sporopollenin consists of the aromatic type, whereas the aliphatic component of fossil sporopollenin is due to early-diagenetic oxidative polymerization of unsaturated lipids. The cuticles of most higher plants contain the aliphatic biopolyester cutin. Additionally, cuticles of drought-adapted, mostly CAM plants, seem to contain the non-hydrolysable aliphatic biopolymer cutan. Only a very few algae are able to biosynthesize resistant, (fossilisable) cell walls: some Chlorophyta, Eustigmatophyta and Prasinophyta produce the aliphatic biopolymer algaenan. Some Dinophyta are also capable of producing algaenan cell walls. Additionally, some taxa produce highly resistant cyst-walls with a high proportion of aromatic moieties. For the morphologically well-preserved fossil material, contamination by organic particles other than the target taxon is hard to eliminate and can contribute to either the aliphatic or aromatic signal. Furthermore, post-mortem migration of aliphatic moieties into, and their condensation onto the macromolecule might occur, e.g. by oxidative polymerization. These phenomena hamper the evaluation of the aliphatic signature of fossil plant material and may for example explain the preservation of initially cutin-based cuticles or non-algaenan containing algae. The extent to which migration and in situ formation of aromatic moieties plays a role in modifying resistant algal macromolecules, notably under elevated temperature and/or pressure conditions, still remains an open question.

Flash pyrolysis/gas chromatography (py/GC) and py/GC/mass spectrometry (MS) have been utilized to characterize the cuticles of invertebrates chemically. Pyrolysis products have been identified and assigned to specific cuticular components. Acetylpyridones, acetamidofuran, 3-acetamido-5-methylfuran and 3-acetamido-(2 and 4)-pyrones are proposed as characteristic pyrolysis markers for chitin. Pyrolysis products displaying ions of m/z 70,154,168,194 are thought to derive from diketopiperazine structures and provide potential markers for proteins and peptides in which proline, alanine, valine, arginine and glycine are the dominant amino acids. These products, constituting specific pyrolysis markers for invertebrate cuticles, may reflect the amino acid composition of their constituent structural proteins. The source of the various pyrolysis products of proteins has been verified by pyrolysis of reference proteins, peptides and amino acid m ' - Chitin [poly-( 1-.4)-P~-2-acetarnido-2deoxyglucopyra-nose] (Fig. 1) is one of the most important biopolymers in the biosphere, occurring in many organisms.' In arthropods, chitin is one of the major constituents of the cuticle,' where it occurs in intimate association with proteins, connected through covalent cross-links involving catechol-amine and histidine/aspartic moieties.' Studies of chitin have utilized a range of analytical method^^-^ to resolve questions relating to its chemical composition and production techn~logy.~ Studies of the chemical structure of arthropod cuticles have focused mainly on insects, and have been advanced by the use of solid-state 13C and/or I5N nuclear magnetic resonance (NMR)?' Information from these studies has included the determination of the general chemical structure of the cuticle, recognition of chitin and other moieties, and investigation of the chemical changes involved in the sclerotization of exoskeletons.' Pyrolysis/gas chromatography (py/GC) and py/GC/mass spectrometry (py/GC/MS) are widely used analytical techniques for the studies of insoluble, polymeric organic materials?-" These techniques allow very rapid screening of complex materials and require only submilligram sample sizes. The facility to accommodate minute samples is critical for the study of fossil materials. The sensitivity of the MS ensures that even traces of the polymers in organic residues can be detected qualitatively.' Chitin and proteinaceous materials have been studied by means of analytical pyrolysis to assess their chemical composition and degradation products."-'7 Nonetheless, use of these techniques in studies of arthropod cuticles, which are highly intractable to Author for correspondence. conventional chemical methds, has been limited. They have, however, been analysed using enzymatic methods." One area in which py/GC and py/GC/MS are proving to be the most appropriate analytical techniques is in the study of the fossilization of animal cuticles.''* Preliminary results indicated that chitin, although relatively resistant to degradation," was not detectabl...