Although the extraordinary preservation of organisms embedded in amber has long been recognized, there has been little awareness until more recently that fossil resins can preserve details of their own molecular structure better than any other form of sedimentary material. In fact, it has been only since the later decades of the 20th century that amber has been analyzed chemically with the goal of defining it as a plant product, a situation related perhaps in part to the development of relevant technologies. Moreover, the importance of amber as a gemstone led to its originally being characterized chemically as a mineral, and different ambers were given mineralogical names such as suc-cinite, burmite, and glessite among many others. In fact, there has been an increase in the number of new mineralogical names in the literature, such as amekite from Nigeria and bitterfeldite from Germany, indicating an interest in describing different kinds of fossil resins (Vávra 1993). The resins are being described as organic minerals, however, using organic geochemical methods of analysis that potentially provide evidence for determining the botanical source of some of them. The use of mineralogical nomenclature is also con-sidered by some to be better than simply naming according to geographic source. Despite some change during maturation, certain resin polymer con-stituents are remarkably stable, enabling establishment of chemical and botanical relationships between some modern and fossil resins. On the other
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hand, as evidence becomes more refined, understanding resin-producing taxa over long periods of time, including possible convergences or parallel evolu-tion of resin polymer structures (Chapter 2), becomes ever more complex.
Because the polymerized terpenoids that constitute the mass of most ambers are largely insoluble, solid-state spectrometric techniques such as infrared spectroscopy (IR), Fourier transform IR (FTIR), nuclear magnetic resonance spectroscopy (NMR), and NMR with cross-polarization / magic-angle spinning (CP/MAS) were primarily used in initial studies of botanical origins (Langenheim and Beck 1965, 1968; Langenheim 1969; Lambert and Frye 1982; Cunningham et al. 1983; Beck 1986). Absolute identities cannot be determined by comparing spectra of modern and fossil resins using these techniques, but a characteristic fingerprint of major constituents can establish relationships at the level of plant family or genus. Identification at the level of species would not be expected. IR remains the most commonly used tech-nique by researchers who do not have access to more expensive, sophisti-cated equipment.
X-ray diffraction has been used to identify the rare individual crystalline components in resin (Frondel 1967a, b). Structures of the small amounts of volatile mono- and sesquiterpenes trapped in amber can also be determined by gas chromatography–mass spectrometry (GC-MS; Thomas 1969, Grantham and Douglas 1980, Mills et al. 1984, Grimalt et al. 1988, Otto et al. 2002b).
Pyrolysis GC (Py-GC) gives more informative fingerprints than FTIR, and combined with MS, Py-GC-MS has enabled even greater analysis of the struc-ture of amber constituents (K. Anderson 1995). Another technique, direct-temperature-resolved MS (DTMS), a sensitive technique for analysis of com-plex mixtures of insoluble organic compounds, has been added to the arsenal of amber analysis (Grimaldi et al. 2000).
Using Py-GC-MS, K. Anderson et al. (1992) provided a chemical classifi-cation scheme for ambers (Appendix 3) that can be related to plant source and also provides some evidence for evolutionary convergence of the skeletal chemistry of the polymers. Diterpenoid labdane polymers (Class I) are partic-ularly important because they characterize ambers occurring in major depos-its. Furthermore, this basic labdane structure occurs in both conifer and angio-sperm resins and is highly conservative across modern and fossil resins. Three subclasses of labdane polymers have been significant in providing informa-tion about the plant source of some ambers. Subclasses Ia and Ib
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ize ambers derived from polymers or copolymers of either communic acid (Figure 1-4) or communol, as first recognized by Carmen et al. (1970). The basic polymer is the same but the diterpenoids are cross-linked with succinic acid in Ia, and not in Ib. Succinic acid thus provides an important additional characteristic for determining botanical source. Although the structure of the polymer in Class Ic is the same as in Classes Ia and Ib, it differs in the stereo-chemistry of the methyl group at carbon 4a and the side chain at carbon 5.
Therefore, the polymer is not derived from communic acid but from ozoic acid (Figure 1-4) and/or its hydroxy derivative, zanzibaric acid, as well as biformenes and related isomers, which again can be significant botanically.
The labdanoid polymer structure in ambers often contains small amounts of occluded nonpolymeric compounds such as monoterpenes. These terpenoids vary greatly in both modern and fossil resins although the relative propor-tions can be useful as an additional characteristic in assigning the botanical source at the level of family or genus. Their intra- and interspecific variation in modern resins is thought to be a part of the plant’s defense (Chapter 5).
Class II ambers, comprising bicyclic sesquiterpenoids, especially cadinene and related isomers, also form ambers that occur in large deposits. Dimeric cadinenes may cyclize to form bicadinanes (triterpenoids). This combination of terpenoids indicates a different group of plants than those characterized by diterpenoids. Phenolic resin polymers are represented by polystyrene (Class III), which is a good indicator of plants with resin containing large amounts of cinnamic acids and esters. Classes IV and V have characteristic nonpoly-mer resin components that often are preserved in small amounts (Appendix 3). Class IV includes cedrane sesquiterpenoids (Figure 1-3) found in fossil leaves. Class V ambers, with primarily abietane and pimarane diterpenoid skeletons (Figure 1-4), have occurred in association with fossilized conifer cones and wood, often suggestive of botanical source, and also as small sol-uble granules in European brown coals (Beck 1999).
Despite increases in knowledge about the chemical properties of resin, there are several dangers inherent in using chemistry as the sole criterion to identify the botanical source of amber. First, in methods that produce a finger-print, such as IR, different spectra may well indicate different plant sources (although not always the identity of a particular taxon), but very similar spec-tra do not necessarily mean the same source. Moreover, the older the amber, the more similar the spectra usually become. Second, techniques based on
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isolation of particular polymerizable compounds must take into account that the same compounds can occur in different resins (Chapter 1). In fact, the occurrence of the same predominant compounds in unrelated plant taxa raises interesting questions of convergent or parallel evolution (Chapter 2).
This does not deny that chemical evidence alone, which may be all that is available, can be significant in suggesting botanical origin, particularly at the level of family and sometimes of genus. Concomitant evidence from included or associated plant remains as well as depositional conditions in some cases, however, is often essential for definite identification of the taxon from which the amber was derived. Lack of correlative information in addition to the com-plexities of evolution within some plant groups result in incompletely solved mysteries regarding the plant source of some significant amber deposits.