Neal S. Gupta

The organic preservation of fossil arthropods: an experimental study

Modern arthropod cuticles consist of chitin fibres in a protein matrix, but those of fossil arthropods with an organic exoskeleton, particularly older than Tertiary, contain a dominant aliphatic component. This apparent contradiction was examined by subjecting modern cockroach, scorpion and shrimp cuticle to artificial maturation (350 °C/700 bars/24 h) following various chemical treatments, and analysing the products with pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS). Analysis of artificially matured untreated cuticle yielded moieties related to phenols and alkylated substituents, pyridines, pyrroles and possibly indenes (derived from chitin). n-Alkyl amides, C16 and C18 fatty acids and alkane/alk-1-ene homologues ranging from C9 to C19 were also generated, the last indicating the presence of an n-alkyl component, similar in composition to that encountered in fossil arthropods. Similar pyrolysates were obtained from matured pure C16 and C18 fatty acids. Py–GC/MS of cuticles matured after lipid extraction and hydrolysis did not yield any aliphatic polymer. This provides direct experimental evidence that lipids incorporated from the cuticle were the source of aliphatic polymer. This process of in situ polymerization appears to account for most of the fossil record of terrestrial arthropods as well as marine arthropods that lacked a biomineralized exoskeleton.

Reinvestigation of the occurrence of cutan in plants: implications for the leaf fossil record

Abstract Cutan, a resistant non-hydrolyzable aliphatic biopolymer, was first reported in the cuticle of Agave americana and has generally been considered ubiquitous in leaf cuticles along with the structural biopolyester cutin. Because leaves and cuticles in the fossil record almost always have an aliphatic composition, it was argued that selective preservation of cutan played an important role in leaf preservation. However, the analysis of leaves using chemical degradation techniques involving hydrolysis to test for the presence of cutan reveals that it is absent in 16 of 19 taxa (angiosperm and gymnosperm), including many previously reported to contain cutan on the basis of pyrolysis data. Cutan is clearly much less widespread in leaves than previously thought, and its presence or absence does not exert any major bias on the preservation of leaves in the fossil record. In the absence of cutan, other constituents—cutin, plant waxes, and internal plant lipids—are incorporated into the geomacromolecule and contribute to the formation of a resistant aliphatic polymer by in situ polymerization during diagenesis.

THE FOSSILIZATION OF EURYPTERIDS: A RESULT OF MOLECULAR TRANSFORMATION

Abstract The fossil remains of eurypterid cuticles in this study yield long-chain (<C9 to C22) aliphatic components similar to type II kerogen during pyrolysis–gas chromatography/mass spectrometry, in contrast to the chitin and protein that constitute the bulk of modern analogs. Structural analysis (thermochemolysis) of eurypterid cuticles reveals fatty acyl moieties (derived from lipids) of chain lengths C7 to C18, with C16 and C18 components being the most abundant. The residue is immune to base hydrolysis, indicating a highly recalcitrant nature and suggesting that if ester linkages are present in the macromolecule, they are sterically protected. Some samples yield phenols and polyaromatic compounds, indicating a greater degree of aromatization, which correlates with higher thermal maturity as demonstrated by Raman spectroscopy. Analysis (including thermochemolysis) of the cuticle of modern scorpions and horseshoe crabs, living relatives of the eurypterids, shows that C16 and C18 fatty acyl moieties likewise dominate. If we assume that the original composition of the eurypterid cuticle is similar to that of living chelicerates, fossilization likely involves the incorporation of such lipids into an aliphatic polymer. Such a process of in situ polymerization accounts for the fossil record of eurypterids.

Molecular taphonomy of graptolites

Graptolites are important fossils in Early Palaeozoic assemblages. Preserved graptolite periderm consists dominantly of an aliphatic polymer, immune to base hydrolysis. It contains no protein even though its structure, and chemical analyses of the periderm of the living relative Rhabdopleura, indicate that it was originally collagen. This anomaly was previously interpreted as the result of replacement by macromolecular material from the surrounding sediment. New analyses suggest that the aliphatic composition of graptolite periderm reflects direct incorporation of lipids from the organism itself by in situ polymerization. A similar process may account for the preservation of most organic fossils.

Taphonomy of Animal Organic Skeletons Through Time

Investigations of organically preserved invertebrate fossils have focused on abundant taxa such as graptolites and arthropods. Analyses have shown that their composition cannot be explained either as a result of decay resistance, or the introduction of macromolecular material from surrounding sediment. The fossilization of organic materials is a result of the diagenetic transformation of lipids in the organism itself by a process of in situ polymerization which generates a composition with a significant aliphatic component. While this process causes the fossilized remains of different taxa, even plants and animals, to converge in composition they may still retain differences following diagenesis. Such chemosystematic signatures have the potential to be used in the identification of organic materials that lack diagnostic morphology. The diagenetic transformation of organic materials in macrofossils is similar to the formation of kerogen – the final composition depends on original chemistry, decay and diagenesis. A better understanding of rates and controls on this process will require more experimental investigation of decay and maturation, as well as analyses of fossils of different ages and from different environmental settings.

MOLECULAR PRESERVATION OF CENOZOIC CONIFER FOSSIL LAGERSTÄTTEN FROM BANKS ISLAND, THE CANADIAN ARCTIC

Abstract The molecular preservation of exceptionally preserved conifer needles from middle Miocene and Pliocene deposits on Banks Island, Canada, was investigated using pyrolysis–gas chromatography–mass spectrometry (Py-GC-MS). Solvent-extracted residues from Miocene Larix, Glyptostrobus, and Pinus, Pliocene Picea, and their associated bulk material, yielded abundant polysaccharide pyrolysis products, such as 2-methylfuran, 2-furaldehyde, and levoglucosan, indicating excellent molecular preservation. Comparison of pyrolysates of individual plant taxa and bulk material from the same deposits revealed the dominance of particular plant taxa in these high latitude floras. Comparison with fossil Lagerstätten from Ellesmere Island (late Paleocene) and Axel Heiberg Island (middle Eocene), both in the Canadian Arctic Archipelago, and Clarkia in Idaho, United States (middle Miocene), demonstrated that the quality of molecular preservation of material from Banks Island is similar to that of Axel Heiberg Island and lies between those of Ellesmere Island and the Clarkia deposit. The ranking of molecular preservation was paralleled by scanning electron microscopy (SEM) observations. Analysis of Larix and Glyptostrobus from different geological ages (Eocene–recent) and locations indicated that age does not correlate with molecular preservation in these fossil Lagerstätten. When relative abundance ratios of vinyl phenol (m/z 91+120), guaiacyl (m/z 109+124), and levoglucosan (m/z 60+73) are used as indicators of the preservation of cutin, lignin, and cellulose, our results suggest that variations in the pyrolysates among several genera reflect different original molecular compositions as well as paleoenvironmental conditions for preservation. The data also illuminate the role of labile biomolecules in the taphonomy of three-dimensionally preserved morphological structures in these Arctic plant fossils.

Fate of Chitinous Organisms in the Geosphere

Organic tissues such as cuticles may survive as organic remains and account for the fossil record of a number of important groups such as graptolites, chelicerates, insects, chitinozoans, ammonite beaks and fish scales. Fossilized cuticles were assumed to be composed of chitin protein complex similar to the living relatives, however, analysis of fossils using a range of mass spectrometric and spectroscopic methods have shown that preserved cuticles include significant amounts of aliphatic hydrocarbon component at times with an aromatic component that is very different to the composition of the cuticle of the living arthropod. Analysis of successively older fossil material has revealed that this transformation to an aliphatic composition is gradual and perhaps time dependant. Taphanomic incubation experiments demonstrate that lipids such as fatty acids are incorporated into the decaying chitin protein exoskeleton as early as a few weeks contributing to the aliphatic component. This is supported by chemolytic analysis of fossils that reveal presence of fatty acyl moieties in the macromolecule. Thus, the aliphatic composition in the fossils is generated in-situ and not from migration from an external source. Many kerogens are similarly aliphatic and serve as a source for petroleum during thermal maturation. In such sedimentary organic matter where the contributing organism does not have a resistant aliphatic biopolymer, in situ lipid incorporation is likely an important mechanism for presence of the aliphatic component in the fossil organic matter.

Molecular Taphonomy of Metasequoia

Abstract Chemical analysis of the leaves of modern Metasequoia and other gymnosperms revealed the presence of structural polyesters, guaiacyl lignin units and polysaccharides, but no long chain recalcitrant aliphatic constituent. Analysis of Tertiary fossil Metasequoia showed the presence of polysaccharides, lignin and a long chain aliphatic polymer. Tertiary fossil conifers similarly revealed the preservation of lignin and a long chain aliphatic polymer (up to C33) with limited preservation of polysaccharides. Experimental maturation of modern pine needles generated a composition with a long chain aliphatic polymer and additional phenolic compounds similar to those seen in the fossils. Experimental maturation of the structural polyester cutin and a pure model C16 and C18 fatty acid mixture yielded fatty acids and an aliphatic polymer less than C20 suggesting that polymers greater than C20 may be incorporated into the fossil macromolecule from longer-chain plant waxes. Thermochemolysis of the fossil conifer revealed fatty acids from C12 to C30 that also occur in internal lipids and the initial structural polyester. This suggests that the formation of the aliphatic polymer in the fossil Metasequoia may have been the result of lipid incorporation, a process likely important in the long-term preservation of gymnosperms and other organic macrofossils.