The conventional geochemical view holds that the chitin and structural protein are not preserved in ancient fossils because they are readily degradable through microbial chitinolysis and proteolysis. Here we show a molecular signature of a relict chitin-protein complex preserved in a Pennsylvanian (310 Ma) scorpion cuticle and a Silurian (417 Ma) eurypterid cuticle via analysis with carbon, nitrogen, and oxygen X-ray absorption near edge structure (XANES) spectromicroscopy. High-resolution X-ray microscopy reveals the complex laminar variation in major biomolecule concentration across modern cuticle; XANES spectra highlight the presence of the characteristic functional groups of the chitin-protein complex. Modifi cation of this complex is evident via changes in organic functional groups. Both fossil cuticles contain considerable aliphatic carbon relative to modern cuticle. However, the concentration of vestigial chitin-protein complex is high, 59% and 53% in the fossil scorpion and eurypterid, respectively. Preservation of a high-nitrogen-content chitin-protein residue in organic arthropod cuticle likely depends on condensation of cuticle-derived fatty acids onto a structurally modifi ed chitin-protein molecular scaffold, thus preserving the remnant chitin-protein complex and cuticle from degradation by microorganisms.

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 degrees 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.

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The fossil remains of eurypterid cuticles in this study yield long-chain (ϽC 9 to C 22 ) 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 C 7 to C 18 , with C 16 and C 18 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 C 16 and C 18 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.

Structural coloration underpins communication strategies in many extant insects but its evolution is poorly understood. This stems, in part, from limited data on how color alters during fossilization. We resolve this by using elevated pressures and temperatures to simulate the effects of burial on structurally colored cuticles of modern beetles. Our experiments show that the color generated by multilayer refl ectors changes due to alteration of the refractive index and periodicity of the cuticle layers. Three-dimensional photonic crystals are equally resistant to degradation and thus their absence in fossil insects is not a function of limited preservation potential but implies that these color-producing nanostructures evolved recently. Structural colors alter directly to black above a threshold temperature in experiments, identifying burial temperature as the primary control on their preservation in fossils. Color-producing nanostructures can, however, survive in experimentally treated and fossil cuticles that now are black. An extensive cryptic record is thus available in fossil insects to illuminate the evolution of structural color.

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