The cuticles of 15 fossil invertebrates ranging in age from Silurian to Cretaceous, and including both marine and terrestrial organisms, have been analyzed using pyrolysis-gas chromatography/mass spectrometry (py-GC/MS). Modern invertebrate cuticles were analyzed in the same way as a basis for comparison. The modern cuticles yielded pyrolysis products derived from chitin and proteins, but none df these components was detected in the pyrolysates of the fossil cuticles. The fossil cuticles fall into two, chemically distinct groups : aliphatic, yielding pairs of n-alk-1-enes and n-alkanes upon pyrolysis, and aromatic, producing pyrolysates dominated by alkylbenzenes and alkylindenes. Aliphatic pyrolysates may derive through polymerization of lipids e.g., epicuticular waxes, during diagenesis. Alternatively the aliphatic moieties found in algae (algaenan) or in plants (e.g., cutan, suberan) may have been incorporated into the animal cuticles by unknown diagenetic processes. Alkylindenes are major pyrolysis products of the fossil cuticles that generate predominantly aromatic components. This association may resolve the enigma of the frequent occurrence of alkylindenes as minor components in the pyrolysates of most types of kerogen. The abundant thiophenes in the same pyrolysates may reflect sulfur incorporation during diagenesis of the original amino sugar (glucosamine) moieties that comprise the chitin biopolymer.