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.