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.