Fig. 5.

Schematic of the proposed transformations of proteins during fossilization in hard tissues. 1 Unaltered tripeptide containing arginine, lysine, and histidine residues. Peptides constitute proteins in fresh tissues. 2 Imidazolone is an early, non-crosslinking product of glycoxidation/lipoxidation reactions and can be found in altered proteinaceous matter of experimentally matured and fossil samples (Fig. 2). 3 A pentosidine crosslink, commonly found in advanced glycoxidation/lipoxidation end products based on originally proteinaceous matter, and in experimentally matured, and fossil samples (Fig. 2). 4 A highly oxidized, polymeric degradation product similar to a melanoidine. Structure may vary. Similar compounds were found in Psammornis (Fig. 2). The colored spheres represent a proteinaceous context (yellow), an oxidatively altered proteinaceous context (orange), a non-proteinaceous N-rich heterocyclic polymer as product of protein transformation (red), and a non-proteinaceous, peroxidized, polymeric scaffold (gray). The protein icons next to the structures illustrate the corresponding degree of tissue discoloration caused by enrichment in each compound as in Fig. 2