amino acid was mapped on a much larger length scale. Com- bining this information with Zn extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) data, we suggested that zinc acts as a cross-linker of Mineralized tissues are produced by most living organisms for load and impact functions. In contrast, the jaws of the clam worm, Nereis, are hard without mineralization. However, they are peculiarly rich in halogens, which are associated with a varie- ty of post-translationally modified amino acids, many of which are multiply halogenated by chlorine, bromine, and/or iodine. Several of these modified amino acids, namely dibromohistidine, bromoiodohistidine, chloroiodotyrosine, bromoiodotyrosine, chlorodityrosine, chlorotrityrosine, chlorobromotrityrosine, and bromoiodotrityrosine, have not been previously reported. We have found that the distributions of Cl, Br, and I differ: Cl is wide- spread whereas Br and I, although not colocalized, are concen- trated in proximity to the external jaw surfaces. By using nanoin- dentation, we show that Br and I are unlikely to play a purely mechanical role, but that the local Zn and Cl concentrations and jaw microstructure are the prime determinants of local jaw hard- ness. Several of the post-translationally modified amino acids are akin to those found in various sclerotized structures of inverte- brates, and we propose that they are part of a cross-linked pro- tein casing.