Article: Original mineralogy of trilobite exoskeletons

The mineralized exoskeletons of well-preserved trilobites are now composed of low-magnesian calcite. However, as this is the only form of calcium carbonate to survive in Lower Palaeozoic rocks, such a mineralogy may be a function of diagenetic processes rather than reflecting primary cuticle composition. Ferroan calcite replacement has previously been used to infer an original high-magnesian calcite mineralogy for trilobite exoskeletons. By contrast, petrographic data involving over seventy trilobite species, ranging in age from Cambrian to Devonian, together with carbon and oxygen stable isotope analyses of specimens from the Much Wenlock Limestone Formation, England (Wenlock), are here used to infer that trilobites constructed low-magnesian calcite exoskeletons. Petrographically, the trilobite cuticles share the same preservational characteristics as low-magnesian calcite organisms such as articulate brachiopods. They also have very similar isotopic signatures to those of brachiopods, yet differ from crinoids which secreted high-magnesian calcite ossicles and now commonly contain microdolomite inclusions and secondary voids. Together, these separate lines of evidence strongly suggest that trilobite exoskeletons originally had a low-magnesian calcite mineralogy.