Exceptionally preserved Cretaceous crabs provide novel insights into the fossilization of arthropod compound eyes

Abstract The arthropod exoskeleton consists of a chitin–protein meshwork that is reinforced by incorporated minerals, such as in decapod crustaceans. Such naturally biomineralized cuticle forms the bulk of arthropod bodily remains in the rock record. However, the extent to which this organic–inorganic composite material is transformed during the fossilization process remains incompletely understood. We examined the composition of Cretaceous crab cuticle, focusing on exceptionally preserved ommatidial lenses. These were compared against optic tissues in extant sesarmid crabs to determine how post-burial processes affect the long-term (geological) survival of corneal cuticle. Our analyses show that rather than calcium carbonate (the presumed main inorganic phase of crustacean cuticle), the exocuticle of sesarmid eyes is strengthened solely by calcium phosphate. The ancient lenses are likewise phosphatized; however, in these, the originally chitinous endocuticle has also been replaced by phosphatic deposits, resulting in a fully mineralized cornea. This finding has implications for interpretations of the optic capabilities of other extinct arthropods; notably the Palaeozoic trilobites, which are generally thought to have possessed mineralized ommatidial lenses in life.