Identification of chitin in 200-million-year-old gastropod egg capsules

Chitin occurs in a variety of invertebrates, especially in arthropod cuticles, but is rarely reported in the fossil record. Although it has been detected in fossils as old as Middle Cambrian and Silurian, the majority of records come from much younger, Cenozoic deposits. In this paper, we report the preservation of chitin in Early Jurassic neritimorph gastropod egg capsules deposited in bivalve shells from prodelta-deltafront and nearshore paleoenvironments of the Holy Cross Mountains, Poland. We used a number of analytical methods to confirm the presence of chitin preserved in these ancient fossils. This is the first record of chitin preservation in Mesozoic deposits that, interestingly, do not follow the conventional Konservat-Lagerstätten manner of preserving soft-bodied and non-biomineralized organisms. We believe that deltaic settings characterized by episodic, high input of fluvial deposits, oligohaline conditions, and oxygen-poor microenvironment within the sediment—as well as early cementation of sediment infilling the shells—were crucial for chitin preservation. The preservation of chitin in such recalcitrant structures as egg capsules and deposits that formed outside conventional Konservat-Lagerstätten conditions renders it likely similar deposits may yield promise for discoveries of similar biological macromolecules.