EVIDENCE FOR SURVIVING CHITIN IN CAMBRIAN TRILOBITES FROM THE CARRARA FORMATION, WESTERN NORTH AMERICA

Abstract As early arthropods, trilobites are widely thought to have had chitinous exoskeletons, because the aminopolysaccharide chitin (C8H13O5N)n is a ubiquitous structural organic component of arthropod cuticle. However, despite the abundance of trilobites in the lower Paleozoic fossil record, chitin has never been detected in these fossils. Here we report detection of chitin in an Olenellus trilobite from the Pyramid Shale Member of the Carrara Formation (514.5-506.5 Ma) at Emigrant Pass, California, USA. By analyzing trilobite cuticle with calcofluor white (CFW) fluorescent staining, Fourier transform infrared (FTIR) spectroscopy, and electrospray-ionization mass spectroscopy (ESI-MS), we found spectral peaks indicative of d-glucosamine, the monomer of chitin. This result contrasts with many previous analyses that produced a negative detection of chitin in a variety of fossils and is consistent with literature published in recent decades reporting positive chitin detections in fossils using modern analytical techniques. Our result appears to demonstrate that chitin might be able to survive in arthropod cuticle longer than previously confirmed, despite conditions of low-grade metamorphism. Because chitin is widely found as a major structural component of many eukaryotes, including a large portion of limestone-building marine invertebrates, the decay timescale of this molecule is fundamentally important for characterizing global biogeochemical cycling of carbon and nitrogen. The array of techniques now available for detecting this extremely common biopolymer may allow new insights into both the structure and function of many fossilized organisms, as well as improved determination of the taphonomic factors governing decay rates of common organic compounds.