Chitin in the fossil record: identification and quantification of d-glucosamine

Abstract

Although a labile molecule, chitin is resistant to decay when complexed with protein. Currently, qualitative evidence for the preservation of chitin rests upon characteristic marker compounds derived through pyrolysis–gas chromatography–mass spectrometry (Py–GC–MS) of fossil arthropod cuticles, supported by a non-specific carbohydrate assay. However, unambiguous confirmation of the survival of chitin polymer requires detection of its hydrolysate monomer, d-glucosamine. We have now developed a GC–MS selected ion monitoring (SIM) method for the identification and quantification of d-glucosamine in fossil materials. Fossils of various ages and depositional settings were investigated and the results compared with those obtained by the Py–GC–MS approach. Specimens from the Rancho La Brea Tar Pits (USA, Pleistocene), showed the greatest degree of preservation: ∼10% (w/w), while fossil insects from Willershausen (Germany, Pliocene) and St Bauzile (France, Miocene) showed chitin to be present in ∼5% (w/w). Fossils from the Oligocene at Enspel, Germany, revealed that more than 0.5% is preserved for 25 million years. The GC–MS–SIM technique confirms the survival of chitin in the fossil record through the explicit identification of the polysaccharide monomer, and supports earlier Py–GC–MS and colorimetric analyses. The presence of other amino sugars of either exogenous (microbial) or diagenetic origin in more ancient specimens was also readily revealed using the GC–MS–SIM approach. This study illustrates the value of using a high-specificity quantitative ‘wet’ chemical approach in combination with Py–GC–MS to further advance the investigation of chitin in the fossil record.