LATE CRETACEOUS MARINE ARTHROPODS RELIED ON TERRESTRIAL ORGANIC MATTER AS A FOOD SOURCE: GEOCHEMICAL EVIDENCE FROM THE COON CREEK LAGERSTÄTTE IN THE MISSISSIPPIAN EMBAYMENT

Chitinous remains of arthropods have been shown to retain their original elemental composition in the fossil record, but the paleoecological utility of these compounds is unknown. In this study, we test for the first time the integrity of stable carbon isotope signatures (δ¹³C) in the organic remains of Mesozoic-age fossil decapod arthropod cuticle, and their utility in paleoecological interpretations. Exceptionally preserved crab and mudshrimp specimens and associated matrix were obtained from the unlithified sediments of the Late Cretaceous Coon Creek Formation Lagerstätte of Tennessee, and variously analyzed using Raman spectroscopy, XRD analysis, EA, biomarker analysis, and bulk and n-alkane δ¹³C analyses. The kerogen R1 ratio obtained from the Raman spectra of the specimens and a low hopane thermal maturity index suggest that samples have been only slightly heated following burial. Comparison of the carbon to nitrogen ratios from the fossil specimens to those from pristine Recent and degraded sub-Recent decapod specimens indicate that the fossil specimens are heavily depleted in nitrogen, but do preserve organic carbon up to 13 weight percent. Mean δ¹³C values obtained from demineralized fossil crab and shrimp samples (−25.3‰ and −25.9‰, respectively) are statistically indistinguishable within and between the taxa, and are comparable to published values for modern confamilial decapods that typically inhabit nearshore or terrestrial environments. In the absence of evidence for diagenetic or microbial isotopic degradation, we consider that the fossil δ¹³C values are reflective of the original biogenic isotope signal. Whereas previously published δ¹³C values from Coon Creek bivalves suggested marine conditions, the decapod δ¹³C values obtained here indicate a predominantly terrigenous diet and potentially a more terrestrially dominated setting for decapods living the Mississippi embayment during the Late Campanian–Early Maastrichtian. This interpretation is further supported by sediment n-alkane δ¹³C values that are similar to the decapod values, as well as the presence of wood and leaf macrofossils in the Coon Creek Formation. We conclude that well-preserved arthropod remains represent a novel isotopic data source for use in paleoecological and environmental reconstructions.