BENTHIC-PELAGIC COUPLING IN THE UPPER CRETACEOUS HIGH ARCTIC; EVIDENCE FROM A SUITE OF UNUSUAL POLAR COPROLITES

Large accumulations of Arctic vertebrate coprolites provide clues about energy flow and feeding strategies in an Upper Cretaceous high latitude marine ecosystem. Exposures of the upper Kanguk Formation on Devon Island, Nunavut show a regressive sequence of sediments from offshore shales to nearshore glauconized sands to terrestrial silts and sands with abundant conifer wood debris. The coprolites are patchily distributed in the greensands and can be locally abundant; fossil burrows, sponges, and Lingula in the same sediments indicate a nearshore environment. Although shape and size (up to 300 ml volume) are variable, the specimens can be characterized by ground mass composition; some are largely phosphatic, whereas others contain high concentrations of greensands.

Most of the phosphatic specimens are highly burrowed and indicate extensive coprophagy by benthic invertebrates. These coprolites contain surprisingly high concentrations of siliceous microfossils. The greensand-filled coprolites are cemented by phosphate and also contain microfossils, as well as crustacean fragments, tiny fecal pellets, and other invertebrate debris. The high sand content in these specimens suggests that the vertebrate fauna included at least one sizeable taxon with a sediment-ingesting feeding habit. Body fossils from the same sediments suggest that sharks, ichthyodectid fishes, and plesiosaurs were possible coprolite producers.

Although Devon Island would have been subject to extreme photoperiodic seasonality in the Late Cretaceous, paleoclimatic models indicate that the climate was considerably more temperate than that of today’s Arctic. The large chunks of fossil wood in the non-marine sediments show that Cretaceous terrestrial habitats differed from modern Arctic environments and suggest that marine ecosystems were also significantly different. The Devon Island coprolites provide evidence for energy flow in a productive ancient ecosystem, and demonstrate intense benthic foraging activity by invertebrate detritivores and vertebrate deposit feeders. Such feeding strategies are consistent with benthic-pelagic coupling in modern Arctic environments, where excess nutrients generated by pulses of planktonic production are transferred to the benthos where they can be exploited over a period of time.