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Seventy-six nautiloids (Order Orthocerida) were located and measured at Nautiloid Canyon, a side canyon of the Colorado River in Marble Canyon, Grand Canyon National Park, Arizona. The fossils occur in a 330m2 exposure within a single, massive bed of fine-grained dolomite at the top of the Whitmore Wash Member of the Redwall Limestone. The consistent morphology, high fossil density, and near-normal size-frequency distribution of the fossils suggest a life, rather than death assemblage. Implosion evidence and non-random orientation data (n=71) are consistent with bodies being in the shells at the time of burial. Life assemblage and intact body evidence suggest that nautiloids were involved in a mass-kill event. The fossil horizon is found on the flank of a broad domal structure which involves the uppermost Whitmore Wash Member and at least 20m of the increasingly cherty sediments of the Thunder Springs Member. The sediments above the fossil horizon exhibit high iron-to-aluminum ratios and a strong depletion in titanium, aluminum, and rare earth elements. Hydrothermal process is indicated by lithologic evidence (dolomite/chert boundary, geochemical signature, and structural mound) coincident with fossil evidence (rapid burial indicated by implosion, currents indicated by shell orientations, and mass-kill evidence). We interpret the nautiloids as being killed in high-temperature, toxic water derived from a hydrothermal source. We then interpret the nautiloids as being transported in a sediment-charged current and deposited and rapidly buried upon a sloping depositional surface on the flank of a building hydrothermal mound.
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