Rare earth element geochemistry of bone beds from the Lower Cretaceous Zhonggou Formation of Gansu Province, China

Auroraceratops rugosus from the Lower Cretaceous Zhonggou Formation, Gansu Province, China is characterized by exquisite external preservation of articulated, often ventrolaterally curled remains found in discrete locations several to hundreds of meters apart. This has prompted a hypothesis that the remains are the result of recurring die-offs and rapid burial in burrows. To test this, rare earth element (REE) concentrations were measured from Auroraceratops bones and surrounding red-bed mudstones via inductively coupled plasma mass spectrometry. Petrographic analysis was conducted to understand the preservation of the bones and surrounding rock. Results suggest early postmortem deterioration of the bone by bacteria and/or fungal hyphae (Wedl type I tunneling and mineralized remains) and mineralization of hyphae in acidic/reducing conditions. This is supported by Ce-enriched, filamentous ferromanganese oxide coatings on the external and internal bone cavities. Deterioration/dissolution of the bone must have ceased for a period of time, and we suggest that desiccation of the carcasses halted the deterioration of bone. A second mineralization phase occurred as remains came into contact with vadose water, as evident by precipitation of micritic calcite, recrystallization of filamentous apatite in the bone matrix, and bone REE patterns indicating equilibration with alkaline/oxidized fluids. Despite poor histologic preservation, the well-articulated, curled-up positioning of the remains, and similarity to remains from the Upper Cretaceous Nemegt Formation of Mongolia, suggests that the Auroraceratops beds are the result of death in burrows in a seasonally arid environment. The similar taphonomic mode to remains from Nemegt may represent a common preservation mode for faunas from extensional basin deposits.