Bacterial origin of iron-rich microspheres in Miocene mammalian fossils

• Iron microspheres in a dinosaur site have been recognized in mammal fossils.
• These microspheres have a distinctive morphology and chemical composition.
• Iron enrichment of typical microbial bone attack is shown here.
• Microspheres originally suggested to be linked to bacterial origin is here confirmed.
• Both dinosaur and mammalian sites are linked to aquatic environments.

In their taphonomic study of a Cretaceous dinosaur fossil from the Gobi desert (Mongolia), Kremer et al. (2012) noted that the histological sections of this fossil preserved within their core iron oxide microspheres containing carbonaceous matter. They interpreted the carbonaceous nature of these structures as organic matter and suggested a microbial origin (probably bacterial) for the structures. Microspheres, similar both in composition and shape, have been identified in fossils from Cerro de la Garita, a Miocene mammalian site in Teruel, Spain. In the latter case, compact bone was also attacked by terrestrially associated bacteria (microscopic focal destruction [MFD]) which were enriched in iron and gives support to the idea that bacteria acted as the biological agent for iron precipitation during soft tissue decomposition in the early stages of bone diagenesis. Subsequent diagenetic episodes of mineralization related to the environmental context differ between these two sites; calcite precipitation at the palaeo-lakeshore of Cerro de la Garita and calcite and gypsum in the Gobi desert study case of Kremer et al. (2012). If the microsphere is bacterial in origin, it may be a useful taphonomic indicator of terrestrial exposure within a transitional environment of land and water.