Embedment cavities in lacustrine stromatolites: evidence of animal interactions from Cenozoic carbonates in USA and Kenya

Embedment structures are formed by the growth of a skeleton-producing host organism around a cavity maintained by an infesting organism. Through the process of embedding, the infesting animal passively produces a secure dwelling structure. This embedment ethology is common in modern and ancient marine settings. Previously unrecognized embedment cavities from ancient lacustrine stromatolites are identified and described here for the first time. Cenozoic lake deposits from the Washakie Basin in Wyoming and Turkana Basin in Kenya contain stromatolites with regularly pitted surfaces, which represent openings to embedment cavities. Stromatolitic laminae deflect downwards adjacent to the cavities. These cavities are circular to oval in cross-section and terminate with a blind base. A lack of branching, basal rootstock, and encrustation by stromatolitic laminae suggest that the cavities were not formed by the overgrowth of plant material, as has been suggested previously. Moreover, the rounded and evenly spaced cavities indicate that these structures are not the spaces between abutted columns of stromatolite material. These findings suggest that some lacustrine stromatolitic ecosystems are more diverse than the body-fossil record demonstrates. It is probable that more embedment structures will be recognized in lacustrine and marine stromatolitic settings.