Geopetal microbial threads (cryptoendoliths) within middle Cambrian brachiopods from North Greenland (Laurentia)

Cavities (crypts) formed postmortally within conjoined valves of acrotretoid brachiopods from the Fimbuldal Formation (Cambrian, Miaolingian Series) of North Greenland contain phosphate encrustations of microbial threads, probably fungi. The planar relationship between these cryptoendolithic masses and the sediment that partially fills the brachiopod interiors defines palaeohorizontal surfaces within geopetal structures. These surfaces parallel the commissural plane, indicating that the dead shells lay in a hydronamically stable position on or within the bottom sediment at the time the geopetals were formed.