We have discovered numerous borings of Trypanites penetrating skeletons and synsedimentary cemented limestones in archaeocyathid reefs of the Forteali formation in southern Labrador. These are, to date, the oldest known macroborings. The discover) of these structures extends the record of large endolithic organisms 100 million years from the Lower Ordovician to the Lowver Cambrian. This immediately postdates the appearance of metazoans with hard parts and confirms that endoliths have played a role in reefformation since the early Cambrian.