Some of the world's oldest macroborings occur in hardgrounds in lower Ordovician (Arenig) limestones exposed on the island of Öland, southern Sweden. The trace fossils, which are described here as Gastrochaenolites oelandicus isp. nov., appear to be dwelling structures excavated in the indurated substrate by invertebrates of unknown taxonomic affinity. They are the oldest examples of this ichnogenus. The appearance of a macroboring life habit at this early time represents a revolutionary new adaptive strategy for inhabiting carbonate hardgrounds. However, this innovative strategy apparently was not successful for the long term, because this particular macroboring taxon seems to have disappeared shortly after its early Ordovician appearance.