Late Cambrian missing link in macroborer evolution preserved in intraclasts

Macroborings are known to be rare to absent in middle to late Cambrian successions, the time interval between the end-early Cambrian archaeocyath extinction and the advent of new boring organisms during the mid-late Ordovician (the Ordovician Bioerosion Revolution). The occurrence of macroborings was controlled mainly by hard-substrate availability; archaeocyath reefs and hardgrounds provided synsedimentarily lithified substrates during the early Cambrian and Ordovician, respectively. In this study, we report bioerosion structures from micritic intraclasts of flat-pebble conglomerates in the Hwajeol Formation (Furongian), Taebaeksan Basin, Korea. Two types of macroborings (Trypanites and cf. Gastrochaenolites) are recognized, both of which penetrate micritic clasts and are filled with bioclastic grainstone matrix. Several lines of evidence, including the sharp boundaries and unaltered shapes of macroborings, as well as the occurrence of macroborings penetrating the coating of iron- and/or manganese oxide-coated micritic clasts, indicate that these macroborings formed after the cementation of micritic substrates. The Hwajeol macroborings would have formed on micritic clasts and/or hardgrounds that were eroded and formed flat pebbles. The presence of bioerosion structures within clasts of the Hwajeol flat-pebble conglomerates supports a previous hypothesis that macroborers survived in hardgrounds during the middle to late Cambrian, after the extinction of archaeocyath reefs. In addition, the Hwajeol cf. Gastrochaenolites is the earliest of its kind, implying that there could be more kinds of macroborings hidden within Cambrian flat-pebble conglomerates.