A proposed ontogenesis and evolutionary lineage of conodont Eurygnathodus costatus and its role in defining the base of the Olenekian (Lower Triassic)

The Early Triassic is a critical interval recording episodic climatic, environmental, and biotic events after the end-Permian mass extinction. A precise global timescale for this epoch is essential to understand the sequence of events during this crisis. The undefined Induan-Olenekian stage boundary (IOB), long disputed, is a crucial element in providing a high-resolution chronostratigraphic framework for the Lower Triassic. Here we report new conodont lineages from the Jianshi section, South China that may contribute to the definition of the IOB. In Jianshi, within a succession of nine conodont zones, a possible IOB is placed at Bed 225 + 40 cm based on the first occurrence (FO) of conodont subspecies Novispathodus waageni eowaageni. New collections include a total of 99 well-preserved Eurygnathodus costatus specimens from Bed 225 to Bed 241, and a single occurrence of Eu. hamadai? from Bed 226. The FO of Eu. costatus occurs in Bed 225 + 10 cm, 30 cm below N. w. eowaageni. The reconstructed ontogenetic series of Eurygnathodus costatus involves four growth stages (A -> B -> C -> D) based on characteristics of the denticles (nodes or ridges), platform, basal cavity, margin profile, and crimp. These preserve ancestral characters that suggest a novel origin for the evolutionary lineage of the genus, namely from Sweetospathodus kummeli -> Eurygnathodus costatus -> Eu. sp. D -> Eurygnathodus hamadai. Seven additional informal species of Eurygnathodus are differentiated as potential biostratigraphic markers. The FO of Eu. costatus may serve as an auxiliary marker for the IOB.