THE OLENEKIAN-ANISIAN/EARLY-MIDDLE TRIASSIC BOUNDARY, AND ASSESSMENT OF THE POTENTIAL OF CONODONTS FOR CHRONOSTRATIGRAPHIC CALIBRATION OF THE TRIASSIC TIMESCALE

Abstract

The conodont Chiosella timorensis (Nogami, 1968) has for a long time been considered to be a suitable biotic proxy for the Olenekian-Anisian/Early-Middle Triassic boundary. The recently acquired ammonoid record around that boundary clearly shows that the FAD of this conodont is located well below the boundary, i.e., in the late Spathian. In the present paper, it is underlined that the conodont Chiosella timorensis was promoted as a proxy for the nominated boundary in the early 1980s when the ammonoid record around the boundary was not yet well established. On the other side, until the mid 1990s the taxonomic definition and the lineage of the conodont Chiosella timorensis were not well stated, and even now there are still controversial interpretations of the taxonomic content of this conodont species. The new data achieved from the ammonoid/conodont record around the nominated boundary, especially in the western USA, and also in the Deşli Caira section in Romania, firmly demonstrate that the conodont Chiosella timorensis is a defunct proxy for the Olenekian-Anisian/Early-Middle Triassic boundary. As a consequence, the present data on the ammonoid-documented Olenekian-Anisian/Early-Middle Triassic boundary requires the recalibration of all physical events that have been tied to the FAD of the conodont Chiosella timorensis. The case of the Albanian Kçira-section, for which the chronostratigraphic interpretation of the ammonoid record is proved incorrect, definitely makes the conodont Chiosella timorensis a defunct proxy for the nominated boundary. Also, the case of the two Chinese sections recently proposed as being “exceptional” GSSP candidates for the Early-Middle Triassic boundary, which is based on an inconsistent ammonoid/conodont biochronology, fully strengthens this conclusion. The history of the controversial usage of the conodont species Chiosella timorensis in defining the Olenekian-Anisian boundary justifies a discussion about the usefulness of conodonts in the chronostratigraphic calibration of the standard Triassic timescale. One may conclude that the conodonts are not qualified, and have not a reasonable potential, to be used to define or to redefine the boundaries of chronostratigraphic units in the standard Triassic timescale, which have been basically defined on ammonoid biochronology.