CONODONTS FROM THE NORIAN TO HETTANGIAN OF SOUTHEAST ALASKA, AND A REVIEW OF JURASSIC CONODONT OCCURRENCES

The Late Triassic was a time of complex environmental and ecological change, culminating in the end-Triassic mass extinction. Conodonts are thought to have gone extinct at the Triassic-Jurassic boundary after a progressive decrease in biodiversity during the Rhaetian. In order to improve our understanding of the timing of extinction and environmental change during the Late Triassic and Early Jurassic, a multidisciplinary study was undertaken at the Carnian – Pliensbachian Grotto Creek section in southeast Alaska. In addition to conodont samples, macrofossils and samples for geochemical and sedimentological analysis were collected. Eight samples over approximately 50 m of the McCarthy Formation contained identifiable conodont specimens, the oldest of which were late Norian (Norigondolella steinbergensis, Misikella hernsteini). Higher samples contained specimens of Mockina englandi and Mockina bidentata, of Norian or Rhaetian age. Specimens of Mockina mosheri morphotype B were recovered from above the first occurrence of the ammonoid Paracochloceras amoenum, indicating the presence of Rhaetian strata. The highest conodont sample at the section originated from the same bed as the first occurrence of the Hettangian ammonoid Psiloceras sp., and included a single conodont specimen referred to Neohindeodella sp.

Conodonts have previously been described from unequivocal Jurassic rocks at the Csővár section in Hungary, with the species Neohindeodella detrei reported in rocks with Hettangian radiolarians. Subsequent re-collection of the section confirmed the presence of Jurassic conodonts, with the recovery of Neohindeodella detrei above Psiloceratid ammonoids and additional Hettangian radiolarians. The restriction of Hettangian conodonts to a single section in Hungary has led to doubts about the significance of these occurrences, whether it be due to re-working or unusual environmental conditions, and most workers maintain that conodonts went extinct at the Triassic-Jurassic boundary. The discovery of an additional specimen of the same genus (Neohindeodella) from undisputed Jurassic rocks at a different location, increases the likelihood that conodonts did survive into at least the earliest Hettangian, and should encourage further study of Jurassic occurrences of conodonts.