PROVINCIALITY IN EARLY LOCHKOVIAN (EARLY DEVONIAN) CAUDICRIODUS SPECIES AND IMPLICATIONS FOR CORRELATION OF THE SILURIAN–DEVONIAN BOUNDARY

The Silurian–Devonian (S–D) boundary interval is uncommonly preserved due to low global sea level followed by a widespread pre-Middle Devonian erosional unconformity. The graptolite Monograptus uniformis is used for boundary correlation, but it is typically limited to deep-water environments. Historically, the conodonts Caudicriodus hesperius and C. woschmidti have been used in shallow marine successions globally, but neither species’ FAD is directly at the S–D boundary. Moreover, species identities and stratigraphic ranges of many earliest Devonian members of Caudicriodus are unresolved. Chemostratigraphic data across the boundary interval demonstrate a global positive δ¹³C excursion (Klonk excursion), which provides a global timeline for accurately calibrating the ranges of species across the S–D boundary. Integrated conodont-carbon isotope studies are the most robust way to correlate the S–D boundary when M. uniformis is not present.

Our research recognizes two new species of Caudicriodus from the lowermost Devonian (Lochkovian) Haragan Formation of Oklahoma that possess a unique middle row of denticles on the spindle, unlike C. woschmidti and C. hesperius. Specimens with the same morphology occur in West Texas and Tennessee, and possibly Virginia and New York. All of these occurrences are temporally constrained to the Klonk interval by δ¹³C data. The geographic distribution of the new species coincides with the Appohimchi Subprovince of Devonian brachiopods, whereas C. hesperius occurs within the Nevada Subprovince and C. woschmidti is found near the Rhenish Subprovince. In contrast to previous reports, Caudicriodus species experienced provinciality even during the earliest Lochkovian as well as later in the Early Devonian. More accurate information regarding conodont taxonomy, biostratigraphy, and geographic distribution from the Early Devonian is needed in order to achieve precise correlation of the S–D boundary.