Two Caudicriodus species from lowermost Devonian strata in southern Laurentia (Texas–Oklahoma–Western Tennessee), C. anitae sp. nov. and C. murphyi sp. nov., are endemic forms that appeared during the positive δ13C excursion associated with the Klonk Event. The other Caudicriodus species that occur during the Klonk Event and that are often used to recognize the base of the Devonian, C. hesperius and C. woschmidti, differ in that the I elements of the southern Laurentia species possess a middle row of nodes on the transverse ridges of the spindle. The apparatus of C. murphyi sp. nov. possesses three groups of elements in addition to the I element: F elements (short wide coniforms); D elements (denticles on the posterior margin) and C elements (slender coniforms with basally flaring costae). Strong provinciality in slightly younger Lochkovian Caudicriodus species was demonstrated earlier, and we propose that Caudicriodus species radiated independently into endemic species in different geographic regions just before or during the Klonk Event. A form originally described as C. woschmidti hesperius by Simpson (1998) is reinterpreted as a late Ludlow–Pridoli species of Praeicriodus, P. simpsoni sp. nov., with an I element and apparatus structure probably ancestral to Caudicriodus.
- Klonk Event
- Silurian–Devonian boundary