Cambrian and earliest Ordovician conodont evolution, biofacies, and provincialism


Conodonts are divided into three groups with different histologies: protoconodonts (most primitive), paraconodonts, and euconodonts (most advanced). The first is poorly known, but paraconodonts included a Westergaardodina and a coniform evolutionary lineage, and each was the ancestor of one or more euconodont lineages. Early euconodonts are thus polyphyletic and included the Proconodontus and Tendonitis Lineages, which appeared in the middle Late Cambrian, and the Fryxellodontus and Chosonodina Lineages, which appeared in the Early Ordovician. Major changes in conodont evolution, biofacies adaptation, and development of provincialism coincided with sealevel fluctuations near the end of the Cambrian (here named the Lange Ranch Eustatic Event, or LREE) and similar fluctuations recorded at the Lower/Upper Tremadoc boundary (here named the Black Mountain Eustatic Event, or BMEE). Protoconodonts and paraconodonts were probably pelagic and cosmopolitan. Genera of the Proconodontus Lineage were probably also pelagic. Some genera of the latter lineage are found only in low- to mid-paleolatitude areas; others were cosmopolitan, including Cordylodus. Genera of the Teridontus and Fryxellodontus Lineages may have been nektobenthic. Some were adapted to warm, high-salinity environments that existed during the LREE, but younger genera probably were adapted to normal salinity and were more widely distributed. No apparent provincialism existed until the appearance of euconodonts, after which two broad faunal realms are distinguishable. The warm faunal realm included shallow seas in low to middle paleolatitudes; the cold faunal realm included high-paleolatitude seas and open-ocean areas. Early euconodonts of the Proconodontus Lineage appeared and quickly became dominant in the warm faunal realm during the latest Cambrian. Much of the preexisting protocondont-paraconodont fauna was displaced from the warm faunal realm but continued to dominate the cold faunal realm through the Early Tremadoc. Major faunal changes occurred in the warm faunal realm as a result of the LREE, and after this event conodonts in this ream consisted for the most part of genera from the Teridontus Lineage. During the BMEE a different euconodont fauna of uncertain ancestry became adapted to the cold faunal realm, after which most of the previously dominant primitive fauna became extinct. Cosmopolitan Cordylodus lived in both faunal realms during much of the Tremadoc, but after it became extinct prior to the Arenig, provincialism was extreme because few species were adapted to both faunal realms. Oneotodus tenuis Miiller is reclassified as the type species of a new genus, Phakelodus.


Geography, Geology, and Planning

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Special Paper of the Geological Society of America