Abstract
A comparison of conodont collections from the Lower Carboniferous indicates that many conodont elements are excellent index fossils. Some of these have a world-wide distribution and always represent the same stratigraphic interval. These, the type A index conodonts, include species of Eotaphrus, Pseudopolygnathus, Siphonodella, and Taphrognathus and are good inter-and intra-regional markers. Type B index conodonts are restricted to specific regions and are good stratigraphic markers within these. Bactrognathus distortus is present in most samples from the middle of the Burlington Formation from that formation's erosional edge in east-central Iowa to just south of St. Louis Missouri, in a few samples from the Floyds Knob Member of the Borden Formation in eastern Kentucky (Goodman, 1975, Baxter 1993a), and in samples from the Andrecito Member of the Lake Valley Formation of New Mexico (Burton, 1964) . This whole region was just south of the equator during the Carboniferous (Baxter, 1993b) . Another type B index conodont is Mestognathus. This taxon is common in collections from Europe, but has also been recovered from exotic, accretionary terrains in Atlantic Canada and Western North America. Type C index conodonts occur word-wide, and although they are good markers within a region, they have different ranges in different regions. Gnathodus texanus and several species of Cavusgnathus range throughout the Visean of North America. In the Midcontinent Region G. texanus occurs throughout this interval but is restricted to the Chester in Western Canada. Cavusgnathus is present throughout the Visean of Western Canada but does not occur below the upper part of the St. Louis Formation in the Midcontinent Region.
Type A index conodonts are essential for word-wide correlation. The other two types are good for correlation within specific regions, but may also serve other purposes. The type B index conodonts may reflect fixed barriers such as restrictive currents or limiting salinity or temperature gradients. This type may also serve to identify exotic terrains and plate movements. Type C index conodonts may be indicative of fluctuations in environmental conditions or may reflect the lateral migration of a taxon.
