One of the most important functions of paleontology in the earth sciences is time correlation of rock strata using taxonomic analysis of fossils in different regions. Comparisons of certain species’ similarities between regions frequently allows for precise age dating and correlation of strata limited only by the presence/absence of species and the speed at which they evolved. Between their first appearance in the early/middle Devonian and their ultimate extinction at the K-T boundary, no other single taxonomic group is as precise or as widely useful for time correlation of strata as the ammonoid cephalopods, an extinct distant relative of the modern chambered nautilus. This is especially true for the Carboniferous Era, where ammonoid change was extremely rapid for reasons that are as yet not fully known, although global climate fluctuation is probably a key driving force.