Analysis of the biostratigraphy and the litho-facies pattern of Upper Cambrian sedimentation upon the craton reveals four synchronous marine regressions and three transgressions attributable to eustatic sea level change. The beginning of each regression relates to the impoverishment or extinction of the existing trilobite biota and the appearance of many individuals of a few unrelated genera and species. Except in near-shore sites the abrupt faunal change preceded by a short time the physical evidence of regression in the sections. The cause of the faunal change and of the regression was a deterioration of world climate.
The first regression occurred during the Bolaspidella Zone and the ensuing faunal impoverishment and change has been used to mark the end of the Middle Cambrian. The beginning of the second regression coincided with the appearance of the oldest genera of the Aphelaspis Zone assemblage on the craton in late Dresbachian; the third with the earliest genera of the Conaspis Zone assemblage in the middle Franconian; and the fourth with the dominance of genera of the Hungaia assemblage on the craton in late Trempealeauan. The small biota of each new assemblage evolved during the regression to occupy the limited habitats of the shrinking environments on the craton and in the adjoining miogeosyncline. During the succeeding transgression some of the genera evolved explosively as a variety of habitats became available in the warming marine waters of the expanding cratonic environment. The Cedaria-Crepicephalus, the Elvinia, and the Prosautya-early Saukja assemblages are the characteristic, highly varied biotas of the transgressions.