Sharks first inhabited the waters during this period of the Paleozoic era named for an English county
... that 
lived in the the waters of muddy estuaries during the Pennsylvanian period, ... 
Age: Paleozoic [DOC] [TLE] Fossils In Millard County, Utah [PAR] Visit Wheeler Amphitheater and Fossil Mountain in Millard County, Utah--two ... 
animal life in Paleozoic Era strata exposed throughout the western United States.