TABLE 1. Ages of some late Cretaceous western interior ammonite-zone boundaries of Gradstein et al. (2004) and this report.

 

Gradstein et al. (2004)
This report
 
W. Interior Ammonite Zone
Age
(Ma)
Duration
(m.y.)
Age
(Ma)
Duration
(m.y.)
 
Baculites compressus
73.50
0.72
73.90
?
 
Didymoceras cheyennense
74.28
0.78
74.50
0.60
 
Exiteloceras jenneyi
75.05
0.77
74.65
0.15
 
Didymoceras stevensoni
75.74
0.69
74.98
0.33
 
Didymoceras nebrascense
76.38
0.64
75.76
0.78

Note: Ages are for base of ammonite zones.