A Cretaceous Time Scale
Copyright © 2001-2009 by Mike Everhart
Last revised: 07/11/2009
The Cretaceous Period was the the last of three periods (Triassic, Jurassic and Cretaceous) that made up the Age of Dinosaurs. It encompasses a segment of time from about 142 million years ago to about 65 million years ago and saw the evolution of most of the dinosaurs that we are familiar with today. In addition, it includes the last 30 million years when mosasaurs evolved and came to dominate the Earth's oceans.
Table 1 gives the approximate duration of ages that comprise the Upper Cretaceous, with remarks regarding mosasaurs.
| Maastrichtian | 65.4 - 71.3 MYA (5.9 MY) | Greatest diversity and distribution; invasion of freshwater habitats |
| Campanian | 71.3 - 83.5 MYA (12.2 MY) | "Second Wave" mosasaurs; Mosasaurus, Plioplatecarpus, Prognathodon, Globidens |
| Santonian | 85.5 - 86.3 MYA (0.8 MY) | Mosasaurs get much larger; world-wide distribution |
| Coniacian | 86.3 - 88.7 MYA (2.4 MY) | "First Wave" mosasaurs; Tylosaurus, Platecarpus and Clidastes |
| Turonian | 88.7 - 93.3 MYA (4.6 MY) | Early mosasaurs; Clidastes and Tylosaurus / Platecarpus precursors |
| Cenomanian | 93.3 98.5 MYA (5.2 MY) | Ancestral mosasaurs (Aigialosaurs) return to the ocean |
Table 2 provides approximate dates for the beginning and end of the various ages that make up the Cretaceous Period.
| Age Boundary | Millions of Years Ago | Remarks |
| Maastrichtian - Paleocene | 65.4 +/- 0.1 MY | End of the Cretaceous - The "K/T Boundary" - Beginning of Tertiary |
| Campanian - Maastrichtian | 71.3 +/- 0.5 MY | Record in Kansas ends in the Campanian Pierre Shale |
| 83 MYA | Deposition of the Smoky Hill Chalk ends; Pierre Shale begins | |
| Santonian - Campanian | 83.5 +/- 0.5 MY | |
| Coniacian - Santonian | 86.3 +/- 0.5 MY | |
| 88 MYA | Deposition of the Fort Hays Limestone ends; Smoky Hill Chalk begins | |
| Turonian -Coniacian | 88.7 +/- 0.5 MY | Deposition of the Fort Hays Limestone begins |
| Cenomanian -Turonian | 93.3 +/- 0.2 MY | Dakota Sandstone / Carlile Shale / Graneros shale / Greenhorn Limestone (approx) |
| Upper Cretaceous above this line - Lower Cretaceous below | ||
| Albian - Cenomanian | 98.5 +/- 0.5 MY | Comanche Sandstone / Kiowa Shale |
| Aptian - Albian | 112 +/- 1.0 MY | No record in Kansas |
| Berremian -Aptian | 121 MYA | No record in Kansas |
| Hauterivian - Berremian | 127 MYA | No record in Kansas |
| Valanginian - Hauterivian | 130 MYA | No record in Kansas |
| Berriasian - Valanginian | 135 MYA | No record in Kansas |
| Tithonian - Berriasian | 142 MYA | Morrison Fm. / End of the Jurassic - Beginning of the Cretaceous |
Adapted from: Obradovich, J. D., 1993. A Cretaceous time scale. pp. 379-396 In Caldwell, W. G. E., and E. G. Kauffman, eds., Evolution of the Western Interior Basin, Geologic Association of Canada, Special Paper 39.