Core Research Center

Cretaceous Ammonites

A Story of Sea Level Changes in the Western Interior Seaway


Cretaceous Western Interior Seaway

Cretaceous Western Interior Seaway. Colorado was covered by a shallow, temperate sea. (Figure by William A. Cobban and Kevin C. McKinney, USGS) (Public domain.)


During the Late Cretaceous the global climate was warmer than today's climate. There was no north polar icecap. Dinosaurs migrated into the high latitudes as the seasons changed. North America was split by the Western Interior Seaway. Colorado was in the center of the temperate sea populated by mososaurs and ammonites.

The ammonite fossils from the Pierre Shale (muds) in Middle Park indicate a shallow marine environment. Obradovich dated bentonites in many of the ammonite fossil localities. The Upper Cretaceous zones within Middle Park range from Cenomanian-Maastrichtian, with dates 95-67 Ma (Obradovich, J.D., A Cretaceous time scale, in Evolution of the Western Interior Basin, edited by W.G.E. Caldwell, Geol. Assoc. Canada Spec. Paper, 39, 379-396, 1993). Cretaceous localities are abundant. There are more than 500 USGS-Denver localities in Grand County alone.

Disk-shaped ammonites (Placenticeras) around the Granby area range in size up to two feet across-about the size of truck tires. The most unusual ammonites are the heteromorphs. The Late Cretaceous environment favored an explosion of morphological experimentation. The heteromorphs evolved from the conservative, basically flat, shell into the ornate spirals typical of Didymoceras. The tight spirals "unwound" into unusual shapes, some like corkscrews, some like fish-hooks. John Stacy worked closely with Bill Cobban and Glenn Scott to reconstruct and illustrate the ammonites. Stacy used a technique called stippling to create dramatic three-dimensional plates for numerous USGS publications.

Ammonite fossil locations near Middle Park CO

Number of USGS-Denver Cretaceous Mollusk Localities in the Pierre Shale (~ 70-67 Ma) (Public domain.)

  • 517 Grand County, CO
  • 312 Boulder County, CO
  • 78 Routt County, CO
  • 29 Summit County, CO

The Pierre Shale deposits are chiefly dark-gray shales. The Pierre muds represent near-shore environments within the Western Interior Seaway. Ammonites populated the shallow sea in present-day Middle Park. The temperate shallow sea was an ideal spawning ground for the giant disk-shaped Placenticeras (some as large as truck tires in the vicinity of Granby) and the corkscrew-like Didymoceras.