Placenticeras meeki ammonite 8 in x 7 in AP73

Placenticeras meeki

Upper Cretaceous Maastrichtian

Pierre Formation

Pennington County, South Dakota USA

This Ammonite reached sizes of up to 40 inches across. Placenticeras meeki is found in the Pierre shale and are the largest ammonites in the Dakotas. Placenticeras meeki has a smooth shell, and a large umbilicus, or central whorl. Based on their narrow keel, lack of nodes and hydrofoil-like shapes, members of this species were probably good swimmers. These animals were not heteromorphic, (did not change shape at maturity) but continued to build their shells on the same radial progression. Many individuals of the Placenticeras genus have been discovered with Mosasaurus bite marks in their shells, and one actually contained a fossil Mosasaurus tooth.