Museum Research Reveals Small Size of Dino-companion Mammals

We’ve known for years that the 20+ types of mammals that are found along side the dinosaurs in the Morrison Formation in western North America are small, but how small? In the Grand Valley, most of our dinosaurs, such as Stegosaurus, Allosaurus, and Apatosaurus, come from the Morrison Formation, a layer of rock well exposed along the base of the Monument and out in Rabbit Valley. But at the Fruita Paleontological Area and at sites in Wyoming and other areas we find a diversity of small mammals mixed in with the dinos — a diversity that matches that of the dinosaurs. How small were they? In a study published last month in the University of California’s paleontology journal PaleoBios, Museum curator of paleontology John Foster compares remains of Jurassic mammals from the Morrison Formation with modern mammals and estimates that the smallest Morrison mammal species (Fruitafossor and Amphidon) probably weighed as much as the tiny Long-tailed Shrew. Most importantly in terms of contrast with modern mammalian faunas, the *largest* mammal in the Morrison world was only as heavy as a small Red Squirrel! It’s a long road from the Morrison to the Serengeti.