See below for our second DinoByte Wednesday re-post. Today, we’ll discuss the geologic time scale as it relates to our field site. Enjoy!
The Earth is 4.6 BILLION years old, and the geologic time scale breaks this long amount of time into smaller units. These units, arranged from longest to shortest, are eons, eras, periods, epochs, and stages. The divisions between units are based upon major geological and paleontological events. At our field site in Hell Creek, Montana, the fossils indicate dinosaurs lived there during the Phanerozoic Eon, of the Mesozoic Era, during the Maastrichtian Stage of the Cretaceous Period.
The Mesozoic Era is the “Age of Dinosaurs,” and the dinosaur fossils we find in the field are 68 million to 66 million years old! We will find dinosaurs during the DIG Field School (as well as turtles, crocodiles, fish, mammals), but one of the things that makes our field site so special is that in this area there are rocks that preserve the last two million years that non-avian dinosaurs inhabited the earth, the layer that shows dinosaur extinction, AND the first one million years of the Paleogene Period.
The Paleogene Period (and the Paleocene Epoch) marks the start of the Cenozoic Era, the “Age of Mammals.” At our field site, in addition to Cretaceous dinosaurs, we find fossils of the mammals and other animals that survived the Cretaceous-Paleogene Mass Extinction that killed the dinosaurs (except birds). That makes this area one of the best places in the world to study dinosaur extinction AND the subsequent recovery of mammals.
Most of the geologic layers in Montana that contain dinosaurs lay at the surface. Next week, we’ll DIG further into rock formations and uncover why these fossils sit at the surface when they are up to 60+ million years old.
P.S. Are you wondering how we can determine the age of the fossils found in the Hell Creek area? These are much too old to use carbon dating, so we can use nearby rocks as clues, particularly layers of ash and coal. Stay tuned for more on dating fossils in a future DinoByte post!
(Most of this information came from the excellent book Dinosaurs Under the Big Sky by Jack Horner)