Tarbosaurus, a large predatory dinosaur and close relative of Tyrannosaurus rex, roamed ancient Asia. Discovered in Mongolia’s Gobi Desert in 1946, this bipedal theropod measured about 10 meters (33 feet) long, stood 3 meters (10 feet) tall at the hips, and weighed up to 5 metric tons. It featured a unique jaw-locking mechanism and notably small forelimbs, proportionally the smallest among tyrannosaurids.
The Cretaceous Period
Tarbosaurus lived during the Cretaceous Period, the final segment of the Mesozoic Era, known as the “Age of Dinosaurs.” This geological period spanned approximately 145 to 66 million years ago, making it the longest period of the Phanerozoic Eon. During the Cretaceous, Earth experienced a warm climate with higher sea levels, often 100 to 250 meters above present-day levels, leading to widespread shallow inland seas. The supercontinent Pangaea continued its breakup, with landmasses drifting toward modern continental configurations. This period also saw the diversification of dinosaur life, the emergence of flowering plants, and early modern mammal and bird groups.
Late Cretaceous Stages
To pinpoint Tarbosaurus’s existence, scientists focus on the Late Cretaceous Epoch. Within this epoch, Tarbosaurus inhabited Earth during the Campanian and Maastrichtian stages. The Campanian stage occurred from approximately 83.6 to 72.1 million years ago, followed by the Maastrichtian stage (72.1 to 66 million years ago). Most Tarbosaurus fossils have been unearthed from rock layers dating to the Maastrichtian, typically around 70 million years ago.
While some evidence suggests its presence might extend into the late Campanian, most findings place it firmly within the Maastrichtian. Tarbosaurus was one of the last large non-avian dinosaurs to thrive before the Cretaceous-Paleogene (K-Pg) extinction event. This event, linked to a large asteroid impact, marked the end of the Mesozoic Era and the reign of non-avian dinosaurs.
Unearthing Tarbosaurus’s Timeline
Scientists determine the age of dinosaur fossils like Tarbosaurus using geological dating methods. Relative dating, primarily stratigraphy, involves studying rock layers; fossils in lower sedimentary layers are older than those in higher ones. This method provides a chronological sequence without exact numerical ages. For precise numerical ages, scientists employ absolute dating techniques, most notably radiometric dating.
Radiometric dating cannot be performed directly on dinosaur bones because necessary radioactive isotopes are not incorporated during fossilization, nor would the bones withstand the heat. Instead, scientists date layers of volcanic ash, igneous rocks found above or below fossil-bearing sedimentary layers. These ash layers contain specific radioactive isotopes, such as potassium-40, which decay into argon at a known rate. By measuring the ratio of parent isotopes to daughter products, paleontologists calculate the age of the volcanic ash, bracketing the age of the fossils found between these layers.
Most Tarbosaurus fossils have been recovered from the Nemegt Formation in the Gobi Desert of Mongolia. This geological formation consists of ancient river channel sediments, sandstones, and mudstones, providing a rich fossil record. While the Nemegt Formation itself has not been directly dated radiometrically, its diverse fossil fauna indicates an early Maastrichtian age, placing Tarbosaurus around 70 million years ago.