Allosaurus was a large, predatory dinosaur. This article explores its geological timeline, the world it inhabited, and the scientific methods paleontologists use to determine these ancient dates.
Allosaurus’s Time on Earth
Allosaurus lived during the Late Jurassic Period, specifically spanning the Kimmeridgian to Tithonian ages, from approximately 155 to 145 million years ago. It was a prominent large predator during this epoch, with numerous fossils discovered. The bulk of Allosaurus remains have been found in North America (Morrison Formation: Utah, Colorado, Wyoming, Oklahoma), with European fossils also recovered from Portugal’s Alcobaça, Bombarral, and Lourinhã formations. Its presence on both continents suggests a wider geographical distribution when landmasses were connected. Its success as an apex predator is highlighted by its fossil record within the Late Jurassic landscape.
The World of the Late Jurassic
The Late Jurassic period had a generally warm and humid global climate, with no polar ice caps and higher overall temperatures than today. This warm climate, likely influenced by elevated levels of carbon dioxide, supported widespread lush vegetation. The supercontinent Pangaea had begun to rift apart, forming Laurasia to the north and Gondwana to the south, with the nascent Atlantic Ocean starting to open. This continental arrangement influenced global weather patterns, leading to less severe seasonal variations.
Plant life was dominated by gymnosperms, including diverse conifers, cycads, and ginkgos, alongside abundant ferns and horsetails. While the first flowering plants appeared, they were not yet widespread or dominant. This rich plant life provided ample food for the large herbivorous dinosaurs that coexisted with Allosaurus.
Allosaurus shared its environment with gigantic sauropods such as Apatosaurus, Brachiosaurus, Camarasaurus, and Diplodocus. Other notable dinosaurs included the armored Stegosaurus and various ornithopods. Smaller theropods like Ceratosaurus and Torvosaurus also inhabited these ecosystems. Early birds, such as Archaeopteryx, also marked this period.
How Scientists Date Allosaurus Fossils
Determining the age of Allosaurus fossils involves scientific methods, as dinosaur bones rarely contain elements for direct dating. Scientists primarily date the sedimentary rock layers where fossils are found. One fundamental technique is relative dating, which relies on stratigraphy, the study of rock layers. This method operates on the principle that in undisturbed sequences, older rock layers lie beneath younger ones, providing a chronological order.
For more precise age determination, scientists use absolute dating techniques, most notably radiometric dating. This process measures the decay of unstable radioactive isotopes (parent atoms) into stable isotopes (daughter atoms) within a rock. Each radioactive element decays at a known, constant rate, referred to as its half-life, acting like a geological clock.
Carbon-14 dating is unsuitable for dinosaur fossils because its short half-life means it can only date materials up to about 50,000 years old. Instead, for materials millions of years old, scientists utilize isotopes with much longer half-lives, such as uranium-238, uranium-235, or potassium-40. These radioactive elements are typically found in igneous rocks, like volcanic ash or lava flows, rather than in the sedimentary rocks where fossils form. Therefore, paleontologists locate volcanic ash layers above and below the fossil-bearing sedimentary strata. By radiometrically dating these igneous layers, they establish a minimum and maximum age range for the dinosaur fossils enclosed within the sedimentary layers.