The Late Cretaceous: Peak Diversity

The Late Cretaceous Period, from approximately 100 to 66 million years ago, represents the zenith of dinosaur diversity and ecological complexity. This was the world of Tyrannosaurus rex and Triceratops, of vast inland seas and towering redwood forests, of ecosystems that in many respects more closely resembled the modern world than anything that had come before. If any moment in deep time deserves to be called the pinnacle of the age of dinosaurs, it is this one.

In North America, the Western Interior Seaway divided the continent into two landmasses: Laramidia to the west and Appalachia to the east. Laramidia, a roughly north-south landmass from Alaska to Mexico, hosted some of the most intensively studied dinosaur faunas in history. It is from this narrow strip of subtropical and warm-temperate land that most of our knowledge of Late Cretaceous dinosaurs derives.

The apex predator of Laramidia — and arguably the most famous animal that has ever lived — was Tyrannosaurus rex, whose fossils date to the final 2 to 3 million years of the Cretaceous. T. rex combined a massive, bone-crushing bite force estimated at 35,000 to 57,000 newtons (far exceeding any other terrestrial predator) with excellent binocular vision, sophisticated olfactory processing, and a body plan optimized for powerful, sustained ambush predation. Its arms were dramatically reduced but not vestigial — the muscles attached to them were among the most powerful for their size in any animal.

Sharing Laramidia with T. rex was an extraordinary diversity of prey. Hadrosaurs — the duck-billed dinosaurs — were the most abundant large herbivores of the Late Cretaceous worldwide. Species like Edmontosaurus and Parasaurolophus formed vast herds that may have migrated seasonally across the landscape. Parasaurolophus had an elaborate hollow cranial crest that functioned as a resonating chamber, producing low-frequency calls for communication within the herd. Hadrosaur teeth were among the most sophisticated dental structures ever evolved, forming dental batteries of hundreds of closely packed teeth that were continuously replaced throughout life.

Ceratopsians — horned dinosaurs — reached their greatest diversity in the Late Cretaceous. Triceratops horridus, with its three facial horns and enormous frill, is the most familiar, but dozens of other species populated Laramidia and Asia. Styracosaurus had an elaborate array of frill spikes; Pachyrhinosaurus had a flattened boss instead of nasal horn; Kosmoceratops had 15 distinct horn and frill ornaments. The evolutionary radiation of frill and horn morphologies strongly suggests these structures played roles in species recognition and sexual selection, analogous to the antlers of modern deer.

Ankylosaurians were the armored tanks of the Late Cretaceous. Ankylosaurus magniventris had a body covered in bony scutes and osteoderms, and a massive tail club formed from fused vertebrae and bony knobs that could deliver bone-breaking blows. Nodosaurids, a related group, lacked the tail club but bore elaborate shoulder spikes.

In Asia, the contemporaneous fauna was distinct but shared some lineages, including tyrannosaurids, oviraptorosaurs, and dromaeosaurids. Gigantic oviraptorosaurs like Gigantoraptor, with its toothless beak and possible feather display structures, thrived in Mongolia. The ankylosaur Tarbosaurus was the Asian ecological equivalent of T. rex.

Gondwanan landmasses — South America, Africa, India, Madagascar — hosted their own unique Late Cretaceous dinosaurs. Abelisaurids like Carnotaurus, with their tiny arms and horn-like cranial ornaments, were the apex predators of the southern landmasses. Giant titanosaur sauropods, including Patagotitan and Dreadnoughtus, continued to reach extraordinary sizes. The Late Cretaceous was not one dinosaur world but many, each isolated continent evolving its own spectacular cast of species.