Triceratops

South Dakota designated Triceratops horridus as its state fossil in 1988, replacing a fossil cycad plant that had held the designation earlier. Triceratops was one of the largest horned dinosaurs that ever lived and one of the most common large dinosaurs in the Hell Creek Formation, which outcrops across the northwestern corner of South Dakota. It was also one of the last non-avian dinosaurs, surviving right up to the mass extinction 66 million years ago.

Triceratops had three horns, including two long brow horns above the eyes and a shorter horn on the nose, plus a broad bony frill extending from the back of the skull. Adults reached up to 29 feet in length and weighed as much as 12 tons. The brow horns on large adults could reach over 3 feet in length. Despite its intimidating appearance, Triceratops was a plant-eater that used its parrot-like beak to clip through tough vegetation.

Scientists still debate what the frill and horns were mainly used for. Candidates include defense against predators like Tyrannosaurus rex, visual display for attracting mates, and recognizing members of the same species. Triceratops lived about 68 to 66 million years ago and vanished in the Cretaceous-Paleogene extinction event that ended the age of non-avian dinosaurs.

South Dakota previously recognized a fossil cycad plant as its state fossil. In 1988, the legislature replaced it with Triceratops horridus, a change that reflected the state's standing as one of the top sources of Late Cretaceous dinosaur fossils in North America. By 1988 the Hell Creek Formation in northwestern South Dakota had already produced numerous Triceratops specimens, establishing the dinosaur as a symbol of the state's paleontological wealth.

Triceratops is one of the most recognized dinosaurs in the world, which made it a natural fit for a state symbol accessible to students and the public. South Dakota's designation came the same year Colorado named Stegosaurus and Utah named Allosaurus, part of a wave of state fossil designations in the late 1980s.

Triceratops fossils in South Dakota come from the Hell Creek Formation, a layer of sediment deposited during the final stages of the Cretaceous period. The formation outcrops across the northwestern corner of the state in Harding, Perkins, and Corson counties, where rivers and wind erosion continuously cut through bone-bearing rock. The area around Buffalo and Reva in Harding County is particularly productive for Cretaceous dinosaur material.

The Museum of Geology at the South Dakota School of Mines and Technology in Rapid City holds one of the strongest Cretaceous fossil collections in the region, including specimens from the Hell Creek beds. The same formation that produces Triceratops in South Dakota also yields Tyrannosaurus rex, Edmontosaurus, and Ankylosaurus.