Thescelosaurus

Thescelosaurus was a robustly built, medium-sized ornithopod dinosaur that inhabited the floodplains and forests of western North America during the latest Cretaceous Period, right up until the end-Cretaceous mass extinction. Measuring approximately 3.5 meters in length and weighing around 80 kilograms, it possessed a stocky body, relatively short but powerful hindlimbs, and a long, stiffened tail that provided balance during locomotion.

The skull featured a small, pointed beak at the front of the jaws, ideal for cropping vegetation, with rows of leaf-shaped teeth behind for processing plant material. Unusually for an ornithopod, Thescelosaurus had bony armor-like ossicles embedded in the skin along its back and flanks, suggesting some degree of protection against predators like Tyrannosaurus rex.

First described by Charles Gilmore in 1913 from a specimen found in Lance Formation deposits in Wyoming, Thescelosaurus was initially overlooked and its scientific name means 'wondrous lizard that was neglected.' The species gained unexpected media attention in 2000 when a specimen called 'Willo' was claimed to contain a preserved four-chambered heart, though subsequent CT studies have reinterpreted this structure as a concretion. Multiple well-preserved specimens make Thescelosaurus one of the best-known small ornithischians from the end of the Cretaceous, providing valuable insight into dinosaur diversity just before the asteroid impact.