Triceratops was a genus of herbivorous dinosaurs that first appeared during the Late Cretaceous Period about 68 million years ago in what is now North America. It one of the last known non-avian dinosaur genera and became extinct during the Cretaceous-Paleogene extinction event 66 million years ago. The frills and three facial horns have been viewed in the past as defensive weapons against predators. More recent theories, noting the presence of blood vessel in the skull bone of the ceratopsians find it more probable that these features were primarily used for identification, courtship and dominance displays much like the antlers and horns of modern reindeer, mountain goats or rhinoceros. Mature Triceratops are estimated to attain lengths of 26 to 29 feet (8 to 9 m), 9.5 to 10 feet (2.9 to 3 m) in height and weighed from 13,000 to 26,000 pounds (6 to 12 tonnes). The skull was among the largest of all land animals. Triceratops species possessed a sturdy build with strong limbs, short hands with three hooves each and short feet with four hooves each. Data from trackways show that Triceratops and other ceratopsians maintained an upright stance during normal locomotion with the elbows flexed and slightly bowed out in an intermediate state between fully upright and fully sprawling, as in the case of the modern rhinoceros.