The postcranial skeleton of the ornithopod dinosaur Tenontosaurus tilletti

ABSTRACT Tenontosaurus tilletti is a moderate-sized ornithopod dinosaur from the Lower Cretaceous (Aptian–Albian) Cloverly Formation in Montana and Wyoming. Tenontosaurus remains are abundant. The vertebral count is 12-16-5-60+, the extremely long tail comprising approximately twothirds the length of the animal. Ossified epaxial tendons are arranged in bundles to each side of the neural spines of the dorsal, sacral, and caudal vertebrae. Ossified hypaxial tendons run longitudinally across the caudal centra and chevrons. The forelimb is relatively long, the humerus is dominated by a strong deltopectoral crest, and the manus is short and broad with a phalangeal formula of 2-3-3-1?-1?. The tibia and femur are subequal in length and moderately robust. The pes has a phalangeal formula of 2-3-4-5-0 with a vestigial metatarsal V. Tenontosaurus is the primitive sister taxon to the iguanodonthadrosaur clade within the Ornithopoda.