STEGOSAUR MARTIAL ARTS: MASSIVE ENLARGEMENT OF HUMERUS PROTRACTORS ("BACKING UP" MUSCLES) SHOULD HAVE AUGMENTED FIGHTING WITH TAIL SPIKES

Most big ornithischian dinosaurs carried defensive weapons. Horned dinosaurs usually had a tall nasal horn and-or tall horns over the orbits, plus a massive beak. Thus the animal could guide blows and bites visually. Stegosaurids carried spikes on the distal tail, far away from the head and therefore harder to aim. The animated 1940 “Fantasia” shows a stegosaurid backing and turning, flexing the torso horizontally, so the tail spikes were thrust more directly forward. Analysis of forelimb muscles persuade us that the basic torso flexure of “Fantasia” is mechanically possible. Unlike most ornithischians, stegosaurids lacked the lattice work of bone rods stiffening the torso; there was little development of a stiff presacral zone of ankylosed vertebrae; the number of mobile torso vertebrae was 18, versus 14 to 12 in horned dinos and ankylosaurids, and the stegosaurid abdomen was far narrower side-to-side than the bulging gut of horned dinosaurs and ankylosaurids. Thus the stegosaurid torso should have been far more flexible.

If stegosaurids backed up and flexed torsos with unusual power, we would expect enlargement of the muscles that pulled the humerus forward and outward. In fact, the origin of the humerus protractors and abductors - the deltoids, the coracobrachialis and supracoracoideus - were expanded into a huge scapular shelf. The insertions also were enlarged into an unusual upper delto-pectoral crest lacking in horned dinosaurs and ankylosaurids. (Cretaceous nodosaurids were armed with a caudal flail and converged on stegosaurids in forelimb muscles but retained a stiff torso). The stegosaurid muscles could have caused a “bulldozer turn”, limbs on one side pulling the body aft, while the limbs on other side pulled it forward. Tail spike accuracy should have benefited from augmented body manuverbility.