JURASSIC TAIL FIGHTERS: BRONTOSAURS AND STEGOSAURS SHOW ADAPTATIONS FOR PRECISION BLOWS

Late Jurassic dinosaurs are famous for caudal weapons -- the fifteen-meter long tail whips of diplodocid sauropods and the quadra-spiked tail of Stegosaurus. Aiming precise tail blows at a maneuvering target requires turning of the body so the eyes can follow the action. Stegosaurs and diplodocids share features for combat choreography. Torsos lack bone tendons and fused vertebrae, so turning is not inhibited. The scapula carries an enormously expanded origin for the deltoid and supra-coracoid muscles, conferring great power for backing up and turning. The clades also share a vertebral geometry, noted by D’Arcy Thompson over a century ago: The neural spines going front to back rise dramatically to reach a maximum over hips and tail base, a cantilevered apparatus that could elevate the forequarters and tail. Sauropod tracks, though common, rarely preserve details that distinguish whip tails from short-tailed species, e.g brachiosaurs. We found skeletons of an early brontosaur in the lower Morrison of Wyoming, in strata that also yield exceptional footprints. Bones and footprints have the squared-off claw tips of brontosaurs instead of the usual rounded apices of other sauropods. The fore track is smaller and much shallower than the hind, matching the disparity in foot bones. The forefoot often did not make any imprint in trackways with deep hind prints, suggesting times when brontosaurs could raise the forequarters to some degree and turn the body to deliver precise tail-whip slaps.

An upper Morrison site in Colorado preserves a fore and aft track couplet matching adult stegosaur feet from Quarry13 in Wyoming. The center hind toe is square-tipped; lateral toes are thinner and claws are more triangular. The fore track appears four-toed but toe V may be coalesced with V. Surprisingly, the fore track is wider and as deep as the hind. Stegosaur wrist bones are blocky and fit snugly together, so the the forepaw could withstand higher stresses in abrupt turns. Furthermore, the stegosaur humerus has a spectacularly more enlarged insertion of the supra-coracoid muscle, indicating even greater turning power. We suggest that stegosaurs “fired” close range precision stabs. Indeed an allosaur pelvis from Wyoming carries a deeply penetrating spike wound.