Both sauropods appear closely matched due to their physical measurements, behavior and adaptations being quite similar.  Sauropods had claws, but their primary weapons were likely their large tails. In terms of power, the slightly bulkier build of the Brontosaurus might provide more stopping power, but Diplodocus’ longer tails and leaner frame give it the advantage of reach and speed. If a Diplodocus could keep out of its enemy’s tail while hitting them with its own, it might win. However, due to their lumbering size and slow nature, a Diplodocus might not be able to outmaneuver a Brontosaurus, and the larger dinosaur could pummel its opponent to submission eventually.