Diplodocids carried the classic sauropod silhouette to its most extreme proportions: a long, horizontally held neck, a body supported on four pillar-like legs, and a tail that thinned into a long, whip-like extension possibly capable of generating a cracking sound if snapped rapidly. Their teeth were slender, peg-like, and restricted to the front of the jaw -- efficient for stripping foliage from branches rather than chewing, since sauropods generally didn't chew their food at all.
The family's fossils come overwhelmingly from the Late Jurassic Morrison Formation of western North America. Diplodocus itself is one of the longest dinosaurs known by body length, though not the heaviest; Apatosaurus was stockier and more heavily built, while Barosaurus carried an even more elongated neck. Together, diplodocids were among the most common large herbivores in their ecosystem, often found alongside stegosaurs and early tyrannosauroid relatives.