One century ago, when paleontologists were still just becoming acquainted with the great dinosaurs of the American West, the skilled paleo-illustrator Charles R. Knight created a curious vision of the long-necked dinosaur Diplodocus. The consensus at the time was that the giant dinosaurs were amphibious—spending much of their time wallowing in swamps and straining soft water plants through their peg-like teeth—but in a scene that also contained this typical image, Knight presented one Diplodocus rearing back onto its tail. This seemed like a very active pose for the sauropod, one that would not become popular until decades later when dinosaurs got a major overhaul in the 1970s and 80s. What compelled Knight to give the Diplodocus a more dynamic position?
The answer can be found in an 1899 paper on Diplodocus by the American Museum of Natural History’s Henry Fairfield Osborn. In studying the dinosaur, Osborn was especially struck by the length of the animal’s tail. Clearly the tapering tail of Diplodocus must have been “of immense service as a propeller in enabling it to swim rapidly through the water,” and the naturalist even speculated that the dinosaur may have been equipped with a “vertical fin” near the tail tip to help move it along. But that wasn’t all. On land, the tail would have served a different purpose:
The tail, secondly, functioned as a lever to balance the weight of the dorsals, anterior limbs, neck and head, and to raise the entire forward portion of the body upwards. This power was certainly exerted while the animal was in the water, and possibly also while upon land. Thus the quadrupedal Dinosaurs occasionally assumed the position characteristic of the bipedal Dinosaurs—namely, a tripodal position, the body supported upon the hind feet and the tail.
Osborn based this supposition on what he thought was a change in tailbone anatomy about halfway down the organ’s length. To him, the posterior half of the tail looked well-suited to supporting the weight of Diplodocus when it reared up on its hind legs. That Diplodocus was capable of such activities was made clear by the relatively lightness of its skeleton compared to the more hefty “Brontosaurus.” “There is a traditional view that these animals were ponderous and sluggish,” Osborn wrote. “ In the case of Diplodocus is certainly unsupported by facts.” If the dinosaur had a relatively light skeleton and looked as if it should have been agile, then why shouldn’t it have been? This sentiment was clearly passed along to Knight, who created many dinosaur paintings for the AMNH and other museums, though Osborn’s idea that some sauropods were graceful was lost in the slew of museum displays and illustrations that showed them as big, slow reptiles. Sauropods remained relegated to the swamp, though it is too bad that Knight never illustrated Osborn’s idea that Diplodocus propelled itself about the Jurassic lakes with a tail fin!