September 16, 2010
Since the early days of paleontology, the posture of dinosaurs and the range of motion they were capable of have been contentious subjects for paleontologists. During the 19th century, especially, the general view of what dinosaurs would have looked like changed no less than three times, and investigations into how these animals moved continue to this day. Among the spate of recent studies on dinosaur flexibility, posture and motion is a new paper by Heinrich Mallison which used the Jurassic stegosaur Kentrosaurus to investigate some of the hypotheses surrounding this armored dinosaur.
Most of what we know about Kentrosaurus comes from the approximately 153-million-year-old Tendaguru Formation in Tanzania. It was there that the German paleontologist Edwin Hennig found numerous isolated bones and elements of disarticulated Kentrosaurus skeletons—in addition to the bones of many other dinosaurs—during the early 20th century; he was also lucky enough to find one partial skeleton of the stegosaur that was suitable for mounting. This specimen, reconstructed with sprawling limbs and a dragging tail, was on display at the Museum für Naturkunde in Berlin for decades. When it was taken apart to restore it in a more accurate posture in 2005, scientists made laser scans of each bone in order to create a digital restoration. It is this digital Kentrosaurus that formed the basis of Mallison’s new study—the closest thing a paleontologist has to a living dinosaur to examine.
In addition to its normal posture and range of motion, Mallison’s study looks at several controversial, little-studied ideas about this dinosaur and its kin. According to Hennig, Kentrosaurus had a squished, lizard-like posture and could not use its spiky tail for defense. In the 1980s, however, paleontologist Robert Bakker went to the opposite extreme, restoring stegosaurs with an erect posture that would have allowed them to pivot and swing their formidable tails at attacking predators. Additionally, Bakker proposed that Stegosaurus and its kin could have adopted a “tripodal” posture in which they reared back to rest on their tails, too, and were much more dynamic animals than envisioned by Hennig and other early 20th-century paleontologists.
Although Mallison stresses that the findings based upon his model are provisional, Kentrosaurus appears to have used different postures for different reasons. While walking, it would have held its limbs erect, but when threatened it was capable of flexing its forelimbs out into a sprawling position to help support itself as it swung its tail at an offending predator. In the latter circumstance, Kentrosaurus would have also been able to extend its neck to look backwards at an attacking dinosaur, though shifting position to keep a predator in view may have created blind spots that would have left this armored dinosaur vulnerable to multiple predators. As far as feeding was concerned, Kentrosaurus was indeed capable of rearing back to rest on its tail, though how often it would have done so—and what sort of food it would have been able to reach by doing so—is unknown. Overall, Kentrosaurus was not as stiff as Hennig proposed. Quite the contrary—this stegosaur was capable of altering its posture to suit a variety of circumstances, and it is likely that at least some of its relatives had similar abilities.
Mallison, H. (2010). CAD assessment of the posture and range of motion of Kentrosaurus aethiopicus Hennig 1915 Swiss Journal of Geosciences DOI: 10.1007/s00015-010-0024-2
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