Cartilaginous Epiphyses in Extant Archosaurs and Their Implications for Reconstructing Limb Function in Dinosaurs
Holliday CM, Ridgely RC, Seldmayr JC, Witmer LM. PLoS ONE 5(9): 15p.
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Dinosaur Articular Cartilage: Hyaline cartilage is a soft tissue vital to the construction and healthy function of vertebrate limb joints. Not only is cartilage responsible for mediating bone growth during an animal’s early years, but it also provides the thin layer of lubricating articular cartilage on the joint’s surface. Even in elephants, the largest terrestrial mammals living today, this layer of articular cartilage is only about 2 mm thick, and being largely without blood vessels, it relies on diffusion to feed the cartilage cells during movement. However the surfaces of many dinosaur bones lack bony condyles necessary for making a knee, for example, the surfaces are often porous and wrinkled, indicating the presence of blood vessels, These soft tissues necessary for joint function are stripped away during fossilization. Thus paleontologists are challenged to accurately infer posture, height, and locomotor behavior in extinct taxa like dinosaurs. Our team, originally from Ohio University, now dispersed to different schools sought to get at this problem by asking:
How much cartilage did dinosaurs have in their joints? How well can we estimate it? What significance might articular cartilages have on our interpretations of dinosaur biology?
Using data from the living relatives of dinosaurs, crocodilians and birds, this study concludes that dinosaurs built their limbs differently than mammals, relying more on cartilage than bone to not only lubricate their joints but also to actually form the articulating condyles and processes that unite and stabilize the knees, elbows, and other limb joints. The bony ends of long bones of dinosaurs indicate that blood vessels supplied these cartilages throughout the animal’s growth, thus enabling these cartilage caps to remain far larger than those found in mammals.
We developed a metric, the cartilage correction factor (CCF) to estimate the amount of cartilage on the ends of long bones. For example applying an Alligator CCF (10%) to Brachiosaurus altithorax, which has a 6ft long femur and about a 4ft long tibia, would add about 1ft of extra cartilage onto the total length of the elements. Adding 1ft extra to the height of a fossil taxon, like the 42ft tall Brachiosaurus, may seem trivial; however distributed evenly on all four joint surfaces results in cartilage caps that are 3 inches thick. That’s really thick cartilage!
We hope this new metric, the CCF, aids paleontologists with their estimates of height by decreasing the amount of error involved in these reconstructions. However, even though we’re losing length and breadth of these elements upon the loss of cartilage, condyles and other articular structures, the most critical features that account for the fit, or congruency of the joints, and therefore posture are lost. Because posture is difficult to estimate, ranges of motion and eventually estimates of speed and locomotor behavior suffer because of the error associated with articulating, or misarticulating these elements. That said, this is not meant to dissuade paleontologists from studying posture and locomotion, but merely to give them more an anatomical leg to stand on.
Funding for this project was provided by National Science Foundation (NSF) DDIG 0407735 (to L.M.W and C.M.H.) and NSF IBN-9601174, IBN-0343744, IOB-0517257 (to L.M.W.) as well as Ohio University Departments of Biological and Biomedical Sciences, Ohio University College of Osteopathic Medicine, and University of Missouri Department of Pathology and Anatomical Sciences.
This project has had a long history, and thanks to the encouragement and patience of numerous people, we are happy to finally see it out.
Please visit the WitmerLab’s Pick & Scalpel Blog for additional information.
This paper was a candidate for the Paleo Paper Challenge 2009 at The Open Source Paleontologist. We almost made it.