The Dread Olecranon of Kentrosaurus

(This was buried in Part 5 of my 2011 review of the Sideshow Apatosaurus maquette, but it’s long deserved to be a post of its own, and now it is. I’m not adding anything new here, just extracting and reposting the relevant bits, for reasons that will become clear in a future post. — MJW)

The Dread Olecranon of Kentrosaurus is something Heinrich Mallison pointed out in the second of his excellent Plateosaurus papers (Mallison 2010: fig. 3).

Heinrich’s thoughts on articular cartilage in dinosaurs are well worth reading, so once again I’m going to quote extensively (Mallison 2010: p. 439):

Cartilaginous tissues are rarely preserved on fossils, so the thickness of cartilage caps in dinosaurs is unclear. Often, it is claimed that even large dinosaurs had only thin layers of articular cartilage, as seen in extant large mammals, because layers proportional to extant birds would have been too thick to be effectively supplied with nutrients from the synovial fluid. This argument is fallacious, because it assumes that a thick cartilage cap on a dinosaur long bone would have the same internal composition as the thin cap on a mammalian long bone. Mammals have a thin layer of hyaline cartilage only, but in birds the structure is more complex, with the hyaline cartilage underlain by thicker fibrous cartilage pervaded by numerous blood vessels (Graf et al. 1993: 114, fig. 2), so that nutrient transport is effected through blood vessels, not diffusion. This tissue can be scaled up to a thickness of several centimeters without problems.

An impressive example for the size of cartilaginous structures in dinosaurs is the olecranon process in the stegosaur Kentrosaurus aethiopicus Hennig, 1915. In the original description a left ulna (MB.R.4800.33, field number St 461) is figured (Hennig 1915: fig. 5) that shows a large proximal process. However, other ulnae of the same species lack this process, and are thus far less distinct from other dinosaurian ulnae (Fig. 3B, C). The process on MB.R.4800.33 and other parts of its surface have a surface texture that can also be found on other bones of the same individual, and may indicate some form of hyperostosis or another condition that leads to ossification of cartilaginous tissues. Fig. 3B–D compares MB.R.4800.33 and two other ulnae of K. aethiopicus from the IFGT skeletal mount. It is immediately obvious that the normally not fossilized cartilaginous process has a significant influence on the ability to hyperextend the elbow, because it forms a stop to extension. Similarly large cartilaginous structures may have been present on a plethora of bones in any number of dinosaur taxa, so that range of motion analyses like the one presented here are at best cautious approximations.

One of the crucial points to take away from all of this is that thick cartilage caps did not only expand or only limit the ranges of motions of different joints. The mistake is to think that soft tissues always do one or the other. The big olecranon in Kentrosaurus probably limited the ROM of the elbow, by banging into the humerus in extension. In contrast, thick articular cartilage at the wrists [of sauropods] probably expanded the ROM and may have allowed the strong wrist flexion that some artists have restored for sauropods. I’m not arguing that it must have done so, just that I don’t think we can rule out the possibility that it may have.

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To see that chunk in context and read more about cartilage in dinosaurs, see the original post — here’s that link again. Throwing in the references to Bonnan et al. (2010) and Holliday et al. (2010) because they’re still relevant, foundational studies.

References

• Bonnan, M.F., Sandrik, J.L., Nishiwaki, T., Wilhite, D.R., Elsey, R.M., and Vittore, C. 2010. Calcified cartilage shape in archosaur long bones reflects overlying joint shape in stress-bearing elements: Implications for nonavian dinosaur locomotion. The Anatomical Record 293: 2044-2055.
• Holliday, C.M., R.C. Ridgely, J.C. Sedlmayr and L.M. Witmer. 2010. Cartilaginous epiphyses in extant archosaurs and their implications for reconstructing limb function in dinosaurs.  PLoS ONE 5(9): e13120. doi:10.1371/journal.pone.0013120
• Mallison, H. 2010. The digital Plateosaurus II: An assessment of the range of motion of the limbs and vertebral column and of previous reconstructions using a digital skeletal mount. Acta Palaeontologica Polonica 55 (3): 433–458.