Osteoderm distribution does not significantly influence the center of mass of stegosaurs

Today, a new paper came out in Fossil Record, a journal that as of Jan 1, 2014 is open access (so go read the paper! It’s free). The paper tests hypotheses about the position of the centre of mass (COM) of stegosaurs and how variations in the distributions of osteoderms (plates and spikes) influences it, and whether rearing behaviour gave stegosaurs especially sturdy forelimbs.

It is a pretty boring paper, overall, and the method used – well, let’s say it conforms to the German proverb of shooting at sparrows with cannons (i.e., using a far too big gun for the purpose; similar to the English phrase to crack a nut with a sledgehammer). Let me explains what the paper deals with, and why a much simpler approach would have been sufficient – and why it was not adopted.

The paper checks two hypotheses previously published by Maidment et al. (2012). In a very interesting and thorough study they found that “the largest stegosaurs have relatively slender humeri in comparison with the smallest members of the clade”, and proceed as follows (Hypothesis 1): “This could be due to changes in the center of mass related to distribution of dermal armor; for example, Kentrosaurus, the smallest stegosaur in the sample, is known to have possessed parascapular spines [Galton (1982)]. Despite the large number of individuals of Stegosaurus known, no parascapular spine belonging to the genus has ever been discovered [Galton and Upchurch (2004)]. The additional mass of these large dermal spines in the shoulder region might have caused the center of mass to be located further anteriorly in Kentrosaurus than in Stegosaurus”.

In short, Hypothesis 1 says that the forelimbs of Stegosaurus were proportionally less strong than those of Kentrosaurus because they did not have to carry as much weight, because Stegosaurus did not have spikes on the shoulders. In fact, the hypothesis is supposed to be a general rule, and Maidment et al. (2012) picked a specific example: S. and K. and shoulder spikes.

Hypothesis 2 deals with the fact that all stegosaurs have proportionally very robust humeri compared to other ornithischian dinosaurs. Maidment et al. (2012) suggest that “[…] the robustness of stegosaur humeri could be related to a specific behaviour. For example, it has been suggested that stegosaurs utilized a tripodal stance […]; perhaps increased stress on the humerus was generated during rearing as a result of pushing off from the ground.”

Both ideas make sense at first glance: more weight means higher forces, and thus more sturdy bones. And higher forces from larger exertion while attaining an unusual pose would also mean more robust bones. So how to test this?

If Hypothesis 1 is correct, we can predict that the additional amount of weight born by the forelimbs is large in relation to the weight that would be born if there was no shoulder spike in Kentrosaurus (i.e., we can check K. with and without shoulder spike), and we can do the same for Stegosaurus. If the spike only adds a few percent to the load, the hypothesis is not supported. So let’s do that:

Figure 1 from the paper shows (top) a 3D model of Kentrosaurus, with the osteoderms distributed the way Janensch and Hennig reconstructed them (Hennig 1925; Janensch 1925) (scale bar 1 m). Also, there are four versions with alternate distributions. Each distribution has its own colour, and the big model has the centre of mass position shown in the corresponding colour for each version. Between red and green the sole difference is that the spike that Janensch put on the hip is moved to the shoulder. Thus, we can see how much of a difference the presence or absence of a should spike really makes – in fact, in this case the effect is exaggerated, because the spike is not “there” or “not there”, but “there” or “elsewhere”, i.e. it moves the COM backwards in the test case where it is not on the shoulder instead of having no influence. And as the enlarged inset shows (scale bar 10 cm) the influence is minimal! Moving the spike moves the COM by a paltry 10 mm, says Table 1 of the paper! That adds a whopping 1% of body weight to the load the forelimbs have to carry, up from 16% to 17%. That is a 6.25% increase.

In the lower half of the figure you can see a Stegosaurus 3D model, in this case with the hypothetical shoulder spike in blue. Adding it moves the COM from the red dot to the blue dot – again only 1% of body weight and a COM motion of this time only 6 mm.

Ergo, Hypothesis 1 is not supported. However, Maidment et al. (2012) gave the shoulder spike as an example only. So what if we move a larger number of osteoderms around? Will that have a big influence on the COM position?

Yes, we can make the COM move quite a bit. However, that requires putting all osteoderms that we know Kentrosaurus had to the neck (light blue), whereas the influence of moving the trunk osteoderms only to the neck, or of moving all tail osteoderms to the tail tip, doesn’t really have a big influence. We can thus conclude that we need to do absurd things to put a lot more (or less) weight on the limbs. Moving a single pair of spikes is not the cause of the more robust forelimbs in Kentrosaurus compared to Stegosaurus.

So what about hypothesis 2? What do the models tell us about it? Both models show a highly similar overall body shape, and accordingly highly similar COM positions. We can now measure the moment arm of the COM versus the hind feet, as well as that of the forefeet. The ratio between the two is nearly 5:1, i.e. a five-fold advantage for a push-up motion. And much of the torque needed to rotate the body up into a tripodal pose as in the picture below would be produced by the hindlimb and tail muscles anyway. The COM position is, in fact, similar to many other big ornithischians, some of which supposedly also fed in a more upright stance. Thus, the claim that rearing forces such sturdy forelimbs onto stegosaurs is also not supported. In sum, this is a seven page paper that simply says “nope, that’s not correct”, but can’t really offer anything new.

Rearing Kentrosaurus digital skeletal mount, from Mallison (2010).

So, why did I say all this work was cracking a nut with a sledgehammer? Because the easy way would have been to simply calculate the volume and from the mass of the shoulder spike. All the osteoderms together make up only ~ 3.5 % of the total body weight of Kentrosaurus! The spike alone, and even a large number of spikes and plates combined, can’t have much effect. With regards to hypothesis 2 it is sufficient to check the published literature, e.g. Henderson (1999), to see what the COM position of stegosaurs is, and that it is not unusual compared to other, slender-forelimbed ornithischians. Therefore, all the fancy 3D modelling is not needed.

Why on earth then did I bother you with it in this post? And why is it in the freakin’ paper? Three reasons, really:

• The 3D models visualize the osteoderm placement variations and their effect on COM position very nicely, making it very easy to grasp what’s going on.
• The method that’s used to calculate the COM positions is very easy and fast, and it is always worth publishing the little tricks. In this case the time saves versus other methods was lost building the models, but you can do more complicated stuff with the method, and save quite a lot of time – if you know it.
• I wrote the paper, and I like my 3D models.

Yeah, the last bit I guess does not come as a big surprise now 😉

I’ll have more to say on this paper, especially on its review history (lessons to be learned by all, and in this case [as normally] I must really laud the reviewers and editors!), but that will have to wait a few days.

References

Galton, P.M. 1982. The postcranial anatomy of stegosaurian dinosaur Kentrosaurus from the Upper Jurassic of Tanzania, East Africa. Geologica et Palaeontologica, 15:139–165.
Galton, P.M. and Upchurch, P. 2004. Stegosauria, p. 343–362. In Weishampel, D.B., Dodson, P., and Osmolska, H. (eds.), The Dinosauria, 2nd Edition. University of California Press, Berkeley.
Hennig, E. 1925. Kentrurosaurus aethiopicus Die Stegosaurier-Funde vom Tendaguru, Deutsch-Ostafrika. Palaeontographica, : 2 Supplement 7:101–254.
Janensch, W. 1925. Ein aufgestelltes Skelett des Stegosauriers Kentrurosaurus aethiopicus HENNIG 1915 aus den Tendaguru-Schichten Deutsch-Ostafrikas. Palaeontographica VII 2 Supplement 7:255–276.
Maidment, S.C.R., Linton, D.H., Upchurch, P., and Barrett, P.M. 2012. Limb-bone scaling indicates diverse stance and gait in quadrupedal ornithischian dinosaurs. PLOS ONE, 7(5):e36904. doi:10.1371/journal.pone.0036904.
Mallison, H. 2010. CAD assessment of the posture and range of motion of Kentrosaurus aethiopicus Hennig 1915. Swiss Journal of Geosciences, 103:211–233. doi:10.1007/s00015-010-0024-2.

Advertisement