Walk softly – and carry an armoured tail

Today, you’ll have to suffer through another post on an insufferably boring paper by yours truly. But I promise the artwork is nice, even awesome! That’s no surprise, as it was created by none other than David Maas of brainpets. Check below the fold for some really cool stegosaur action shots!

But  before you rush off for the visual gratification, suffer through a short summary of the paper first.

The Museum für Naturkunde in Berlin features in its Dinosaur Hall several wonderful mounts of Tendaguru dinosaurs. Most famous it the giant sauropod Giraffatitan (previously thought to belong to Brachiosaurus), but there are also mounts if Dicraeosaurus, Elaphrosaurus, Dysalotosaurus - and, almost hidden away under the tail of a cast of Diplodocus, the spiked stegosaur Kentrosaurus aethiopicus Hennig, 1915.

The mounted skeleton of Kentrosaurus aethiopicus Hennig, 1915 in the Museum für Naturkunde Berlin. Image from http://en.wikipedia.org/wiki/File:Fossil_Kentrosaurus_aethiopicus_in_Museum_f%C3%BCr_Naturkunde_Berlin_001.JPG

Weird, weirder, stegosaur

Kentrosaurus is a close relative of the much better known North American genus Stegosaurus. Stegosaurs are odd animals: their thighs are longer than the rest of the leg, the head is tiny, the forelimbs are extremely robust, and all over they are covered by bony spikes and/or plates. There has been much debate on what those were for, with the plates interpreted as a sort of heating mechanism, cooling mechanism, protective armour or display.I don’t want to repeat all the arguments for and against all these hypothesis here, suffice it to say that the spikes at the tail tip have seen slightly less controversy. Quite clearly they could not serve as heating or cooling mechanisms, so the main fight was over the flexibility of the tail: was it sufficient to whack the spikes into an attacker’s face, or was the tail too stiff? While researchers still quibbled, the great artist Gary Larson in one of his famous Far Side cartoons, “solved” the case by having a caveman give a lecture to his peer, in which he explains that the spike end of a stegosaur tail is called a “thagomizer“, after the late Thag Simmons.

The question of whether the tail was used for whacking was practically put to rest when Ken Carpenter and colleagues published a wonderful fossil find: a tail vertebra of Allosaurus that had been smashed, and started to heal – around what had obviously been a stuck piece of a stegosaurian tail spike (Carpenter et al. 2005)! Ken and his colleagues also made some rather rough calculations about how hard a Stegosaurus could whack things, and found out that the answer was “pretty darn hard”! And as far as mobility of the tail is concerned, that was also figured out, but quite recently, and by none other than me (Mallison 2010).

Some questions left to answer

This left just a few questions unanswered. First of all, Kentrosaurus is a lot smaller than Stegosaurus, and looks a bit different. How much damage could its tail cause, and how did different shape and the probably slightly different arrangement of the tail spikes influence the batting? And, I’m sorry to say so, but I had to ask how the errors in Carpenter et al. (2005) influenced the results. So, having those wonderful high-resolution scans of the Berlin material on hand that Dave Mackie (then with RCI) made, and having computer programs that allow modelling tail strikes in pretty good detail, I did CAE modelling of tail strikes Kentrosaurus aethiopicus.

I guess at least 50% of you now say “Hu? CA-what?” So I’ll explain.
CAE is short for Computer Aided Engineering – using computer programs to do typical engineering tasks such as prototyping of machines, cars, etc., comparing physical prototypes to the digital ones and series production to prototypes (this is then termed COQ, Computer Aided Quality Assurance), and so on. Often, CAE refers to FEA (Finite Element Analysis), but there are other things to do with it – for example the well known CAD (Computer Aided Design).

What I did was using a CAE program to compute how Kentrosaurus wagged its tail: how fast, and with how much of a punch.

The business end of Kentrosaurus aethiopicus, better known as "The batter from Hell". From Figure 3 of Mallison (2011)

The baseball batter from hell? Or just a feeble failure?

Things weren’t as easy as I had initially hoped. First of all, the more I researched the topic, the more did I become convinced that the tails of dinosaurs are usually reconstructed much too thin. And that means that there was way more force available to swing the tail that I had at first assumed – but also way more weight to move! Others have come to similar conclusions before, most notably my friend Vivian Allen (Allen et al. 2009) and Scott Persons (Persons 2009; Persons and Currie 2011). And another ugly problem became apparent: how much force can a muscle really produce? Carpenter et al. (2005) had sued two values to bracket a range, but more recent research suggested that this range was much too high. In the end I had to do a lot more work than initially planned, but it was great fun. And the results?

If we pick one of the likely scenarios it tells us that it would be a very bad idea to get hit by an all-out tail strike. A human would likely not survive such a hit, and even a large theropod might get such a mighty whack across the face, or a spike deeply pushed into the lungs, that survival seems unlikely. When David Maas and I planned the artwork for the press release (and this post), David therefore suggested a scene in which the predator has been noticed by the intended prey, and wisely chooses to walk away, rather than risk a fight.

Game over! The subadult allosaur has been spotted, and walks away rather than trisking a fight. The Kentrosaurus has lifted its head high up so that it can look over its back. This means that there is no dead zone, and a predator can't easily sneak up on it.

The danger zone, in which the tail tip was fast enough to cause serious damage can be easily visualized by drawing a coloured circle on the ground, as David did in the next image.

The juvenile allosaur nearly comitted a deadlymistake. A small step to the left saved his life. In a moment the tail spikes of the kentrosaur will miss him by a whisker. Getting hit at over 40 mph would have been fatal.

REFERNCES:
Allen, V., Paxton, H., and Hutchinson, J.R. 2009. Variation in center of mass estimates for extant sauropsids and its importance for reconstructing inertial properties of extinct archosaurs. Anatomical Record, 292:1442-1461.

Carpenter, K., Sanders, F., McWhinney, L.A., and Wood, L. 2005. Evidence for predator-prey relationships. Examples for Allosaurus and Stegosaurus, p. 325-350. In Carpenter, K. (ed.), The Carnivorous Dinosaurs. Indiana University Press, Bloomington.

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.

Persons, W. 2009. Theropod tail muscle reconstruction and assessment of the locomotive contributions of the M. caudofemoralis. Journal of Vertebrate Paleontology, 29 Supplement to No.3:164A.

Persons, W. and Currie, P.J. 2011. The Tail of Tyrannosaurus: Reassessing the Size and Locomotive Importance of the M. caudofemoralis in Non-Avian Theropods. Anatomical Record, 294:119-131.

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5 Responses to Walk softly – and carry an armoured tail

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  5. Colin Waite says:

    Fascinating. About time we started reading about the strength and ability of the Stegosaurid species.
    Even as a child I never believed them to be so slow and clumsy when you just had to look at the elegance of their skeletons.

    Kentrosaurus looks beautiful and clearly is NOT to be messed with. Im sure even other herbivores stayed away from them around mating season.

    Lord knows HOW they mated.

    Good work.

    Colin.