It’s time for more Plateosaurus!

I can hardly believe that I have neglected to post Plateosaurus stuff for nearly three weeks by now! No, I am not mentally ill, but apparently close to 🙂

So, assuming you all have read all my papers on this topic in detail (cough cough), I’ll post on something that usually gets neglected, dismissed, never-thought-about, and in all other imaginable ways forgotten. And that despite the fact that it is cheap, highly informative, and easy to create and use, barely anyone does it.

What I am talking about is having a look at dinosaur skeletons from the top, front and behind, any odd point of view. Especially behind, because, “ass” is most of what dinosaur locomotion is about!

Butt view of GPIT/RE/7288 Plateosaurus engelhardti in the Department of Geosciences’
of the Faculty of Natural Sciences of the Eberhard-Karls-Universty Tübingen paleontological museum (or rather, public collection if the bean counters get their
way). Previously known as “GPIT Skelett 1” and “GPIT1”.

OK, that one skeleton AGAIN! It is in all my papers, all my talk, even in the header of this stupid blog – why oh why do we need another picture of it? What’s so special about this view?

Most people think of dinosaurs in lateral view. Maybe a tad anterior, maybe somewhat from below if it is a large predator or a sauropod. Maybe from the front, if that view is dominated by a huge mouth full of banana-sized teeth. But a butt view? A dino mooning you? That’s WAY too boring, at first glance.

However, if we consider what a butt really is (and why us humans are usually attracted to very muscular ones, with males have an additional preference for a side helping of small doses of well-distributed fatty tissue), then it becomes obvious that a butt is central for walking and running. And in practically all amniotes this is true: the butt plays a key role in locomotion, so that the shape of the butt tells you if an individual is fit (and thus fast) or not.

Let’s have a look at a section of the above image. I painted in (with a fancy brush variation the program calls “Van Gogh small”) the attachment sites of the caudofemoralis longus (CFL) muscle on the femur (4th trochanter) and the tail (exemplarily on two verts, lateral faces of haemal arches, lateral faces of centra, underside of transverse processes [admittedly, the CFL doesn’t really attach there. It just runs along it). And the dotted line traces the contours of the muscle, at a “best guestimate” level.

Yeah, it is thicker than your thigh (unless you happen to be an ex-gubbenator of CA). And it is the main limb retractor, in an animal that has very limited extension power in the knee and ankle. It is, therefore, the thing that drives this animal.

I base the above reconstruction line on three lines of evidence: own inspection of a bunch of X-rays of living tailed amniotes, Viv Allen’s work on this topic , as well as the studies of Person & Currie (all dissection based), and my very limited understanding of bone formation. In short, in extant tailed amniotes the tail muscles extend way beyond the bones, which is visible in X-rays and was found by Viv and Scott Persons in dissection studies. Viv’s paper (Allen et al. 2009) gives percentage ranges of how far, Scott’s paper (Persons & Currie 2010) tells you how to model the shape of the CFL. And because bone grows only from certain stimuli, “pull or push”, it is pretty obvious that the transverse processes and haemal arches (chevrons) can’t extend to the skin.

by the way: if you want to have a really good laugh at wikipedia (or a good cry), check out their article on the caudofemoralis! At least, do so before I get to it. My poor to-do list for wikipedia keeps growing almost as fast as my kids. *sigh*

So, here’s the Allosaurus cast in the MFN, again seen butt-first. Go ahead and reconstruct that CFL yourself.

What I hope you’ll notice right away is that the CFL, while being a huge muscle in Allosaurus, is quite a bit smaller proportionally than it is in Plateosaurus. That’s because theropods use their limbs slightly different than sauropodomorphs do – for more info come see my SVP talk.

What’s also pretty clear: the tail and hind limb are not separate entities. We tend to section an animal in our heads into

• skull,
• neck,
• forelimb,
• trunk,
• hindlimb,
• tail.

In fact, for reptiles (excepting tailless ones = birds) the functional units are

• skull,
• neck with neck base on body,
• forelimb,
• trunk,
• hindlimb and tail base,
• distal tail.

Functionally, the base of the tail can even be seen as part of the trunk, with the hips an oddity in it from which the hind limbs sprout. But that again would suggest that tail and hind limb are not one part, when in fact the CFL unites them into the “engine” of the dinosaur.

OK, so a butt view can make obvious to us how big a muscle the CFL is, and that makes obvious to us how different from a tail-less mammal a dinosaur is. But that’s not all the above shots show. Check out the shape of the ischia. See how they form an arch for eggs and faeces to pass through? And how they then bend medially to give the knees more room? That’s a good thing to remember: a dinosaur is only as wide at the hips as is needed for an egg + bones and muscles to run the limbs. Mammals, in comparison, are wide enough for an baby + bones and muscles. And the bigger a mammal is, the bigger the babies get, proportionally, and the wider its ass. Non-avian dinosaurs are very very slim, much more so than many people realize, and their lateral balance is therefore a lot better when running. That’s because they can move the leg fore and aft with minimal adduction and have the foot under the center of mass, whereas bipedal mammals (i.e., us freaky humans) need to do a lot of swaying and gyrating of the hips. The opposite is true when standing: the wheel base of a dinosaur is much narrower than that of a mammal, thus they are more likely to unbalance. Life’s a compromise.

Another thing to note is the shape of the ribcage. High-oval cross section in Plateosaurus, with the belly ending at the huge pubes. That’s not distortion from post-mortem damage to the skeleton, e.g., by compression. Such a narrow and high body is typical for rather rapid runners among today’s larger animals. Think horse, not hippo. The MFN Allosaurus, in comparison, has a rather barrel-shaped chest. That’s odd, and may have to do with the cast. I seem to remember many mounts with narrower chests. I can’t really tell for sure on my pics, but the AMNH mount seems high-oval, too.

For the finale here’s an anterior view of the pelvis of the old Berlin Giraffatitan mount. Notice how derived sauropods broaden their hips compared to early sauropodomorphs! If you take the weaker tail and odd ilium shape into account, this animal looks almost as if it wanted to “go mammal”!