Monday, November 19, 2007

Nigersaurus...and a question...












*Credit: Photo © Gabrielle Lyon, courtesy Project Exploration

Science has always been my passion. As a young kid I would capture and observe small animals from the stream in our back yard. Lightning, tornadoes and other natural phenomena fascinated me; I put pictures of extreme weather on my walls and tried, with no luck, to go on a tornado chase for my fifteenth birthday. I told my parents, all throughout my childhood, that I wanted to be a paleontologist.

So I often wonder why I majored in writing and not in a science. And the answer is actually very simple: I was scared. Science is intimidating. It's an arena where everyone is trying to prove you wrong. It's not static. It's dominated by some of the most brilliant minds. It's a field that requires a certain kind of thinking.

I was afraid of science. I was afraid of being proven wrong, of asking the wrong questions. And sometimes I still am. So I have a question. One that may have a painfully obvious answer to some of you. I've been afraid to ask it, but I also want to be a good scientist, and I figure asking questions is the best way to become a good scientist. So here goes...

Nigersaurus had a very pneumatic vertebrae, which made the skeleton quite light (relatively). But Nigersaurus was also very small (also relatively!). So the question (not mine) is, "Why did a small sauropod need a light skeleton?"

My question is, could it be a form of protection? If Nigersaurus was small compared to most sauropods, and most sauropods got their protection from their size alone, would a lighter skeleton allow for a faster sauropod? One that would not have a size advantage?

And that leads me to more questions, as follows:
  • In general, are lighter-boned animals faster than animals with dense bones?
  • Is bone density related to muscle size? I'm assuming that small bones have small muscle attachment sites and than light bones can only handle a certain amount of stress caused by muscle contraction.
  • Would a small sauropod be fast enough to outrun a carnivorous therapod?
  • If not faster, what sort of adaptations would a small sauropod have for defense?
I also wonder, could the light skeleton mean that Nigersaurus was a slender animal? Perhaps it was not as robust as other sauropods, especially given its feeding habits and did not need an especially strong frame to carry its weight.

Anyway, I hope that doing this will help me get over my fear of asking questions and maybe stimulate some good conversation. I'm always afraid to ask questions like this, but really, what do I have to lose?

Reference

Sereno, P. C., Wilson, J. A., Witmer, L. M., Whitlock, J. A., Maga, A., Ide, O. & Rowe, T. A. (2007). Structural extremes in a Cretaceous dinosaur. PLoS One (11): published online.

3 comments:

Jerry D. Harris said...

So the question (not mine) is, "Why did a small sauropod need a light skeleton?"

Well, maybe it didn't, per se...maybe it only had a highly pneumatic skeleton because its ancestors did (and it's true that diplodocoids in general, as far as they are known, have highly pneumatic skeletons, although finding one in a basal diplodocoid like Nigersaurus is pretty cool!). But pneumaticity does more than just lighten the skeleton!

Having a system of air sacs has respiratory benefits as well -- they enable an organism to take in much more air (and thus oxygen) than they would be with the kinds of crappy two-way lungs we mammals are stuck with. To be sure, the air sacs themselves are not capable of oxygen exchange (at least, they're not in any living organism, including birds) -- that's still the purview of the lungs alone. But, assuming that the respiratory system in a sauropod was similar to that of a bird (a fairly safe bet in general, if not in many details), they continually funnel air through the lungs to be processed for oxygen. This is a more efficient means of powering the metabolism on the go.

Another nifty hypothesis that's been floated about lately -- and one I find quite compelling -- is that the air sac system is also a terrific heat dump. Having your body lined with comparatively cool air (cooler than your internal temperature, at any rate) means that air taken in can be heated up by the body, then exhaled, taking the heat with it. This is a nifty way of avoiding overheating. This is particularly important in flying birds, which are riddled with air sacs, because continual operation of flight muscles generates a LOT of heat, but is also useful in large animals simply because their surface area:volume ratios are low, so their bodies don't eliminate heat as efficiently as smaller animals can. Granted, Nigersaurus wasn't huge, even by sauropod standards, but it was still big. If it was more endothermic/tachymetabolic/
homeothermic, this is exceptionally important, but even if it wasn't, the fact that it was an herbivore means that its guts would have been rife with fermenting bacteria (to break down the consumed plant matter), and that fermentation process is also highly exothermic, meaning that some system would have been needed to dump the generated heat. Being more "warm-blooded" and having an internal fermentation vat would make this especially valuable!

The question, in a much broader sense, then, is "which of these was the strongest selective factor in the initial evolution of postcranial pneumaticity?", and that's a question that can't be easily answered, in indeed only one thing was selected for and all other functions were exaptations. Personally, I think it's most likely that all of them in tandem made them selective!

In general, are lighter-boned animals faster than animals with dense bones?

Hard to answer if only because there are no examples of which I can think of animals that have similar enough bauplanes (overall body layouts), but one with pneumatized bones and the other without, that can be readily compared this way. I think the intuitive answer is "Yes," given that in our experience, lighter cars, airplanes, etc. are generally faster than heavier ones, all other factors (like engine power) being equal. But it's hard to compare, say, a pelican (very highly pneumatized, to the point where its arm bones are translucent!) with a cheetah since their modes of locomotion are so different. With a lightened skeleton (and rest of the body -- the air sacs are much more prevalent than just inside the bones, at least in modern birds -- they surround bone, muscle, etc. as well, and some are even subcutaneous!), Nigersaurus would have had less concern about weight, but the structure of sauropod limbs is such that sauropods in general weren't designed for fast movement -- things like limb bone proportions, the general inflexibility of the wrists and ankles, etc. are all hallmarks of slow-moving ("elephantine") animals. Nigersaurus weren't no speed demon!

Is bone density related to muscle size? I'm assuming that small bones have small muscle attachment sites and than light bones can only handle a certain amount of stress caused by muscle contraction.

I don't know that density is related to muscle size, but while it's certainly true that bigger bones can devote more surface area to muscle attachment, thereby enabling larger muscles overall, a number of other things need to be considered here. First, muscle itself is really heavy, so having bigger muscles, while certainly more powerful, isn't always the best idea. Evolution, in principle, works to ensure that muscles follow basic mechanical principles as closely as possible: optimal lever arm lengths to muscle size so that the smallest possible muscle has the strongest possible effect, alleviating the need for muscles to simply get big in order to become more powerful. (The same is true for the shapes of the bones, the positions of the muscle insertions, etc.) Second, a bone need not be solid, or even thick-walled, to efficiently anchor a muscle. Indeed, the overall shape of a bone or series of bones can say a lot about where the stresses acting on it are maximized (see, for example, work by Emily Giffin on theropod skulls for some cool work along these lines), but even on a microscopic scale, the orientations of the individual crystals that make up the bone tend to be oriented to most efficiently resist the stress. Architects and engineers have discovered these same principles for making, say, buildings -- it's why, for example, a support beam can be I-shaped instead of a big, solid rectangular block: forces are concentrated in small areas, so the substance (bone, steel, whatever) only needs to have good structure in those places. This is, in large part, why sauropod vertebrae are basically constructed of a series of thin sheets: the sheets are where the muscles exert force, but all the surrounding space can be devoted to other stuff -- like air sacs -- that are light. In short, it's not a simple matter of "small bones = small muscles, big bones = big muscles" -- it's possible to have more efficient muscles and stress-resistance without resorting to simply increasing bone size.

Would a small sauropod be fast enough to outrun a carnivorous therapod? If not faster, what sort of adaptations would a small sauropod have for defense?

Almost certainly not, with some variation between the particular sauropods and theropods under consideration. Elephants can't outrun lions or tigers, either, but that doesn't mean that they're basically walking buffets waiting to happen, either. Speed isn't the only defense! If Nigersaurus (and other rebbachisaurids) had the kinds of elongate distal caudal vertebrae that their flagellicaudatan (Diplodocidae + Dicraeosauridae) relatives do, it's possible they used their tails as defense mechanisms. Living in groups is also a pretty good defense -- makes it hard for a predator to move in and target a single individual, and it's conceivable (though not really testable) that sauropods were communal enough that if one were attacked, others would come to its defense (like parent antelope frequently do when their young are attacked by African predators today). Maybe sauropods had some kind of stink gland like a skunk to dissuade predators -- again, not really testable, but not impossible, since some other reptiles have similar mechanisms (like garter snakes). Who knows?

could the light skeleton mean that Nigersaurus was a slender animal?

Guess it depends on what you mean by "slender"!

Having scented talk of sauropod pneumaticity, I fully expect a pack of rabid, slobbering Matt Wedels and Mike Taylors to be along any moment to make further comment...! ;-D

Amanda said...

Awesome! That's just what I was hoping for (rabid Wedels and Taylors). And of course, someone with more knowledge to enlighten me. I can almost guarantee that I will remember everything you've said in this comment much better than I would remember something I simply read in a book. Thanks a lot!

Zach Miller said...

I have nothing to add to that. Jerry did a wonderful job explaining it--much better than I could have.

I would like point out, though, that the most pneumatic portion of Nigersaurus is its skull, which is not only filled with holes, but the bones themselves are incredibly thin-walled. One of Sereno, et al.'s concerns was how Nigersaurus' skull construction could have withstood the forces generated by biting vegetation! While it's telling that the most "solid" part of the skull was the maxilla-premaxilla-dentary complex, the muscles of the jaw were weak, and the bones those muscles attached to were similarly weak. Thus, a diet of young ferns and softer, perhaps water-based plants were recommended.