What allowed sauropods to get big, and what kept them from getting any bigger?

This post started out as a comment on this thread, kicked off by Dale McInnes, in which Mike Habib got into a discussion with Mike Taylor about the max size of sauropods. Stand by for some arm-waving. All the photos of outdoor models were taken at Dino-Park Münchehagen back in late 2008.

I think it’s all too easy to confuse how big things do get from how big they could get, assuming different selection pressures and ecological opportunities. I’m sure someone could write a very compelling paper about how elephants are as big as they could possibly be, or Komodo dragons, if we didn’t have indricotheres and Megalania to show that the upper limit is elsewhere. This is basically what Economos (1981) did for indricotheres, either forgetting about sauropods or assuming they were all aquatic.

Truly, a mammal of excellence and distinction. With Mike and some dumb rhino for scale.

In fact, I’ll go further: a lot of pop discussions of sauropod size assume that sauropods got big because of external factors (oxygen levels, etc.) but were ultimately limited by internal factors, like bone and cartilage strength or cardiovascular issues. I think the opposite is more likely: sauropods got big because of a happy, never-repeated confluence of internal factors (the Sander/et al. [2008, 2011, 2013] hypothesis, which I think is extremely robust), and their size was limited by external, ecological factors.

Take a full-size Argentinosaurus or Bruhathkayosaurus – even modest estimates put them at around 10x the mass of the largest contemporary predators. Full-grown adults were probably truly predator-immune, barring disease or senescence. So any resources devoted to pushing the size disparity higher, instead of invested in making more eggs, would basically be wasted.

If there was reproductive competition among the super-giants, could the 100-tonners have been out-reproduced by the 70-tonners, which put those extra 30 tonnes into making babies? Or would the 100-tonners make so many more eggs than the 70-tonners (over some span of years) that they’d still come out on top? I admit, I don’t know enough reproductive biology to answer that. (If you do, speak up in the comments!) But if – if – 70-tonners could out-reproduce 100-tonners, that by itself might have been enough to put a cap on the size of the largest sauropods.

Another possibility is that max-size adult sauropods were neither common nor the target of selection. In most populations most of the time, the largest individuals might have been reproductively active but skeletally-immature and still-growing subadults (keep in mind that category would encompass most mounted sauropod skeletons, including the mounted brachiosaurs in Chicago and Berlin). If such individuals were the primary targets of selection, and they were selected for a balance of reproductive output and growth, then the few max-size adults might represent the relatively rare instances in which the developmental program “overshot” the selection target.

Dave Hone and Andy Farke and I mentioned this briefly in our 2016 paper, and it’s come up here on the blog several times before, but I still have a hard time wrapping my head around what that would mean. Maybe the max-size adults don’t represent the selective optimum, but rather beneficial traits carried to extreme ends by runaway development. It seems at least conceivable that the bodies of such animals might have been heavily loaded with morphological excrescences – like 15- to 17-meter necks – that were well past the selective optimum. As long as those features weren’t inherently fatal, they could possibly have been pretty darned inefficient, riding around on big predator-immune platforms that could walk for hundreds of kilometers and survive on garbage.

What does that swerve into weird-but-by-now-well-trod ground have to do with the limits on sauropod size? This: if max-size adults were not heavy selection targets, either because the focus of selection was on younger, reproductively-active subadults, or because they’d gotten so big that the only selection pressure that could really affect them was a continent-wide famine – or both – then they might not have gotten as big as they could have (i.e., never hit any internally-imposed, anatomical or biomechanical limits) because nothing external was pushing them to get any bigger than they already were.

Or maybe that’s just a big pile of arm-wavy BS. Let’s try tearing it down, and find out. The comment thread is open.

References

• Economos, A.C. 1981. The largest land mammal. Journal of Theoretical Biology 89(2):211-214.
• Hone, D.W.E., Farke, A.A., and Wedel, M.J. 2016. Ontogeny and the fossil record: what, if anything, is an adult dinosaur? Biology Letters 2016 12 20150947; DOI: 10.1098/rsbl.2015.0947.
• Sander, P.M. and Clauss, M. 2008. Sauropod gigantism. Science 322(5899):200-201.
• Sander, P.M., Christian, A., Clauss, M., Fechner, R., Gee, C.T., Griebeler, E.M., Gunga, H.C., Hummel, J., Mallison, H., Perry, S.F. and Preuschoft, H. 2011. Biology of the sauropod dinosaurs: the evolution of gigantism. Biological Reviews 86(1):117-155.
• Sander, P.M. 2013. An evolutionary cascade model for sauropod dinosaur gigantism-overview, update and tests. PLoS One, 8(10) p.e78573.