This Saturday the World’s Biggest Dinosaurs exhibition opens at the American Museum of Natural History and I don’t mind telling you, I’m this close to embarrassing myself out of excitement – something I never thought would happen to me regarding sauropods – you live and learn.
Sauropods were superficially morphologically conservative – they were the Long-Necks, the Littlefoots, the Dinos – well, Dino was more likely a prosauropod, but you get the idea. You know the body plan. Nevertheless, there have been about 120 different sauropod genera named.You also, presumably, know that they got big. Really big. Big enough that I want to quote The Hitchhiker’s Guide to the Galaxy right now. We’re talking head-to-tail lengths of 35m1 and body masses up to 70 tonnes2. Compare this to an elephant; mass of 10 tonnes, height of 4 meters (the largest terrestrial land mammal ever, Indricotherium was a little bigger). Even the other groups of dinosaurs didn’t get that big.
So why did Sauropods get so big?
There’s an informally recognised trend in evolutionary history – Cope’s Rule, that says lineages tend to increase in body size. The basal forms of many major animal groups were small in size: small animals usually have shorter lives which means they have more potential to evolve into a variety of forms to fill a variety of ecological niches. But once new forms evolve, there are many advantages into becoming as big as possible: defence against predators (or ability to eat more things, if you are a predator) is a big one. As is the ability to reach unavailable food (hello, Mr. Giraffe). Big animals tend to live longer, can maintain body heat better and have better reproductive success (a lower birth-to-death rate).
It seems a good idea – and one generally held up in nature – to be as big as possible. You might expect from that that elephants, komodo dragons and Triceratops became as big as they possibly could. The real question, therefore, is not so much why as it is how. There was something special about Sauropods.
Nature, not Nurture
It’s a good idea, at this point, to rule out environmental conditions as an explanation for big sauropods. After all, arthropods (insects, spiders, millipedes) achieved some relatively spectacular sizes during the pre-dinosaur Carboniferous, at a time when oxygen levels of the planet spiked massively3. Sauropods, though, were the largest herbivorous animals on the planet for 100 million years, during the Mesozoic, which had no such oxygen surplus, and there are no patterns of sauropod size that match any conceived environmental factor. We also cannot attribute it to any particular diet – sauropod teeth, mouths and necks are too diverse for that. Whatever it was that enabled sauropods to become giants, it was in the sauropods themselves.
This is where I got interested. I’m currently working as an educational intern at the AMNH, and part of my job involves creating curricula to tie in with exhibits. So I started researching gigantism in sauropods, starting with the Science article by Sander and Clauss4 which gave four particular features that may have contributed to hugeness: 1) a small head and long neck, 2) a bird-like respiratory system, 3) A high metabolism that actually slowed down over the animal’s life time, and 4) a ‘primitive’ reproductive strategy.
Don’t Chew Your Food
When you eat, say, cookies, your jaw moves in a roughly circular motion, incorporating sideways motion as well as an up an down action. Holding the food in your cheeks, you can really grins your molars against it and get it already on the way to digestion before sending it down you oesophagus. It’s a pretty neat system; as omnivores we can get a head start on hard-to-digest plant foods. Herbivores, such as cows, chew even more. Sometimes they chew their food more than once. Ornithischian dinosaurs such as Triceratops also had good molar-like tooth setup and cheeks in which to chew their food. Others like Psittacosaurus swallowed stones to create what’s known as a gastric mill – extra crushing action in the stomach.
Sauropods lacked both of these apparatuses – they needed, therefore, their giant thick-in-the-middle barrel body to digest the food they swallowed. But not having complicated chewing equipment let their head remain relatively tiny, a lightweight organ on the end of a neck that is free to grow longer and longer. A long neck on a grazing animal means it can reach more food – either out of reach of other animals, or just by having a grazing circle of massive radius, but it can’t have a long neck without a small head to put on the end – a Triceratops neck would snap if it didn’t carry the weigh of its head frill over its shoulders.
Breathe in, breathe out.
Breathe in. Go on, you were going to do it anyway. Your chest probably expanded, your diaphragm contracted, and you filled your lungs with fresh air from the environment. Your bronchioles fill with that air, and exchange oxygen with carbon dioxide with your blood. Now breathe out – the air is expelled from your lungs, with more carbon dioxide and less oxygen content, but it’s the same air, more or less – you, like all mammals, have a bidirectional flow of air through your lungs. It goes in, it goes out.
Birds, however, have a unidirectional flow of air through their lungs. Their trachea may bring air in than out, but the flow of air through their lungs is always fresh and oxygen rich. This is a more efficient system in terms of oxygen absorption, and helps birds obtain more oxygen from hypoxic environments like high altitudes (an occupation risk for a bird), and maintain a higher metabolic rate. The system also utilizes a succession of air sacs extending throughout the body, which coincidentally makes their body lighter, and a pneumatic bone structure formed by lots of smaller holes throughout the skeleton.
What do birds have to do with dinosaurs? Well, birds are dinosaurs. Specifically, birds are Theropod dinosaurs, the sister group to sauropods and also including the largest terrestrial carnivores ever to have lived. The respiratory system that helped birds take off may also have given saurischian dinosaurs a skeleton that was light for their size, and a highly efficient oxygen intake that would make up for the enormous length of neck that air has to get down to get to the lungs – there’s pretty much no way a mammalian bellows lung would work like that. The evidence that saurischian dinosaurs (theropods + sauropods) at least had a system like this is building up.5
Slow down with old age
Efficient lungs enable a fast metabolism. Fast metabolism means fast growth. Fast growth means large sizes can be reached quickly. Sauropod eggs were tiny – not compared to extant birds eggs, but compared to the adults that laid them. We have a specimen on display the AMNH that’s about 30cm in diameter. This is important for the point below, but also meant that juveniles themselves were tiny – 10kg hatchlings that could grow to 70,000kg adults – that’s seven thousand times as big. A ten stone adult human is only twenty times the mass of a 7lb newborn baby. Bone histology suggests that this growth happened fast6, reducing the time at which the babies are at risk from predators, before enjoying the benefits of large size.
Only, a fast metabolism like mammals and birds causes problems the larger the animal gets. It means an animal produces a lot of body heat, and this is actually a disadvantage for large animals. Large animals lose heat at a slower rate than small animals (small animals have more skin per unit of body mass from which to lose heat). High metabolisms also need more food, which is harder to manage at large sizes. But maybe sauropod metabolism slowed down with age – a fast growing baby becomes a slow growing adult as it becomes more and more giant.
Quantity over Quality
But back to those tiny babies – there’s another implication in the small size of sauropod eggs, and that’s that they’re easier to produce. Every egg needs to contain enough materials to create a young animal and see that organism through to hatching, and those materials come at the expense of the mother: the bigger the baby the more investment it represents on behalf of the parents. Large mammals, such as elephants or humans, invest massively into their offspring, and produce very few offspring as a result of that; the total fertility rate of human populations is very clearly linked to projected life expectancy: a baby born in Europe or North America is very likely7 to reach adulthood, so we invest in that child because our evolutionary fitness depends more on the quality of that child’s life than the number of children we have. This strategy is also seen in some birds, such as raptors.
The alternative to a few high quality offspring is to have as many as possible and play the odds that at least two will make it to maturity8. Sauropod nesting sites with many small eggs show that the dinosaurian giants employed this strategy; each eggs takes the minimum amount of resources away from the mother, and she can get on with the process of eating a lot of food and breathing unidirectionally. With many babies, genetic diversity of each generation is increased, and the species itself becomes more resistant to extinction – small fast growing juveniles make a population quicker to recover from depletion events. This would have been helpful over the 100 million years for which sauropods dominated the ecosystems.
So there you have it: A mixture of ‘primitive’ reproductive and digestive strategies combined with an awesome breathing technique could have given sauropods what they needed to get so very huge. And all this time I thought they were boring. And what will my kids be doing in their class? Well, they’ll be building their own dinosaurs, of course!
Hone DW, Keesey TM, Pisani D, & Purvis A (2005). Macroevolutionary trends in the Dinosauria: Cope’s rule. Journal of evolutionary biology, 18 (3), 587-95 PMID: 15842488
Sander, P., & Clauss, M. (2008). PALEONTOLOGY: Sauropod Gigantism Science, 322 (5899), 200-201 DOI: 10.1126/science.1160904
Wedel, M. (2009). Evidence for bird-like air sacs in saurischian dinosaurs Journal of Experimental Zoology Part A: Ecological Genetics and Physiology, 311A (8), 611-628 DOI: 10.1002/jez.513
1Diplodocus. My figures here are conservative, and I’m citing dinosaurs known from more complete skeletons, because even conservative numbers are pretty awesome in this case.
2Futalognkosaurus, or the better known, but less complete Argentinosaurus.
5Wedel, M. J. (2009). Evidence for bird-like air sacs in saurischian dinosaurs. Journal of experimental zoology. Part A, Ecological genetics and physiology, 311(8), 611-28. doi: 10.1002/jez.513. (http://sauroposeidon.wordpress.com/publications/)
6Sander, P. Martin (2000) ; Longbone histology of the Tendaguru sauropods: implications for growth and biology Paleobiology, 26(3) 466-488. DOI: 10.1666/0094-8373 (http://paleobiol.geoscienceworld.org/cgi/content/abstract/26/3/466)
7Can we assume statistics here aren’t necessary?
8For a stable population, an average of two offspring per breeding pair should reach sexual maturity for each generation. Even if the pairs themselves aren’t stable, with an equal sex balance that’s two offspring per female.