Photograph by Tadek Kurpaski, image from Wikipedia
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Apatosaurus was a huge dinosaur, but some sauropods grew even larger.
Photograph by Tadek Kurpaski, image from Wikipedia

Dinosaur Reproduction, Not Ancient Gravity, Allowed Super-Sized Sauropods to Evolve

Why was Supersaurus so big? This impressive, 100-foot-plus sauropod was one of the largest creatures to ever walk the Earth – far larger than any terrestrial animal alive today. What could account for such superlative size? Baseball player Jose Canseco offered his own hypothesis on Twitter a few days ago.

After promising to share some of his thoughts on gravity, Canseco cemented his reputation for weird tweets by outlining his thoughts about why there’s nothing quite so big as Supersaurus around anymore:

Plenty of bloggers picked up on the armchair speculation, and science communication star Bill Nye even chimed in. But there was something totally missing from the various responses – science.

Writers with time to kill picked out Canseco as an easy target worth a bit of ridicule, but neglected to explain why the sports star was wrong. Snark is easier than substance. That’s a shame, especially given the growing amount of literature about why dinosaurs – and sauropods in particular – got to be so mind-bogglingly enormous.

Canseco didn’t have all his facts straight. No dinosaur ever grew to be 200 tons in weight. Yet his off-the-cuff comments about gravity aren’t so unusual as they might initially seem. Researchers have at least considered the speculative idea. In a 2011 paper on sauropod gigantism, paleontologist P. Martin Sander and coauthors briefly debunked the proposal that Jurassic gravity was weaker. Sauropods didn’t grow so large because of reduced gravity, greater oxygen content in the prehistoric atmosphere, or because of an overabundance of food, the paleontologists concluded. More subtle factors accounted for big body size. A characteristic the dinosaurs shared with birds and the way sauropods reproduced made their Mesozoic growth spurt possible.

Before getting to the oddities of sauropod biology, though, let’s dispense with the idea that there was something about Earth’s environment that drove dinosaurs to gargantuan sizes. My friend and fellow science writer Matthew Francis covers the gravity angle over at his blog – there’s no evidence that the Earth’s gravity was weaker during the heyday of the dinosaurs. Even as continents shifted around, the Earth’s gravity didn’t change. Canseco’s idea of “nimble” dinosaurs sprinting around in low gravity isn’t grounded on any actual evidence. (Canseco didn’t consider what would happen to all the smaller dinosaurs that lived during the same time as Supersaurus. I can’t help but imagine Stegosaurus happily leaping and bounding over Jurassic plains in reduced gravity.)

And there’s no reason to think that giant dinosaurs got an assist from higher oxygen levels, either. Even though Canseco didn’t bring up this idea, some of his fans did in response to his ancient gravity tweets. The idea goes back to the connection between increased oxygen and the evolution of giant insects during the Carboniferous, over 300 million years ago. While oxygen composes about 20% of the air we breathe now, Carboniferous air contained about 30% oxygen. This boost is connected to better breathing efficiency among ancient insects and arthropods, which allowed the invertebrates to grow larger.

Some paleontologists speculated that oxygen might be behind the evolution of huge dinosaurs, too, but the hypothesis has been totally undermined by evidence from the geological record and dinosaur anatomy. Geochemical evidence has shown that Jurassic and Cretaceous air contained about as much oxygen as today, if not a little less. More than that, the dinosaurs did not need increased oxygen to adequately nourish their bodies.

As part of their respiratory system, sauropods had a complex network of air sacs that gave them two advantages. Not only did the air sacs allow the dinosaurs to breathe more efficiently – more like birds than mammals – but the soft tissues invaded bone to make the skeletons of these dinosaurs lighter without sacrificing strength. Indeed, even at around 100 feet long, Supersaurus has been estimated to weigh in between 35 and 40 tons. That’s quite hefty in absolute terms, but consider that the largest African elephant on record weighed about 12 tons, and the extinction rhino Paraceratherium – about 26 feet long and 16 feet tall at the shoulder – weighed about 18 tons. You’d think a dinosaur about four times as long as Paraceratherium would be much heavier – 72 tons or more – but Supersaurus and similar dinosaurs were relatively light. Air sacs allowed sauropods to escape some of the physical constraints that have limited the evolution of mammal body size over the past 66 million years.

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Sauropod neck length, presented to scale with the necks of other organisms (at left).

Sauropod experts Michael Taylor and Mathew Wedel just published a paper outlining how these air sacs, and other biological features, allowed sauropods to evolve extravagantly-long necks. While just part of the whole animal, the long necks of sauropods greatly contributed to the overall size of these dinosaurs. Indeed, Taylor and Wedel estimate, the neck of Supersaurus probably stretched about 50 feet long, nearly half the animal’s entire length. The ability of this dinosaur, and other sauropods, to support such a long neck relied on several different features.

Compared to a giraffe, sauropods had a more stable, sturdy body that was better able to support ridiculously long necks and tails. But an even more critical feature was that sauropod had small heads. Unlike big mammals – which typically have big, heavy heads set with large teeth necessary for chewing – sauropods had tiny skulls with teeth that could only grip and crop food. Sauropods couldn’t chew, and instead relied on some other mechanism in their guts to totally break down food. Mammals have to balance neck length with having heavy heads, while sauropods avoided this problem by getting around chewing. (Although exactly how these dinosaurs processed the massive amounts of plant food they needed is a mystery.)

Furthermore, indentations and pockets in sauropod neck bones show that these dinosaurs had a bird-like air sac system – soft tissues that eliminated some of the mechanical and physiological problems of breathing with an extra-long neck. For example, sauropods undoubtedly breathed through their trachea like other vertebrates, and this structure faces certain physical constraints. In a long, narrow trachea, Taylor and Wedel point out, it’s difficult for an animal to inhale quickly. But to widen an elongated windpipe creates the problem of “tracheal dead space”, in which previously used air may be re-inhaled with each new breath and therefore cut breathing efficiency. Birds are able to eliminate such dead space by virtue of their air sacs, and so it’s likely that the same held true for sauropods. The posture, diet, physiology, and air sac systems of sauropods removed evolutionary pathways that have blocked mammals from attaining truly long necks.

Of course, not all sauropods were giants. Some of the smallest were between 13 and 17 feet long, including dwarfed varieties that evolved on islands (such as Magyarosaurus). Understanding why sauropods evolved such insanely long necks is part of the key to understanding why they loomed so large over the Mesozoic landscape, but it’s not the entire story. In fact, the reason sauropods could attain such large sizes has just as much to do with reproduction as skeletal architecture and specialized soft tissues.

The biggest dinosaurs started off very, very small. Sauropod mothers laid clutches about 10 eggs at a time in small nests, and the embryonic dinosaurs developed in eggs about the size of a large grapefruit. Once they hatched, these little dinosaurs grew at an absolutely fantastic rate. Based upon sprawling nesting grounds where many sauropods came to lay their eggs, these dinosaurs might have had a reproductive strategy similar to extant sea turtles. Rather than investing a great deal of energy into one or two offspring that required a great deal of care, sauropods regularly flooded the environment with clutches of offspring that were probably on their own from the very start. As suggested by paleontologists Christine Janis and Matthew Carrano in 1992, and more recently supported by a study conducted by Jan Werner and Eva Maria Griebeler, this way of reproducing may have removed constraints that keep mammals relatively small by comparison.

Elephants, giraffes, and other large mammals reproduce in a different way. Big mammals usually carry a single offspring internally for a long period of time, and the need for protection and milk means that mammalian youngsters continue to be an energy drain after birth. The costs of carrying and caring for a single, large baby are staggering, and long pregnancies run the risk of potentially fatal complications. All these factors conspire to create a reproductive threshold that mammals just can’t cross. For mammals to get much larger, they would have to carry young for longer and likely provide increased parental care. The extinct Paraceratherium and steppe mammoth, two of the largest land mammals ever, might represent how large it’s possible for mammals to get without fundamentally changing the way large mammals reproduce.

By externalizing birth and development, sauropods and other dinosaurs were able to sidestep the costs and risks that constrain mammal size. For dinosaurs, mechanical and other biological constraints might have prevented them from becoming even larger – the amount of time it would take for nerve impulses to travel to a 100-foot-long dinosaur’s brain for example. The fact that all the genera that are contenders for the “largest dinosaur of all time” title – including Argentinosaurus, Supersaurus, and Diplodocus – top out around 100 to 110 feet in length might indicate that these dinosaurs were reaching the anatomical ceiling of how large it was possible for them to get.

But let’s be clear about sauropod size. Biological quirks such as air sacs and laying lots of little eggs allowed sauropods to grow to large size, but these features did not drive dinosaur inflation. There were titanic dinosaurs as well as tiny ones. Dinosaurs did not experience the same barriers as mammals, and therefore evolved a greater range of body sizes. The evolutionary driving forces behind the evolution of truly huge body size are not clear, and likely differed from one group of dinosaurs to the next. Paleontologists have determined the features that made it possible for a creature as spectacular as Supersaurus to exist, but the reason why the dinosaur’s lineage ended up pushing biological boundaries of body size are still unknown.

Will creatures the size of Supersaurus ever walk the Earth again? Perhaps. Paleontologists have now outlined some of the major features that allowed huge body sizes to evolve. An egg-laying quadruped with a sturdy torso and some sort of efficient breathing system would be a good candidate. Sadly, though, there’s nothing alive today that quite fits that description. Mammals certainly don’t, and, as Taylor and Wedel pointed out in their study, the evolutionary pathways presently open to birds are limited by their bipedal posture. If anything on the scale of Supersaurus is going to evolve again, it’s many millions of years off.

Animal life was evolving for over 380 million years before the origin of sauropods, and there hasn’t been anything so wonderful as Supersaurus and Diplodocus in the last 66 million years. Maybe the behemoths will end up being a rare, contingent product of evolutionary unfolding that will never be matched. Still, thanks to the wonderfully rich fossil record, we can imagine the heyday of giants as we admire what remains of enormous dinosaurs.

References:

Sander, P., Christian, A., Clauss, M., Fechner, R., Gee, C., Griebeler, E. Gunga, H., Hummel, J., Mallison, H., Perry, S., Preuschoft, H., Rauhut, O., Remes, K., Tütken, T., Wings, O., Witzel, U. 2011. Biology of the sauropod dinosaurs: the evolution of gigantism. Biological Reviews. 86, 1: 117-155. 10.1111/j.1469-185X.2010.00137.x

Taylor, M., Wedel, M. 2013. Why sauropods had long necks; and why giraffes have short necks. PeerJ: e36 http://dx.doi.org/10.7717/peerj.36

Werner, J., Griebeler, E. 2011. Reproductive biology and its impact on body size: Comparative analysis of mammalian, avian and dinosaurian reproduction. PLoS One. 6, 12: e28442. 10.1371/journal.pone.0028442