How Brachiosaurus (and Brethren) Became So Gigantic

By Brad Balukjian

I was 12 years old, sitting in a movie theater in Warwick, Rhode Island, when Steven Spielberg changed movies forever. His Jurassic Park made Jaws look like a silly hand puppet and ushered in the modern era of computer-generated special effects, for better

or worse.

But for that iconic scene when the paleontologists laid eyes on living dinosaurs for the first time

, Spielberg had a crucial decision to make—what type of dinosaur would appear first, bending imaginations and searing its place in cinematic history? Would he go with the ultra-kinetic, flesh-rending T. rex? Or maybe a more subdued Stegosaurus? Much to my delight, he chose a sauropod, the clade

of lumbering vegetarians that dominated for 120 million years as, unequivocally, the largest land animals ever. Specifically, a Brachiosaurus, one of the few sauropods that probably used its long neck to browse treetops rather than holding it parallel to the ground. (Kudos to Spielberg et al. for getting this scientific detail right!) I’m not sure what dictated Spielberg’s decision, but sauropods’ sheer size—up to 90 tons and 130 feet long—probably had something to do with it. (Contrary to popular belief, most dinosaurs were not gigantic.) And that gargantuan size is what inspired the new PLOS ONE sauropod collection (“Sauropod Gigantism

”), organized by evolutionary biologist Martin Sander of the University of Bonn. Sander and 13 other researchers united to answer one question: how did these thunder lizards get so freaking big—and its shuddering corollary—why didn’t they get any bigger?

Traditionally, paleontologists studying non-avian dinosaurs (remember, birds are dinosaurs

) were fairly limited by the fact that their study organisms perished 65 million years ago when an asteroid struck Earth (or when they developed cataracts, got out-eaten by an army of caterpillars, or suffered low sex drive

). It’s hard to do experiments when all that’s left of your animal is a fossilized femur. Or is it? The collection authors pulled some Jurassic Park-esque wizardry of their own, using lasers, computer models, and detailed examinations of living relatives (birds, crocodiles, even mammals) to resurrect sauropods and better understand how they lived and how they got so big. For example, William Sellers and his team used a LiDAR laser to scan the entire 130-foot length of the Argentinosaurus huinculensis skeleton housed in Museo Municipal Carmen Funes and build a 3-D digital replica. They then added muscle tissue by examining the relationship between muscle mass and action in such extant animals as hare, reindeer, and greyhounds. Finally, they used their computerized re-creation to simulate how A. huinculensis could have walked while supporting its 83-ton girth. http://www.youtube.com/watch?v=a1OP-fKcjHc In another study, Tom Schanz and colleagues experimentally analyzed the footprint of an African elephant to show that the weight of the elephant can be calculated from the geometry of its footprint and properties of the soil, suggesting that the same could be done for long-gone sauropods.

In the collection overview, Martin Sander lays out his overarching hypothesis for sauropod gigantism. Over deep time, lineages evolve according to the interplay of two main factors: key innovations, in which lineages gain some new anatomical or behavioral character that allows them to flourish and expand their niche, and historical contingency, in which all past and existing characteristics constrain where the lineage can go next. For example, if you’re a small plant-feeding reptile with 50 million years of history, you probably are not going to suddenly become a vicious predator due to the physiological constrains imposed by your own past. In other words, in the card game of life, you still have to play some of the hand you’re dealt, although you can always improve your position when you draw your next card. Sometimes, historical contingency can play to your advantage, specifically when some already existing trait gets co-opted for a purpose for which it did not originally evolve; this is called exaptation. According to Sander’s evolutionary cascade model, sauropods started out small, but became giants through a combination of exaptation and key innovation. As each co-opted or novel trait provided some adaptive advantage, it led to the emergence of another adaptive trait, and so on through a cascade of evolutionary changes that ultimately ended with one group of freakishly large dinosaurs.

“We’re taught in science to prefer simple solutions based on the principle of parsimony,” said Sander. “So you could explain gigantism just by looking at the trait of having many small offspring. But our model shows us there were probably several factors.” So what were the traits that got sauropods so big? Sander highlights five that each set off multiple evolutionary cascades ending in gigantism: having many small offspring, lacking a gastric mill (a grinding apparatus in the stomach), not chewing food, having a bird-like lung, and having high metabolism. While the evidence is stronger for some of these traits than others (e.g., not chewing is widely accepted while high metabolism is more controversial), new data provided in several of the collection articles have bolstered Sander’s hypothesis. “What we don’t know yet is the relative importance of the cascades,” Sander pointed out.

You may be wondering how something like not chewing food could lead to a 90-ton dinosaur. Fair enough. In this particular evolutionary cascade, not chewing meant that sauropods could consume massive amounts of food per day, as they didn’t waste any time masticating. Their guts were huge, but research shows that they didn’t spend a whole lot of time digesting it, allowing for fast turnover. The lack of chewing also meant that sauropod heads could remain small, with no need for large jaw muscles. The small head opened the door for the evolution of a long neck (a small head could be easily supported), which allowed sauropods to become energy-efficient eating machines. Standing in a single spot, sauropods could browse a vast swath of vegetation without ever having to change positions. All of this adds up to the ability to ingest and process incredible amounts of food, producing massive body size.

This begs the question: once the sauropods bit the asteroid dust 65 million years ago, why didn’t their furry contemporaries, the mammals, go big? Why aren’t there 90-ton elephants roaming Africa? Several things limited mammalian body size, chewing being one of them. The musculature required for chewing led to larger head size, and the time spent chomping reduced the amount of food mammals could consume in one day. Although one might imagine a dinosaur-sized elephant with a proportionately enormous head, at a certain point, gravity takes over and keeps things from getting too out of hand. Gravity also imposed an upper limit on sauropod size. Interestingly, Sander points out that no studies to date have provided convincing evidence that sauropod size had anything to do with different environmental conditions in the distant past, such as elevated levels of atmospheric oxygen or carbon dioxide. With all the new insight coming from the innovative techniques used in this collection’s studies, our understanding of sauropod biology can only get more and more realistic. In fact, that Brachiosaurus may look downright prehistoric by the timeJurassic World

hits theaters in 2015.

Brad Balukjian blogs on the PLOS Network. He has a PhD in Environmental Science from University of California, Berkeley and was a AAAS Science Writing Fellow at The Los Angeles Times. Find him on Twitter @BradBalukjian

Read the full PLOS ONE “Sauropod Gigantism” collection here.