by Brian Switek
When considered as a whole, all of the largest dinosaurs seem to be around 100 feet long, and maybe a little more. (Only Amphicoelias is estimated to be considerably bigger.) If so many of the largest dinosaurs topped out at around the same size, this might represent the upper boundary of just how large a dinosaur could get before collapsing upon itself.
And not all sauropods were big. Indeed, most species never approached that 100-foot upper bound, and on prehistoric islands, some stranded populations of sauropods even became dwarfs that reached only about 20 feet in length. What makes a sauropod a sauropod isn’t size, but anatomy. And these dinosaurs were weird. As paleontologists discover more, the dinosaur lineages Cope, Marsh, and other researchers found early on seem rather plain, but I still believe that those sauropods were some of the most bizarre dinosaurs of all.
Consider Apatosaurus. This hefty herbivore, familiar to just about everyone who’s ever heard the word “dinosaur,” might seem rather vanilla. Lacking spikes, horns, armor plates, a crest, or any other bizarre ornamentation, the elegant sauropod might not strike us as nearly as strange as an armor-encased ankylosaur, or as unusual as a feather-covered, sickle-clawed deinonychosaur. But their familiarity belies how little we know about the animal’s natural history. Sauropods are so marvelous not just because of their size, but because they are improbable creatures in so many ways. Almost everything about sauropod biology is a puzzle, especially their most distinctive feature.
The necks of sauropods are ludicrous monuments to evolution. Any giraffe would be green with envy at the ability of these dinosaurs to pluck fodder from high in the trees or sweep their heads over fern-covered savannas to suck up mouthful after mouthful of vegetation. Their necks also beautifully demonstrate the jury-rigged nature of evolution while simultaneously refuting the notion that some divine Artificer intelligently designed organic life. As Matt Wedel remarked in a recent paper on these dinosaurs, part of their necks were a fantastic “monument of inefficiency.”
Evolution does not operate in the best of all possible worlds. As the paleontologist Stephen Jay Gould remarked in his famous essay on the panda’s “thumb”—a modified wrist bone used by the black-and-white bear to grip bamboo—“Odd arrangements and funny solutions are the proof of evolution—paths that a sensible God would never tread but that a natural process, constrained by history, follows perforce.” The recurrent laryngeal nerve of sauropods is another beautifully complicated example of circuitous anatomical solutions. The unwieldy nerve wasn’t unique to huge dinosaurs, but a trait they shared with all other four-limbed vertebrates, inherited from a common ancestor.
Over 375 million years ago, creatures like the flattened “fishapod” Tiktaalik formed the basis for what would eventually become a fantastic radiation of animals called tetrapods. From early amphibians to dinosaurs to vertebrates that secondarily lost their limbs—snakes and whales, for example—tetrapods are a widely varied group of animals. And all tetrapods share the recurrent laryngeal nerve. This string of neurons—which runs from the brain down along the neck, and allows both sensation and motor response by the larynx—started off as a close connection between the head and heart among the early four-limbed fish that dragged themselves through the Devonian mud. As tetrapods flourished and forms with longer necks evolved, however, the nerve had to stretch to keep brain and throat in contact—and a twist in the nerve’s pathway made the structure twice as long as it had to be. The nerve takes an extended path from the brain into the chest cavity, where it loops around a series of major blood vessels called aortic arches and winds back up the length of the neck, a course resembling a drawn-out U. This wasn’t an issue for creatures with compact bodies, with brains not so very far from their hearts, but it becomes ridiculous in extremely long-necked tetrapods. A giraffe with an eight-foot-long neck carries a laryngeal nerve about sixteen feet long. This boneheaded “design” is a hallmark of evolution—what already existed is modified and co-opted into new forms.
Sauropods had the most stupendously inefficient nerve paths of all. The longer the neck, the further the nerve has to stretch, and sauropods unquestionably had the longest necks of any creatures. According to Wedel’s calculations, an adult Supersaurus is estimated to have had a neck about 46 feet long and would therefore have had a recurrent laryngeal nerve almost as long as its body at 92 feet long. “The existence of 28 m neurons in the RLN of Supersaurus may seem fantastic,” Wedel writes, “but they appear unavoidable given what we know of tetrapod embryology and evolution.”
Wedel also points out that sauropods most likely had even longer nerve cells. Just think about the nerves at the end of a sauropod’s tail that would have had to convey signals triggered by touch all the way back to the dinosaur’s brainstem. Wedel notes that such a connection suggests that that the largest of modern-day whales have neurons that reach over 90 feet, and slightly larger sauropods would have required even longer ones. No one has actually seen these cells, but, as Wedel hypothesizes, such fantastically long neurons should exist. The neurons in a dinosaur like Supersaurus could have been the longest cells of all time, creating quite a problem for these dinosaurs. If Amphicoelias was a real dinosaur and truly reached over 160 feet, Wedel points out, then even nerve impulses traveling at over 330 feet per second would have required several tenths of a second to reach the brain. While this doesn’t mean that sauropods were so slow that predatory dinosaurs could nibble on the tails of the giants without the behemoths knowing, there definitely would have been a delayed response to signals from the outside world. As Wedel notes, this might be a hint that the largest dinosaurs were reaching the upper limit of how big it was possible to get.
And for their size, sauropods had ridiculously small heads. I have an Apatosaurus skull cast in my living room. Even though my wife specifically told me, “Don’t bring home any dinosaurs,” when I went to the estate sale of the former state paleontologist of Utah, James Madsen, I couldn’t resist the full-size replica of an Apatosaurus skull. The carbon-copy cranium belonged to an animal more than 80 feet long, yet I was able to comfortably cradle the dinosaur’s head in my arms as I happily walked it out to my car. This is a tiny head for such a big animal. And, even worse, this was not a skull suited to chewing. Much like its contemporary cousin Diplodocus and other sauropods, Apatosaurus had only a short row of pencil-shaped teeth at the front of its square muzzle. How did Apatosaurus consume enough food to fuel itself? I’m an unabashed fan of evolution, but sometimes I wish nature’s mysteries weren’t so damn hard to solve.
Our mammalian bias often gets in the way of understanding dinosaurs. We’re chewers, and we expect dinosaurs to have done the same. But that wasn’t the case at all. Dinosaur jaws plucked, sliced, cleaved, ripped, and otherwise cropped food, but then they immediately horfed their meals down. Sauropods must have been champs at this. Apatosaurus didn’t stand on Jurassic floodplains grinding down ferns and conifer branches like some sort of Jurassic cow. The dinosaur’s bad table manners allowed the sauropod to suck up the vast amounts of succulent green food it required to survive. Exactly how much food Apatosaurus and similarly sized dinosaurs needed is a matter of physiology, though, and that is one of the most frustrating of all dinosaur mysteries.
When I first met dinosaurs, the Dinosaur Renaissance was still in the process of revitalizing them. Plodding Mesozoic idiots on television and in my library books were gradually being replaced by colorful, clever dinosaurs that looked limber enough to turn cartwheels over the ancient landscape. In the pop-sci parlance of documentaries, dinosaurs had gone from being typical cold-blooded reptiles to hot-blooded creatures unlike anything before or since. But while paleontologists generally agreed with the new image of dinosaurs as far more complex than anyone had previously understood, they fought like hell over the particulars of dinosaurian biology.
Unless you’re describing a romance novel, “hot-blooded” is an awful term. It doesn’t tell you anything about an animal’s biology. With a more or less constant body tem
perature of about 98.6 degrees Fahrenheit, I’m unquestionably hot-blooded, but a lizard that lies out in the sun for long enough will also warm to an active, hot-blooded state. An animal’s physiological profile isn’t equal to its body temperature. The distinction lies in the varied biological mechanisms involved in how that body temperature is maintained. For dinosaurs, this means we have to figure out whether they regulated their body temperatures internally, whether they maintained constant body temperatures, and whether they had high or low metabolic rates. Those three features create an outline of a creature’s physiology, and paleontologists have struggled to understand these details of dinosaur biology.
Lacking a time machine and a thermometer, we can’t measure dinosaurian temperature directly. That may be for the best. The paleontologist Edwin Colbert and his collaborators had enough trouble taking the temperature of American alligators in an experiment meant to sketch what kind of physiological strategy dinosaurs employed. Three decades before the dinosaur temperature debate exploded, Colbert and colleagues traveled to Florida to measure the body temperatures—via the cloaca—of various small American alligators as the scientists moved the archosaurs between sun and shade. Since alligators are ectotherms, and therefore rely on their environment to regulate their body temperature, the scientists wanted to see just how fast the crocodylians warmed up and cooled down as a way to create a model for how much time even larger dinosaurs might have spent sunning themselves. The researchers even went as far as to create little wooden armatures—which looked like torture devices—that they used to manipulate little alligators into dinosaur-like positions to see if posture made any difference. The illustration of the boxes included in the paper looks like a snapshot from an alligator crucifixion.
The experiment didn’t go exactly as planned. Two alligators died from prolonged exposure to the sun. Even supposedly sun-loving archosaurs could fatally overheat. And, not surprisingly, warm-up and cool-down time varied based on the alligator’s size. The smaller alligators warmed up and cooled down faster than their larger counterparts, thanks to their smaller volume. (It’s the same concept that explains why a small plate of leftovers heats up fast in the oven but a plump Thanksgiving turkey takes hours.) The results didn’t solve any dinosaurian mystery. When Colbert applied the patterns to dinosaur body sizes, Apatosaurus-class sauropods would have required a whole day in the sun before they fully warmed. And the same principle held for cooling down. If an Apatosaurus started to overheat, the dinosaur would have had an awful time trying to dump the excess heat and might have died, just like the small alligators. An ectothermic lifestyle reliant on sunbathing just didn’t work for big dinosaurs, and small dinosaurs were so lightly built and agile that they didn’t seem to need any warm-up time. Even as paleontologists, artists, and animators still imagined that sauropods required warm ponds to survive, evidence was mounting that dinosaurs needed a fresh look.
Dinosaur imagery changed before paleontologists really understood how sauropods and their various relatives functioned. The paleontologist Bob Bakker, in particular, pointed out that dinosaurs grew fast, had upright limb postures consistent with an active lifestyle, and had population structures more reminiscent of mammals than reptiles—all clues that dinosaurs may have kept their internal fires burning at high, constant temperatures. All those lines of argument still hold true. And that’s not all. Many dinosaurs were covered in insulating feathers, and entire communities of dinosaurs flourished in polar habitats, where they undoubtedly faced long, snowy winter nights. The more we learn, the more it’s apparent that dinosaurs had the internal mechanisms to run hot. Dinosaurs were definitely not sluggish ectotherms restricted to a perpetual muggy Mesozoic summer, as brought to life in Disney’s Fantasia and Rudolph Zallinger’s Age of Reptiles mural at Yale’s Peabody Museum of Natural History.
Mammals have even helped researchers investigate the physiology of dinosaurs. In 2012, a landmark study of mammal bones showed that ruminants—hoofed herbivores with an even number of toes on each foot—had lines of arrested growth in their bones. These bands are signs of a seasonal slowdown, and naturalists had previously thought that they were present only in ectothermic organisms whose physiology fluctuated with the surrounding environment, such as crocodiles. Since dinosaurs had these same lines, some paleontologists had suggested that dinosaurs were more like reptiles than like mammals or birds, but the new study struck down this line of argument. As the paleontologist Kevin Padian remarked in an opinion piece on the research, the study showed that dinosaurs “were anything but typical reptiles.” Now we know that at least some mammals, too, grow rapidly when times are good and slow their growth during dry or cold seasons, when resources are scarce.
Together, these lines of evidence strongly suggest that dinosaurs were active, fast-growing creatures that would have required high metabolic rates. As the paleontologist Stephen Brusatte wrote after reviewing the evidence accumulated so far, “What seems clear … is that dinosaur physiology and metabolism was more similar to that of living birds and mammals than living reptiles.” Much of dinosaur physiology remains the subject of fierce debate, but with a tenure spanning the past 230 million years, dinosaurs must have had a physiology that was immensely adaptable.
Of course, there was no single physiological profile that fit all dinosaurs. Dinosaurs were so diverse and disparate—just think of the differences in size and shape between Supersaurus, the tiny fluffball Sinosauropteryx, and the heavily armored Kentrosaurus, to pick just three of hundreds of genera—that lineages varied just as dramatically as living mammals do (say, a bat, a dolphin, and an elephant). And sauropods are among the most vexing of all. Little sauropods grew at such an astonishing rate that there’s almost no way they could have done it without having highly active metabolisms and probably maintaining high body temperatures—but that ability could come at a cost at larger sizes.
Some paleontologists have highlighted sauropods as perfect examples of gigantothermy—a strategy in which body temperature remains more or less constant because of sheer size, rather than metabolism. A dinosaur the size of Apatosaurus would have had a hard time gaining or losing heat, but if the dinosaur was ectothermic and conserved heat rather than actively generating it, the giant would have had a little more physiological leeway before being in danger of overheating. As a corollary to that, some researchers have cast sauropods as giant walking compost heaps—kept warm by the breakdown of all that plant material inside them. But these views still see sauropods as huge reptiles rather than as the unique creatures they were. From indentations on their bones, we know sauropods had systems of air pockets along their vertebral columns—especially their necks—similar to the air sacs branching from the respiratory systems of today’s avian dinosaurs. These air-filled structures not only made the dinosaur’s skeleton lighter, but may have acted as a kind of air-conditioning system, as it does in birds, allowing sauropods to cope with excess heat while on the move.
Admittedly, the physiology and biology of sauropod dinosaurs is a fast-changing area of research. This much is clear, though—whatever sauropods did, it worked. They were not an evolutionary fluke, but a group in which truly gigantic size evolved multiple times, culminating in some of the largest vertebrates of all time.
As we wonder about how, we face the equally daunting question of why some sauropods became giants. Over the years, authorities have suggested an array of different ideas: that size increase was a defense against predators; that the Mesozoic atmosphere contained more oxygen, and therefore let dinosaurs breathe more efficiently; or even that the pull of the Earth’s gravity wasn’t as intense in the past. None of these environmental factors stand up to scrutiny. The real secret of sauropod size is an irony: they got so big because they started off small.
Contrary to the sweet stories of doting dinosaur parents in The Land Before Time, and even in the less well-known Baby: Secret of the Lost Legend, sauropods did not have single offspring that they attentively nurtured. In reality, Little
foot’s mom would have actually had many more offspring and cared for them considerably less. From fossil eggs and nests, we know that mother Apatosaurus, Supersaurus, and Argentinosaurus laid clutches of a dozen or two relatively small eggs.
I didn’t really understand just how small baby sauropods were until I got a chance to visit the finished version of the AMNH’s World’s Largest Dinosaurs exhibit. Huge models of dinosaur organs and a life-size sculpture of Mamenchisaurus dominated the show, but behind the exhibits showcasing the features of the largest dinosaurs, a small display presented a model of a sauropod nest. The eggs were about the size of grapefruit, and the exhibit’s baby sauropods could have curled up in the palm of my hand. The tiny dinosaurs would have provided little more than a quick snack for a passing theropod, or, as shown by lovely fossil finds in India, constrictor snakes that slithered through sauropod nesting grounds.
By starting out small, researchers have found, sauropods were freed from the biological constraints that have limited mammal body size. These constraints explain why there aren’t any Apatosaurus-sized mammals walking around today. Taking a cue from fossil mammal expert Björn Kurtén, in 1990 the Brown University paleontologists Christine Janis and Matthew Carrano looked at the different ways the largest dinosaurs and the heftiest land mammals reproduced. While sauropods laid sizable clutches of relatively small eggs that they tended fairly briefly, if at all, elephants, giraffes, and other large mammals gestate small numbers of offspring—typically just one—for very long periods of time. And, after birth, mammal babies continue to be an energy draw on the mothers as they require milk and attention. These features of the way mammals reproduce—long pregnancies during which much can go wrong, and infants with intense and prolonged energy needs—put limits on mammals that dinosaurs did not experience.
