In the public imagination then, dinosaurs were plodding, thunderous monsters, cold-blooded and stupid. Even paleontologists had lost interest in these “symbols of obsolescence and hulking inefficiency,” Ostrom’s student Robert T. Bakker later wrote. “They did not appear to merit much serious study because they did not seem to go anywhere: no modern vertebrate groups were descended from them.”
—RICHARD CONNIFF, THE MAN WHO SAVED THE DINOSAURS, 2014
THE NEW CONQUEST OF CENTRAL ASIA
The ambitious paleontologist Henry Fairfield Osborn, head of the American Museum of Natural History, wanted to explore the long-neglected Gobi Desert region for fossils of early humans (see chapter 14). Like many scientists of his time, he believed that humans first evolved in Eurasia, not in Africa as Darwin had suggested (because our closest relatives, chimps and gorillas, live there). Anthropologists were convinced that modern humans evolved in the rigorous climates of Eurasia, and their deeply ingrained racism prevented them from considering that humans might have originated in Africa, where the “inferior” races were found. Fossils like “Java man” and “Peking man” (now both considered Homo erectus), plus the hoax known as “Piltdown man” (“discovered” in 1912 but not exposed until 1953), only whetted his appetite for reaching this untapped resource of fossils.
Once World War I had ended and the economy was booming, Osborn set about raising funds among his rich relatives and friends, swaying them with promises of amazing fossil finds of early humans. Finally, in 1922, the American Museum sponsored one of the most ambitious scientific expeditions ever attempted (figure 17.1). Led by the legendary explorer Roy Chapman Andrews, the expedition traveled to China and Mongolia with a huge caravan of 75 camels (each carrying 180 kilograms [400 pounds] of gasoline and other supplies), three Dodge touring cars and two Fulton trucks, and a large party of scientists, guides, and helpers. The party included not only Andrews but also paleontologist Walter Granger, a veteran of many fossil-hunting expeditions in the United States and elsewhere, who was experienced in hunting fossils in China, as well as two geologists (Charles P. Berkey and Frederick K. Morris). Many other assistants were needed to drive the trucks and cars and camels, cook the food and set up the camp, and act as guides and interpreters. Osborn told Andrews, “The fossils are there. I know they are. Go and find them.”
Figure 17.1
The Flaming Cliffs of Shabarakh Usu, Mongolia, with the American Museum camel caravan in the foreground. (Image #410767, courtesy of the American Museum of Natural History Library)
The “biggest scientific expedition ever to leave the United States” passed through a gate in the Great Wall of China on April 21, 1922, and headed into Mongolia. Aided by their cars, they traveled 426 kilometers (265 miles) in just four days, which was much more efficient than any previous expeditions mounted on horse or camel. Over the course of the many expeditions, they faced bandits, blinding sandstorms, blistering heat and freezing cold, and lots of dangerous vipers, but the trip went off with relatively few problems.
THE REAL “INDIANA JONES”?
Roy Chapman Andrews (figure 17.2) was a flamboyant and colorful character. One of the last classical “scientific explorers” who was not a scientist with a doctorate in a particular specialty, Andrews’s gift was in raising funds, leading and organizing the trips, and in conveying the excitement of his many exploits to the general public through his popular books and lectures (which further aided in fundraising). Despite his image, Andrews was not trained as a paleontologist, nor was he good at collecting fossils. In the Mongolian expeditions, the real paleontologists urged him not to try to excavate the fossils before they got there to do the job properly because he often damaged the fragile specimens. When someone botched the collection of a fossil on the trip, or butchered it trying to get it out of the ground, they would say it had been “RCA’d” (after the initials for Roy Chapman Andrews).
Figure 17.2
Roy Chapman Andrews. (Courtesy of Wikimedia Commons)
Despite his limitations, Andrews was a bold and fearless leader. Several times Andrews scared off Mongolian bandits by shooting before they could draw their weapons (and occasionally using guns to intimidate corrupt border guards or greedy officials). In one incident, he charged the bandits with his car, shooting as he approached, and the bandits fled as their horses were spooked. Many people consider him the model for the “Indiana Jones” character played in the popular movie series by actor Harrison Ford, but George Lucas, Steven Spielberg, nor anyone else connected with the films has ever confirmed this. In fact, they have said that Indiana Jones is based loosely on a number of heroic movie explorers of the Silver Screen that they grew up with (although some of those may have been inspired by Andrews).
Born in Beloit, Wisconsin, in 1884, Andrews taught himself marksmanship and taxidermy, and he earned a degree from Beloit College (paid for by his earnings from taxidermy). When he talked his way into the director’s office and asked to work at the American Museum as a taxidermist, he was told there were no openings, so he started as a janitor. While mopping floors, he earned a master’s degree in mammalogy at Columbia University. In 1909 and 1910, he joined an expedition on the USS Albatross in the East Indies, where he collected lizards and snakes and studied mammals. In 1913, he was on the crew of the schooner Adventuress in the Arctic, where they hoped to obtain a bowhead whale specimen. That effort failed, but they filmed some of the best footage of seals ever seen. In 1916 and 1917, Andrews led the American Museum’s Asiatic Zoological Expedition through western and southern Yunnan Province in China, where he collected many specimens and developed valuable skills and contacts in Asia.
By 1920, Andrews was planning the first of several American Museum expeditions to Mongolia. The first, in 1922, was a short exploratory trip to find out whether there were any fossils at all. They were so successful that that they returned in 1923, 1925, 1928, 1929, and 1930. Almost immediately after arriving in Mongolia, fossils were found, and their second expedition found spectacular dinosaur bones and the first dinosaur eggs. This made the expeditions world famous and helped fundraising for three additional expeditions. They found not only dinosaurs but also the first good specimens of tiny mammals from the Age of Dinosaurs. But by the time of the last expedition, the political situation in Mongolia had deteriorated so badly that no further expeditions were possible.
By the 1930s, Andrews’s ability to mount great expeditions to Asia had ended. The Great Depression worldwide made it impossible to raise funds to mount another trip. Many of the formerly rich museum donors had lost their fortunes in 1929 and 1930, and some of the museum’s investments had become nearly worthless as well. By 1932, the museum was so strapped for funds that it canceled all fieldwork entirely and cut its staff to the bone. In addition, tensions between China and Japan were rising as the Japanese prepared to invade China and other parts of Asia.
Andrews spent much of his time in the 1930s writing books about his exploits; he was also designated one of the first “Honorary Boy Scouts” and served as president of the Explorers’ Club (1931–1934). In 1935, Andrews was appointed director of the American Museum of Natural History, replacing Osborn, but he was unable to do much to help the museum during the depths of the Depression. Despite his great skills in organizing expeditions and raising money for them, he proved to be so inept at running the museum that the trustees replaced him in 1941. It took two more directors, plus the end of the Depression and World War II, for the museum to recover its strength. Andrews retired to Carmel-by-the-Sea, where he lived out the rest of his life writing popular books until his death in 1960 at the age of 76.
WALTER GRANGER, PALEONTOLOGIST
The other key figures in the American Museum’s Central Asiatic Expeditions of the 1920s were Walter Granger (figure 17.3) and Henry Fairfield Osborn (see figure 14.1). Granger was the main paleontologist on all of the expeditions, and there could not have been a more competent person assigned to the task. Born in Vermont in 1872, he was one of five children of Civil War veteran and insurance agent Charles H. Granger. Like Andrews, he developed an early talent for taxidermy, and by 1890, when he was only 17, he got a job doing taxidermy at the American Museum. Within a few years, he went on field expeditions to the American West searching for vertebrate fossils with the American Museum paleontologists. After two field seasons (1894 and 1895), his fossil-hunting talents were better appreciated, and he was transferred to the Department of Vertebrate Paleontology in 1896. Granger discovered the legendary Bone Cabin Quarry near Laramie, Wyoming, in 1897, which he worked for the next eight field seasons. The site yielded thousands of bones representing 64 species of dinosaur, including the mounted skeletons of the big Apatosaurus and Stegosaurus on display at the museum today.
Figure 17.3
Walter Granger, the chief paleontologist on the expedition. (Courtesy of Wikimedia Commons)
After working Bone Cabin Quarry from 1897–1906, Granger accompanied Osborn on an expedition to the Eocene-Oligocene Fayûm beds of Egypt in 1907. This was the first American Museum fossil trip outside North America. They made many important discoveries that complemented what had been described by British Museum paleontologist Charles W. Andrews just a few years earlier. By this point, Granger was promoted to assistant curator, and his flexible schedule allowed him at least five months in the field every year to find more fossils. He continued to write two or three scientific papers each year as well.
In 1921, Granger began explorations in China, which eventually led to the discovery of “Peking man” in the Zhoukoudian caves near Beijing. This work in China laid the foundation for negotiations to go through China to Mongolia that allowed the 1922 American Museum expedition to succeed. When he finished the expeditions, he was promoted to curator of fossil mammals, allowing him to continue to work on his many amazing discoveries until his death at age 68 of heart failure in 1941. Entirely self-taught, Granger never earned a formal academic degree; this was rectified in 1932 when he received an honorary doctorate from Middlebury College.
Andrews, Granger, and American Museum preparator Peter Kaisen were in the badlands of Outer Mongolia on August 11, 1923, late in the first field season, when Kaisen spotted bone weathering out of the rock. He and Granger collected it, protected it with a plaster jacket, and shipped it to New York at the end of the season. When it was prepared, the specimen turned out to be a nearly complete skull of a small predatory dinosaur (badly crushed), plus a large sickle-shaped claw (figure 17.4A). In 1924, Osborn described the specimen as Velociraptor mongoliensis (fast robber from Mongolia). Nothing else was known of the fossil, so it was impossible to tell much about the rest of the dinosaur. In fact, nothing more would be known about this head and claw until the Polish-Mongolian expeditions in the early 1960s. When those specimens were found, it turned out that the huge sickle-shaped claw was on the second toe of each hind foot. The rest of the dinosaur was built rather lightly, with a long thin neck, a long narrow tail, and strong forearms and hands with sharp curved claws. Complete specimens showed that the biggest Velociraptor was the size of a large turkey (figures 17.4 and 17.5), with a total length of only 2 meters (7 feet) and weighing about 15–20 kilograms (33–44 pounds).
Figure 17.4
(A) The original type skull of Velociraptor mongoliensis. (B) The famous “fighting dinosaurs” specimen found by the Poles in Mongolia, which entombed a Velociraptor attacking the head of Protoceratops. (C) The mounted skeleton of Velociraptor. ([A] Courtesy of Wikimedia Commons; [B] courtesy of D. Fowler; [C] photograph by the author)
The “fighting dinosaurs” was most famous specimen the Poles found in Mongolia (figure 17.4B). A Velociraptor skeleton was buried in a collapsed sand dune and fossilized in a pose attacking a Protoceratops. The specimen vividly shows that Velociraptor had gripped the edge of the frill of the prey with its hands, and its sickle-clawed feet were slashing the throat of the Protoceratops and that the Protoceratops had a bite on the arm of Velociraptor. Eventually, over a dozen different genera of extinct animals like Velociraptor would be discovered and placed in a group known as the dromaeosaurs (“running lizards” in Greek). The group got its name after the first genus to be discovered, Dromaeosaurus albertensis, found in 1914 by Barnum Brown in the Upper Cretaceous beds of the Red Deer River of Alberta (figure 17.5).
Figure 17.5
Comparison of the size of different dromaeosaurs. On the man’s arm is the four-winged Microraptor gui. At his feet is the Canadian Cretaceous taxon Dromaeosaurus albertensis, which gives the group its name. To the right is the large Austroraptor cabazai. Below it to the right is the turkey-sized Velociraptor mongoliensis, followed by the largest dromaeosaur of all, Utahraptor ostromi. On the extreme right is Deinonychus antirrhopus. (Courtesy of Wikimedia Commons)
Meanwhile, another important discovery, the third dromaeosaur to be found, would put Velociraptor and the dromaeosaurs into context.
“TERRIBLE CLAW”
The discovery of the fossil that changed our understanding of dromaeosaurs like Velociraptor can be traced to one man: John Ostrom (figure 17.6). Born in New York in 1928, Ostrom planned on being a doctor like his father until he read George Gaylord Simpson’s 1944 book, Tempo and Mode of Evolution. This changed his life plans completely, and he set out to make a career in paleontology. He got his BA degree in 1951 at Union College, but in 1950 he was a field assistant to his idol George Gaylord Simpson. This helped him become a student at the American Museum under Ned Colbert, where he began working on dinosaurs. After getting his PhD from Columbia University, Ostrom taught at Brooklyn College (1955–1956) and Beloit College (1956–1961) before becoming the curator of fossil reptiles at the Peabody Museum of Natural History of Yale University in 1961. There he inherited the Marsh Collection, and he spent the rest of his career there, from 1961 until he retired for health reasons at age 65 in 1993. From the 1960s to the 1990s, he mentored a lot of PhD students at Yale, including many American dinosaur paleontologists. If they were not Ostrom’s students directly, they were students of Peter Dodson of the University of Pennsylvania, one of Ostrom’s first acolytes. Ostrom passed away at age 77 in 2005 after years of suffering from Alzheimer’s disease.
Figure 17.6
John Ostrom with a sculpture of his most famous discovery, Deinonychus. (Courtesy of Wikimedia Commons)
I remember John well, both as a friend and as an inspiration. In 1994, I invited him to give a talk on birds and dinosaurs for a short course on vertebrate evolution that his former student Robert Schoch and I had organized, and his presentation was the highlight of the session. I can still see him giving talks with the excitement in his voice, a twinkle in his eye, looking down at his audience through the half-rimmed glasses perched on his nose, and giving us all a sly grin. In fact, nearly all of my generation of vertebrate paleontologists knew him well as a jovial, provocative, and inquisitive man who never accepted dogmatic answers and handled his forays in major controversies without losing his calm or the smile on his face. He was famous not only for his discoveries and insight but for his unsparing search for the right answer, and his unflinching honesty and integrity.
Early in his career, John embarked on a series of field seasons working in the Lower Cretaceous Cloverly Formation of the Bighorn Basin in Montana. Barnum Brown (chapter 21) had shown Ostrom what turned out to be an early specimen of a dromaeosaur that he never got a chance to describe before he died. Brown found it in the Cloverly Formation in Montana, but it sat in the American Museum unstudied for decades. Ostrom was looking for Early Cretaceous dinosaurs, which were virtually unknown in North America (in contrast to the abundant Wealden dinosaurs of Britain, or the huge diversity of Lower Cretaceous dinosaurs in Asia). As recounted by Richard Conniff in “The Man Who Saved the Dinosaurs”:
But dinosaurs had begun to look a lot more interesting one afternoon in late August, 1964, near Bridger, Montana. Ostrom and his assistant Grant E. Meyer had been walking a landscape of prairie punctuated with rocky, eroding outcrops, considering sites for the following summer’s fieldwork, when they spotted a large, clawed dinosaur hand protruding from the earth on a slope just below them. They scrambled down, dropped to their knees beside it, and because they hadn’t brought their toolkit, began digging excitedly with their hands, and then with their jackknives, turning up a scattering of the serrated teeth of a predator. Next day, returning with proper tools, they unearthed an astonishing foot. Two of three toes had ordinary claws. But from the innermost toe, a sharp sickle-shaped claw curved murderously up and out. It had a slashing arc, Ostrom later calculated, of 180 degrees. Hence the eventual name Deinonychus, or “terrible claw.” Ostrom and his crew spent two full field seasons digging at the site and three years in study and reconstruction at the Peabody, working with more than a thousand bones from at least four individuals of the same species. Then in 1969, Ostrom announced what he called a “grandiose” conclusion: that foot was “perhaps the most revealing bit of anatomical evidence” in decades about how dinosaurs really behaved. In place of the plodding, cold-blooded dinosaur stereotype, Deinonychus “must have been a fleet-footed, highly predaceous, extremely agile, and very active animal, sensitive to many stimuli and quick in its responses,” Ostrom wrote.
Deinonychus was not only an amazing creature (figures 17.5 and 17.7), but its anatomy completely forced a rethinking of the “slow sluggish” dinosaur image. Its tail was long, straight, and pointed, and was held rigid by a truss of crisscrossing struts of bone from the vertebrate (now turned to stone). With such a rigid structure, the tail could not have dragged on the ground, instead serving like a tightrope walker’s balancing pole. Deinonychus was completely bipedal, yet to use the huge slashing claws on its feet it would have to leap up and strike with its entire foot. This simply was impossible for a sluggish reptile that was slow and inactive.
Figure 17.7
Deinonychus: (A) mounted skeleton in a dynamic pose; (B) reconstruction showing the feathers that would have covered its body. ([A] Photograph by the author; [B] courtesy of N. Tamura)
This is the animal that thrilled movie audiences watching the Jurassic Park movies—except instead of calling it by the proper name, Deinonychus, author Michael Crichton and director Steven Spielberg opted to call the dinosaur Velociraptor. According to some accounts, Crichton was misled by a 1988 book by dinosaur artist Greg Paul, who falsely argued that Velociraptor and Deinonychus were the same dinosaur, making Velociraptor the first valid name. Other accounts suggest that Crichton just thought Velociraptor was easier to read, spell, and pronounce or that it sounded cooler than the correct name.
Unfortunately, the movies got the science completely wrong. First of all, the actual Velociraptor was the size of a large turkey (figure 17.5). In addition, Velociraptor is only known from Mongolia, yet the expedition finds it in “Snakewater, Montana” (which was actually filmed in Red Rock Canyon State Park, California, where the beds yield Miocene mammals, not dinosaurs). Third, Velociraptor and most small predatory dinosaurs had feathers. There are even specimens of Velociraptor from Mongolia with quill knobs on their arm bones showing where their largest feathers attached. Thanks to Crichton and the movies, the general public now has a slightly more accurate image of what dinosaurs (especially the “raptors”) looked and acted like, but everyone attaches the wrong name to the animal that has become so famous. For example, the Toronto NBA team is called the “Raptors” but shows images of the large dromaeosaurs like Deinonychus—even though the name they use is that of the turkey-sized Velociraptor.
This small but bad choice by Crichton and the moviemakers still drives paleontologists crazy! The other annoying mistake is the fact that Velociraptor/Deinonychus had feathers (figures 17.5 and 17.7B), something we’ve known since 1996. The moviemakers refuse to put feathers on their dinosaurs, so the science is not up to date in the last three Jurassic Park movies. Only the first movie was relatively accurate for its time.
HOT AND COLD RUNNING DINOSAURS?
The beginning of the Dinosaur Renaissance, or the change in the way we thought of dinosaurs—from slow sluggish lizards dragging around in swamps to active animals—can be traced to Ostrom’s work. In 1963, he wrote a paper arguing that duck-billed dinosaurs were not slow, stupid swamp dwellers but active land-based herbivores that could be compared to buffalo. His discovery of Deinonychus in 1964 accelerated his general rethinking of how dinosaurs were built. If Deinonychus was an active, hopping, fast-running, jumping, and slashing predator, then maybe the rest of the dinosaurs were more active and agile than once thought. Ostrom also pointed out that dinosaurs all had fully upright posture, with their limbs beneath their bodies, so they were never slow, sprawling, sluggish creatures like crocodilians or lizards. Instead, they seem to have been active fast runners, more like elephants and mammals, and especially birds. Soon other discoveries, such as additional dinosaurs with rigid tails and trackways that showed how fast dinosaurs could run and how they never left tail drag marks, confirmed the “running dinosaur” model.
Once Ostrom began to think about dinosaurs as active creatures, the next obvious question was, “What was dinosaur physiology like?” If Deinonychus and many other dinosaurs were fast and agile and active, wouldn’t they also be warm blooded? Ostrom cautiously suggested these ideas in a famous paper given at the first North American Paleontological Convention in Chicago in 1969, and he talked about it occasionally afterward. But his former student at Yale (and later Harvard PhD) Bob Bakker aggressively marketed the idea of “hot-blooded dinosaurs” until he was world-famous for it, even publishing cover articles in National Geographic and Scientific American. During the entire 1970s and early 1980s, the battle over “hot and cold running dinosaurs” raged in scientific meetings and in publications and even in popular books. Eventually, however, the topic reached a limit of how much we could really know about ancient extinct animal physiology. Richard Conniff described it this way:
Bakker had latched onto many of Ostrom’s ideas as an undergraduate and, to Ostrom’s occasional chagrin, he ran with them. Bakker—“the infamous Bob Bakker,” as Peter Dodson, another former Ostrom student, says—became the outspoken advocate of dinosaurs as active, warm-blooded, and even “superior” animals. “Where John was cautious, Bob was evangelical,” Dodson and Philip Gingerich later wrote. “Each deserves considerable credit for revolutionizing our concept of dinosaurs.” In his book The Riddle of the Dinosaur, science writer John Noble Wilford added that Bakker “was the young Turk whose views could be dismissed by established paleontologists. Ostrom, however, could not be ignored.” Late in 1969, Ostrom took the challenge directly to the North American Paleontological Convention in Chicago, declaring in a speech that there was “impressive, if not compelling” evidence “that many different kinds of ancient reptiles were characterized by mammalian or avian levels of metabolism.” Traditionalists in the audience responded, Bakker later recalled, with “shrieks of horror.” Their dusty museum pieces were threatening to come to life as real animals.
The problem with talking about dinosaur metabolism is that vertebrate physiology is more complex and not amenable to oversimplifications such as “cold-blooded” animals and “warm-blooded animals.” There are actually two components to physiology: the source of the heat and whether the heat is regulated. Animals that get their body heat from the environment are called ectotherms, and those that burn food to create body heat through metabolism are called endotherms. Animals that let their body temperatures change with the surrounding temperature are called poilkilotherms, and animals that try to hold their body temperature constant are called homeotherms.
In the modern world, the boundaries seem pretty clear: all living birds and mammals are homeotherms and endotherms, and the rest of the animals are all poikilotherms and ectotherms. Homeothermic endotherms can live in almost any environment, no matter how hot or cold, but they pay a heavy price in that they burn most of the food they consume for metabolism. Poikilothermic ectotherms regulate their body temperature by moving in and out of hot and cold areas, and if it gets too cold or too hot, they die. For example, a desert lizard typically has a higher body temperature than a “warm-blooded” mammal like you when it is running or active—but it regulates its temperature by shuttling between sun and shade, or burrowing down into the cool sand, not by burning food.
But even with these broad generalizations, there are exceptions that are informative. For example, ectotherms like pythons can generate body heat by shivering when they are incubating their eggs, and sea turtles, tuna, sharks and even some insects are capable of some endothermy. Many homeotherms (such as the platypus, sloths, and certain rodents, shrews, and small birds) let their body temperature fluctuate tremendously, as do animals that go into torpor when they hibernate. These animals allow their body temperature to drop as they go into their suspended animation state. At the other extreme of body size, camels are famous for letting their body temperature cool down during the cold desert night. With their large body mass and small surface area, it takes a long time for the heat of the desert to warm them up. They can even let their bodies reach unusually high temperatures at the end of the day because they are about to cool down in the cold desert night.
During the peak of the controversy, there were lots of arguments back and forth about the possible evidence for dinosaur endothermy. French paleophysiologist Armand de Ricqles pointed out that dinosaur bones have large canals for blood vessels inside them, called Haversian canals, a feature found in mammals but not in reptiles. But it turns out that the presence of these canals is also affected by body size and rate of growth. Some large ectotherms (sea turtles, tortoises, and crocodilians) also have them, but some small birds, bats, shrews, and rodents may not have them. The presence of Haversian canals seems to be more an indicator of rapid growth to large body size than an explanation for endothermy. Dinosaurs are now known to have had extremely rapid growth after they hatched, which better explains the Haversian canals.
Bob Bakker’s favorite argument was talking about ratios of biomass of predators in a food pyramid to the biomass of prey. When the predator is an endotherm (say, a lion), the biomass of prey species needs to be about 10 times that of the biomass of lions, because most of the lion’s food goes to body heat. In other words, the predator/prey biomass ratio is about 1:10. If the predator is an ectotherm (say, a crocodile), it eats rarely and doesn’t use its food for body heat but for activity and growth, so the biomass of predatory crocodiles to the biomass of prey species can be almost equal (the predator/prey biomass ratio is 1:1). When you look at the predator/prey ratios in Early Permian assemblages from Seymour, Texas (preyed upon by the fin-backed protomammal Dimetrodon), Bakker claimed that the ratio was about 1:1, but for most dinosaur faunas, the prey biomass is about 10 times that of predators.
This sounds convincing at first, but on closer examination it breaks down. Too many factors bias which animals are fossilized and which are not, so you cannot interpret fossil collections as perfect reflections of what was originally living. Museum collections tend to be highly biased because the collectors are after only the spectacular skulls and other diagnostic parts, and they don’t take an unbiased sample of what was present in the collecting area. Lots of things just don’t fossilize well, or are overabundant or rare, and these factors may have nothing to do with biology. For example, there are numerous examples of dinosaur quarries that are nothing but predators (such as the Falcarius quarry in Utah or the Cleveland-Lloyd Quarry in Utah, which is full of allosaurs). The famous La Brea tar pits in Los Angeles have far more predators and scavengers (primarily dire wolves and saber-toothed cats, plus vultures and predatory birds) than then do prey species. If you took this overabundance of predators at face value, it would suggest a world entirely full of carnivores who were mostly cannibals, with almost no prey species to eat.
Many other arguments were debated back and forth, and the “hot-blooded” dinosaur controversy raged for several decades, but it now seems to be resolved. So were dinosaurs endotherms or ectotherms? The answer is, “It’s complicated.” Certainly the smaller predatory dinosaurs (like the “raptors” of Jurassic Park fame) were endotherms. With their small body size and high levels of activity, they would need a high metabolism to be successful. Indeed, there is good evidence that “raptors” and most predatory dinosaurs (including even T. rex) were covered by a downy coat of feathers for insulation; these animals were not slow and stupid but active and smart and warm-blooded.
But for huge dinosaurs like the sauropods, size presents a different problem. At such large body sizes, they have a relatively small surface area compared to their huge volume (see chapter 9). Remember, area only increases as a square, but volume increases as a cube, so the volume increases much faster than the surface area in larger sauropods. They had no obvious ways of rapidly gaining or losing heat from their bodies.
The living elephants are a good example of this physiological dilemma. At its huge size, an elephant must spend much of its time in mud or water or resting in the shade to dump excess body heat. Its huge ears are primarily used as radiators to shed heat from its body. Most sauropods would have had even greater difficulties if they were endotherms and generating body heat from their metabolism of food. Instead, such large beasts kept warm because of the warm climates around them. With their large size, they would have gained or lost body heat only very slowly, so they could obtain a stable warm body temperature by sheer size alone. This strategy is known as “inertial homeothermy,” or “gigantothermy,” and it probably characterized all of the larger nonpredatory dinosaurs, including sauropods, and possibly stegosaurs, horned dinosaurs, duckbills, and many others.
It is instructive to consider the big mammals, the largest living endotherms, as a reality check. The largest living endotherm is the elephant, which is nearly at the thermal limit for an endotherm. Most of its day is spent finding places to cool down, and it feeds mostly in the cool of the night. Some of the largest extinct mammoths and the gigantic hornless rhinoceros known as Paraceratherium are about twice as large as the living elephants, suggesting that there is a size limit for a land-dwelling endothermic animal. No land mammalian endotherm has ever gotten bigger. Yet some of the huge sauropods are 10 times the mass of elephants or Paraceratherium, and there is even less convincing evidence that they had efficient heat-regulating surfaces that would allow endothermy to work for an animal with such an unfavorable surface area to mass ratio.
However, the idea of inertial homeothermy in these animals has been challenged recently by Martin Sander and colleagues. They argue that the long necks and tails of sauropods gave them much more surface area than has been suggested, all of which could have helped radiate some of their excess body heat. They also point to the enormous air sacs and passages that connect to the lungs, which could have allowed for passing the air from their bodies in a one-way flow system similar to modern birds, ventilating a lot of body heat from their core body temperature.
At the moment, it’s hard to decide which system makes more sense. No one has done the rigorous physiological calculations and modeling to test Sander’s model and establish whether sauropods had enough surface area in their skins and respiratory system to ventilate their huge furnaces of bodies running as endotherms. For that matter, no rigorous studies to see whether inertial homeothermy is a viable option have been done either. For now, we’ll just have to reserve judgment until rigorous studies have been done to show which model is more plausible.
FOR FURTHER READING
Alexander, R. McNeill. Dynamics of Dinosaurs and Other Extinct Giants. New York: Columbia University Press, 1989.
Bakker, Robert T. “Anatomical and Ecological Evidence of Endothermy in Dinosaurs.” Nature 238, no. 5359 (1972): 81–85.
——. The Dinosaur Heresies. New York: Morrow, 1986.
Carpenter, Kenneth. The Carnivorous Dinosaurs. Bloomington: Indiana University Press, 2005.
Chinsamy, Anusuya, and William J. Hillenius. “Physiology of Nonavian Dinosaurs.” In The Dinosauria, 2nd ed., ed. David B. Weishampel, Peter Dodson, and Halszka Osmólska, 643–659. Berkeley: University of California Press, 2004.
Colbert, Edwin. Men and Dinosaurs: The Search in the Field and in the Laboratory. New York: Dutton, 1968.
Conniff, Richard. “The Man Who Saved the Dinosaurs.” Yale Alumni Magazine, July/August 2014. https://yalealumnimagazine.com/articles/3921-the-man-who-saved-the-dinosaurs.
Desmond, Adrian. Hot-Blooded Dinosaurs: A Revolution in Paleontology. New York: Dial Press, 1976.
Farlow, James. “Predator/Prey Biomass Ratios, Community Food Webs and Dinosaur Physiology.” In A Cold Look at the Warm Blooded Dinosaurs, ed. Roger D. K. Thomas and Everett C. Olson, 55–83. Boulder, Colo.: American Association for the Advancement of Science, 1980.
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