|Bringing Home the Brontosaur|
Allosaurs may have been attentive parents that dropped chunks of meat into the mouths of their young, but large and small tooth marks seen together on the bones of prey also suggest that adults and juveniles ate dinner side by side.
Bob Bakker bends over a dissecting microscope in a makeshift laboratory at the Wyoming Dinosaur International Society, meticulously cleaning dinosaur and crocodile teeth that have not seen daylight in 147 million years. His scope sits on a counter beside dirty coffee mugs, cooking utensils, and a microwave oven crowned with a toy mastodon. All around are plastic refrigerator dishes and Quaker Oats containers filled with drill bits, dental picks, razor blades, and fossils. Listening to National Public Radio, he ignores the clutter and contentedly focuses on the task at hand. Right now he is going to work on a peanut-size piece of gray mudstone, scanning for evidence of a Jurassic-era struggle.
"Who's the chewer and who's the chewee?" Bakker asks as he examines a tiny black, fossilized triangle. Compared with the giant skeletons that spin museum turnstiles, this embedded crocodile tooth is an unprepossessing fleck. Seen through Bakker's eyes, however, it becomes a remarkable bridge between past and present. Shed teeth could tell where a long-dead creature lived, what it ate, how it migrated, even what kind of company it kept.
"Crocs are the ideal test, because Jurassic crocs look like modern ones. The crocs are our control," Bakker says. The pattern of their behavior suggested by the fossil record, he finds, closely matches that of today's crocodiles. If the fossil evidence can be trusted for ancient crocs, it should be valid for extinct dinosaurs as well. That tantalizing connection is what keeps Bakker glued to the microscope. Ever since he fell in love with dinosaurs as a boy in New Jersey, he has wanted to understand what those petrified remains were like when they were living, breathing, fighting, bleeding creatures. Teeth may tell him.
By analyzing "dino junk"his name for about 2,000 dino and croc teeth, plus bits of chewed-up bone and turtle shell that he has gathered from fossil beds in Como Bluff, WyomingBakker has sketched a life portrait of the giant carnivorous dinosaurs that once terrorized Earth. The species he has the most evidence for is Allosaurus,
a slightly smaller but equally fierce forerunner of Tyrannosaurus rex.
In Bakker's view, these killing machines were surprisingly social animals with complex behaviors: When allosaurs weren't tackling plant eaters 10 times their size, they were dragging "bronto brisket" back to the lair and feeding it to their young. In the dino-eat-dino world, adult allosaurs guarded their offspring, which lived at home until fully grown. When the dry season came and food became scarce, "Mom and Dad and the kids went off and spent summer at the lake," Bakker says. With typical irreverence, he refers to his discovery as dinosaur family values.
Although his language is jovial, Bakker's mood is intense as he continues to scrape away at the chunk of rock, slowly exposing the crocodile tooth. His gray bird's-nest beard dangles beside the microscope. His meaty hands rest under the lens in the sunspot of an old Tensor lamp goose-necked down toward the diminutive fossil. Only his fingers flex. When he reaches a stopping point, he looks up, happy: "This is my world. Some days I get to do it all day, most days for only an hour. An hour spent collecting specimens in the field requires up to 20 hours of cleaning. Everybody likes to collect. Not everybody has the patience to clean."
Staunch self-sufficiency defines the Bakker style. The reason his room at the Wyoming Dinosaur International Society is full of kitchen implements is that it is, in fact, the kitchen of his home in suburban Boulder, Colorado. Upstairs is the main workspace, where he analyzes, draws, and writes up his findings. He has the reputationunfair, he saysof spurning peer-reviewed journals. He does little to hide his disdain for the "pompous, priestly language" of the academic establishment. There is no mistaking Bakker's slatted stool for a university chair in paleontology, or his crowded kitchen for the granite halls of a museum.
"He's brilliant, and he's willing to go out on a limb," says John McIntosh, a vertebrate paleontologist and retired professor of physics at Wesleyan University in Connecticut, who has been a close friend for more than 30 years. Many of Bakker's colleagues ask to remain anonymous when they talk about him. A specialist in dinosaur anatomy offers this assessment: "He's a wild man. He's a maverick and incredibly bright. His only failing is he'll never back down. Vertebrate paleontologists are very full of themselves. He likes to needle them, partly because they are
so full of themselves." An assistant collection manager at a major museum says, "If he were more cautious and more of a team player, his reputation would be better. But he's a lone wolf."
Still, this wolf isn't baying at the moon. As a Yale University undergraduate, Bakker wrote a paper challenging the prevailing view that dinosaurs were squat, stodgy lizards. He concluded from anatomical studies that the alleged sluggards were actually "fast, agile, energetic creatures." In earning his Ph.D. from Harvard University in 1976, Bakker expanded the case for warm-bloodedness. A lengthy 1975 article in Scientific American
helped reignite interest in the bygone behemoths and legitimize the then-radical idea that birds evolved from a branch of dinosaurs. "In terms of public awareness, that was the turning point," says Philip Currie, the dinosaur curator at the Royal Tyrrell Museum in Drumheller, Alberta. Bakker consolidated his thinking in a 1986 best-seller, The Dinosaur Heresies.
In 1995 Bakker wrote an adventure novel, Raptor Red,
starring a resourceful Utahraptor. Raptor Pack,
an illustrated summary of his research aimed at young readers, will be published this month.
When he is not writing, Bakker is out collecting. For nearly 30 years, he has crisscrossed the desolate high prairie of Como Bluff. The site, a 10-mile succession of outcrops almost 7,000 feet above sea level, contains one of the first known mass dinosaur graveyards. In 1868 paleontologist Othniel Charles Marsh rode the transcontinental railroad out to unexplored Wyoming Territory. By the late 1870s, Marsh and his men were extracting a mother lode of dinosaur skeletons, including the first named Brontosaurus,
from Como Bluff's Jurassic strata. For the next century paleontologists had their hands full just piecing together the bones, describing dinosaur anatomy, and identifying species.
"What has lagged behind is putting the dinosaurs into the environment," Bakker says. Back when he was a graduate student, scientists commonly believed that giant sauropods, such as Apatosaurus
languished in swamps because their legs could not support their massive body weight on dry land. Studying the heft of brontosaur leg bones made Bakker suspect otherwise. On one of his earliest Como expeditions, he worked with paleoecologist Kay Behrensmeyer. "Kay looked at the sediments and said, 'These aren't swamps; these are dry floodplains.' She was absolutely right. So my conclusion was, these guys didn't live in swamps. They lived on dry turf," he says.
In the Jurassic, Como Bluff contained several sharply contrasting habitats located within a relatively small area, making it the perfect place to reconstruct the lifestyles of the dead and famous. As Bakker continued to comb the region from the 1970s to the 1990s, he became singularly adept at distinguishing species not only by their bones but by smaller bits, especially teeth. By noting the type of sediments in which teeth from different species were found, he began to see a way to illuminate the behaviors of the extinct creatures he had loved since boyhood.
What makes the leap possible is a trait shared by dinosaurs, crocodiles, and many other nonmammalian vertebrates. Instead of growing one set of adult teeth, these creatures replace their teeth continuously. As the roots dissolve, each surface tooth is eventually loosened and pushed out by a developing tooth rising from below. Critical to Bakker's research is the proposition that teeth are usually shed during eating, when a chomp of the jaw causes the loosened crown to fall out. "Dinosaurs left behind shed teeth where they fed, both babies and adults," he says. If so, each tooth contains a message: A dinosaur ate here. Moreover, such teeth are easy to identify. A shed tooth looks different than one that was in the jaw at the time of death. The latter has its root attached forever, even if the tooth eventually falls out of the desiccated jawbone. "A shed tooth is like a footprintif the fossil record is faithful, it should map out dinosaur life journeys," says Bakker.
This smallish Jurassic croc weighed in at about 150 pounds. Shed teeth indicate its young had to fend for themselves. Adult teeth are blunt, designed to crush turtles and big snails. "But hatchlings had delicate, pointy teeth, because baby crocs go after bugs, frogs, and small mammals," Bakker says.
But is it faithful? This is where crocodiles come in. Today's crocs are little changed from those of the Jurassic. Bakker realized that if he could match the pattern of fossil croc teeth from Como Bluff with the behavior of the modern animals, that would validate the dental evidence. Some of the Como sites were lake beds in the Jurassic, and others were dry floodplains"surf and turf," in Bakker's cheerful terminology. Shed teeth from crocodiles, he predicted, should be rare in turf sites and plentiful in surf sites. Hatchling teeth should be found at surf sites that were close to shore and not at sites that represent deep water. And all the fossil remainsshed teeth, teeth attached to roots, and skeletal bonesshould be found together, because crocodiles don't migrate. "By gum, the fossils show that," Bakker says.
Each dinosaur at Como Bluff carved out a unique ecological niche. "When I went to school, people said you couldn't tell families of meat eaters apart by their teeth, but they really have different shapes," Bakker says. "Allosaurs have coarse serrations down the trailing edge; ceratosaurs have fine serrations. You don't need a microscopeyou can feel them." Allosaurus teeth are also thicker in cross section, befitting the top predator of the Jurassic west.
Ceratosaurus probably thrived on fish by the water's edge, where Bakker finds most of the dinosaur's shed teeth. Tooth shapes hint at the diversity of behaviors among Jurassic predators, he says. "Ceratosaur teeth are sharp, long, delicate, and rarely worn. Ceratosaurs were pretty careful in their chewing; they were making filets. Megalosaurs had thick, coarse teeth, good for crushing stuff. Allosaur teeth are sort of in the middle."
Bakker began intensively collecting shed teeth, both dinosaur and crocodile, in 1990. Although parts of Como Bluff have been scoured for souvenirs by amateur paleontologists and trinket hunters, most of the ridge is private ranchland that has been protected by its owners since the 1890s. Early in his career, Bakker won access to those ranches in return for a promise to preserve significant finds at museums within the state. In addition to allosaurs, he has found remains of two other large late-Jurassic predators, Megalosaurus
and Ceratosaurus. "Ceratosaurus
was a wonderful and unusual design. It was the only carnivore with a big horn on its schnoz, and it had huge, incredibly sharp teeth. I saw that in a book when I was in the fourth grade, and I thought that was really neat. So, naturally, when I started leading my own expeditions to Como, I wanted to find one. Well, we didn't find any. We were finding lots of beautiful allosaur shed teeth," Bakker says. At a location known as Nail Quarry, 98 percent of the teeth were from allosaurs.
As it happened, the first beds Bakker excavated in Como were turf sites. When he and his students began to dig at a prolific surf site called Claw Quarrybingo! Ceratosaur shed teeth. "I said, 'Wow, we haven't been finding ceratosaur teeth because we've been looking in an allosaur habitat.' Then it dawned on me. It's been known for a long time that ceratosaurs had a low, long body and a deep, flexible tail. A really good swimmer. Duh! That's why we're finding its teeth at surf sites. Allosaurs were much leggier and stiffer-bodied. A ceratosaur could swim rings around an allosaur. That was the first great epiphany: Ceratosaurs preferred different habitats from allosaurs."
Eventually, Bakker excavated 33 separate sites in Como Bluff. As digging proceeded, he started investigating a question that had long perplexed him: Did allosaur parents feed their young? "The books I grew up with said that the chicks of carnivorous dinosaurs had to catch prey all by themselves, the way baby crocodiles do," Bakker says. The teeth tell a different tale. Under the microscope, Bakker could see that baby allosaur teeth are miniature replicas of the adult teeth. To him, that similarity carries a significant message: "They ate the exact same thingbig hunks of meat. If baby allosaurs were feeding by themselves on little things, they'd need different teeth." Sure enough, bones of prey exhibit both baby and adult tooth marks. Equally important, the baby shed teeth are always found with adult shed teeth, implying that adult and child were eating in the same place. The family that shed together, fed together.
In similar fashion, Bakker concluded that allosaurs maintained guarded lairs where they ate and protected their young until they were fully grown. As Bakker uses the term, lair
means not a nest or a den but a feeding area, potentially something as simple as a shaded spot on the plain. These sites contain abundant allosaur shed teeth mingled with bones from giant herbivores' meaty parts, such as the rump, thigh, and upper tail. Feet, which offered less nourishment, are missing. The sediments at these sites are fine-grained, meaning they were deposited by slow-moving water. "These carcasses didn't wash in," Bakker says. "It looks like somebody dragged them in."
At Como Bluff, Bakker has found whole teeth, but virtually no shed teeth, from Camarasaurus and other giant herbivores. He takes this strange absence as a sign of mass migration. "They were just passing through," he says. "They weren't stopping to feed. It wasn't Jurassic Park; it was Jurassic Parkway." At some Cretaceous sites, in contrast, shed herbivore teeth are quite common. Those locales may have been more favorable environments where plant-eating dinosaurs stayed put for long periods.
Piled with pungent carcasses, lairs almost certainly would have attracted other predators, hoping to poach some food or make a meal of the vulnerable babies. But the sites that are rich in allosaur teeth do not contain shed teeth from other predators, Bakker found. Evidently, the beasts maintained good security. Teeth from all growth stages appear together in these lairs, hinting that allosaurs, like modern hawks and eagles, maintained extended families in which older siblings sometimes helped their parents raise the next generation.
Bakker sometimes casts himself in the role of a detective reconstructing a crime scene, referring to predators as "perps," prey as victims or "vics," and shed teeth as "fired bullets." He makes another analogy that may be even more apt: "It's like biblical archaeology, where context is everything." Bakker has a deep interest in Scripture and theological history. In his college days, he preached on the sidewalks of Upper Broadway in New York City. His fascination with archaeological reconstructions has also taught him to be skeptical about giving too much weight to a single piece of evidence, no matter how spectacular. "If you were to come from a dig with one beautiful pot, you'd be drummed out of the corps. Where did it come from? What was around it? What was the trace of the house in which you found it? The object out of context is worth very little," he says.
To Bakker, the same holds true in paleontology. A pristine and complete fossil skeleton is not only a raritya freak of nature arising from sudden death and immediate burial in the proper kind of sedimentit is also a near-useless prize. "It tells you very little because it was never part of the food chain. The better looking the specimen, the less information it contains. You want chewed-up junk," he says. "Much more interesting."
Recently Bakker pieced together all of that junk evidence into a provocative scenario of allosaur family life in Como Bluff. With its subtropical climate, Jurassic Wyoming had a wet season and a dry season. The wet season, as Bakker pictures it, was a time of abundance. Food and drinkherbivorous dinosaurs and waterwere plentiful on the open meadows and savanna. Allosaurs reigned supreme, judging by the prevalence of shed teeth and the many bones from dismembered herbivore carcasses.
|Allosaurs probably dragged bits of carcasses back to their lairs. That behavior would explain the concentration of large, chewed bones at certain sites. |
In the dry season, the long-necked brontosaurs and armored stegosaurs probably trundled elsewhere, creating hard times for the allos. Bakker wondered how they survived. Did they hunker down and make do with small prey? Did they scavenge? Or did they pack up and spend summer at the lake? As Bakker compiled his census of allosaur teeth at the Como Bluff turf sites, he found a strange gap in the sample. The distribution of allosaur shed teeth was not even across all size ranges, as is the case with crocodiles, which stay all year in one locale. Rather, there was a significant deficit of teeth from juveniles that were about one-third of their adult size.
Bakker set off on the trail of the missing teeth. One possibility is that the allosaurs had vamoosed and that the fast-growing juveniles had shed those teeth while the pack was at its summer home. So Bakker looked at teeth from primarily aquatic sites, such as the edges of ancient lakes. When he toted up the allosaur teeth from those surf sites, he found a bulge in the size distribution that coincided with the gap at the turf sites. Now the evidence fell into place: This is where the young allosaurs were hanging out when they were one-third grown.
Summer at the lake was no vacation. Judging from the shed teeth, all the local predators gathered there, drawn to bodies of water large enough to withstand evaporation. Ceratosaurs were in their element, hunting lungfish. Megalosaurs, big bone-crunchers with heavy-duty teeth, were also butting in. Bakker imagines the scene from the point of view of a juvenile allosaur: "You're in a dank place that smells of mud and rotting fish. You smell all the other species of predators. Mom and Dad are nervous. There are a lot more fights. You're still fed, but totally different food. Chunks of fish, bits of turtle and crocodile. Smaller portions. And there's danger from adult crocodiles. Get too close to the water, and one of your siblings disappears in a swoosh of muddy foam. For a couple of months, maybe half a year, you're there. Totally stressed. Maybe you're growing, but not as fast as you were. Then the spring rains come, big thunderstorms, and you and your parents return to where you were. Now it's back to bronto brisket, your favorite."
Bakker tells a good story. "His lectures get people really energized," says Kay Behrensmeyer, now at the Smithsonian Institution. She has remained friends with Bakker since they were graduate students at Harvard. "But the scientific community, at least, often walks away wondering, 'How much of this can we really believe, in terms of documentation?' I've always found Bob provocative and intellectually stimulating. The downside is that he hasn't documented a lot of his ideas in peer-reviewed journals. He did a lot of publishing early on, but subsequently he hasn't kept pace." Philip Currie of the Royal Tyrrell Museum echoes this concern. "There's no reason Bob couldn't publish in peer-reviewed journals more often, and I wish he would, frankly. I think sometimes he's just not patient enough for the peer-review process," he says. Bakker responds that he does submit his work for review but that he prefers museum bulletins and symposia proceedings because "they publish longer papers and place more emphasis on long-term field work." His latest, most complete shed-tooth study will appear in September as a peer-reviewed monograph published by the University of Indiana Press.
Some of Bakker's colleagues also question the way he interprets his data. Jack Horner of the Museum of the Rockies, who gained fame for unearthing the world's largest T. rex
skeletons, doubts that shed teeth can identify a dinosaur feeding ground. "Dinosaurs shed their teeth on a regular basis. It was like a conveyor belt. Just because you find shed teeth doesn't mean they were feeding on something," he says. Brent Breithaupt, director of the Geological Museum at the University of Wyoming, is as familiar as anyone with Como Bluff, but he, too, is not yet convinced: "What I have one of the biggest problems with is the idea that 150 million years ago an activity occurred right there
at a given spot. If we're talking about footprints, you can say, 'Yes, a dinosaur was here.' But I still believe that the teeth and bones were deposited somewhat randomly through a change in velocity of a stream, or something like that."
|A juvenile allosaur confronts a crocodile defending its territory during the dry season. "Being the top predator is tough," Bakker says. "We see evidence of broken bones, compression fractures, bite wounds, bacterial infections that eat away the jawbone. Our allosaurs just get whacked."|
Bakker is undaunted. "Sure, some teeth were shed away from feeding sites, but we find only a few such cases. The key point is that predators leave copious shed teeth where they feed heavily," he says. "At the risk of sounding like a proud parent, I can say that the patterns we've shown to exist right now have never been documented by anybody, anywhere." These patterns cannot describe a literal day in the life of a dinosaur, Bakker agrees, but his real interest lies in developing a time-averaged portrait of dinosaur behavior. Behrensmeyer, for one, is impressed with Bakker's studies of crocodile teeth, which show that the animals' lifestyles remained consistent for tens of millions of years. After a conversation with Bakker last fall, she began collecting and measuring crocodile teeth to guide her study of the environment of human ancestors in Africa. And Bakker's notion of dinosaur family life has inspired other researchers. In Canada, Argentina, and Japan, paleontologists have found bones of adult and juvenile carnivorous dinosaurs mixed together "in concentrations that are just too high to be coincidental," Currie says.
Every time he hops into his dusty red Toyota 4Runner for the three-hour drive from Boulder to Como Bluff, Bakker says, "I feel like I'm going home." Roaming over the vast stretches of desolate rolling prairie and stark outcrop, he muses on the stories still buried in the rock. "Eventually, I think we will be able to tell female shed teeth from male. The females should be more calcium stressed at certain times, when they're growing embryos, and that should turn up in the growth rings."
Bakker's confidence is unshakable, his energyat age 58inexhaustible. One day last winter, he spent six hours prospecting at one of his richest surf sites, "squatting and scraping, scraping and squatting," as he puts it. Before he started, he challenged his helpers: "We don't stop for lunch until we find a lungfish jaw." The sun was bright, the wind cold and steady. Chopping the loose, eroded soil with screwdrivers and sifting it with fingers, no one found anything nearly as dramatic as a lungfish jaw. For Bakker, the exercise was productive all the same. He is testing his potential successors, looking for patient workers who will put in long days to add a few brushstrokes to the allosaur family portrait. "Can I put my e-mail address in the story so graduate students can write to me if they want to do this?" he asks. Bakker can be reached at email@example.com
The Museum of Vertebrate Paleontology at the University of California at Berkeley maintains an exhaustive compendium of links to dinosaur-specific Web sites: www.ucmp.berkeley.edu/diapsids/dinolinks.html
Bakker's "Inside the Head of a Tiny T. rex" (Discover,
March 1992, page 58) relates his and his colleagues' research into the structure of dino skulls. Using CT scansand coat hangersto look into the interior of dinos' braincases, he explores the evolutionary relationships between carnivorous dinosaurs, both large and small, and their avian descendants. This article is available at www.discover.com
For more from Bakker, see "Jurassic Sea Monsters" (Discover,
September 1993, page 78), Bakker's discussion of Jurassic sea monsters and how extinction patterns of these ancient swimmers differ from those of land-dwelling dinosaurs. This article is available at www.discover.com