Computer scientist Ken Stevens explores the quality of vision in meat eating dinosaurs using models and lasers. Stevens believes that dinosaurs, like the Tyrannosaurus Rex, had wide stereoscopic vision, which made them formidable hunters rather than scavengers.
Full Text :COPYRIGHT 1998 Kalmbach Publishing Company
Stevens is seeing what dinosaurs saw. His view flesh or spied its dinner with the eyes of a killer.
Ten pairs of beady reptilian eyes stare placidly at Kent Stevens as he works to uncover the sixty-five-million-year-old secret they withhold. He ogles them right back and strokes the bony brows of the heads where they rest. If only he could glimpse the past and see what they saw.
But these archaic orbs are ready-made glass eyes inside sculpted dinosaur models born of clay. At first glance, they lend few clues about how the monsters of the Mesozoic surveyed their world -- whether Tyrannosaurus rex, for example, scoped its landscape with acute depth perception, calculating the distance to its prey as a keen hunter would, or whether the animal's limited vision could manage only to search out another beast's bloody kill site and get the leftover victuals to its mouth.
Yet these heads divulge much.
"The models helped me estimate how the bumps adorning the nose and brows had an effect on vision," says Stevens, a computer scientist at the University of Oregon.
Knowing how vision was affected offers some clues about how dinosaurs lived their lives. Using lasers and the modeled heads, Stevens has, for the first time ever, mapped the field of view for T. rex and other sharp-toothed dinosaurs such as Velociraptor and Troodon. His results show that these meateaters had wide binocular vision, with both eyes working together to provide depth perception -- a characteristic of many modem hunters. Scientists have long suspected dinosaurs might have possessed this trait, but they had no supporting evidence.
Stevens has the evidence. He also thinks the faces of some meat-eating dinosaurs evolved in ways that enhanced this binocular vision. And these theories help bolster arguments that T. rex was a predator and not the scavenger some think it was.
"I think it's just one more line that emphasizes the fact that T. rex had a much better set of senses than once thought," says paleontologist Phil Currie of the Royal Tyrrell Museum in Alberta. "I think it could help settle some of the disputes."
It was a movie -- the 1994 blockbuster hit Jurassic Park -- that got Stevens thinking about settling those disputes. In one scene, a raging T. rex breathes down the shirts of a few petrified park visitors. Actor Sam Neill, portraying a paleontologist, says, "Keep absolutely still. His vision is based on movement." They do, and the beast turns to pluck a quivering lawyer off a toilet seat. Dinner.
Stevens, who has a research background in artificial intelligence and has studied human vision for twenty-five years, thought the paleontologist's comment was absurd. It implied that T. rex did not have depth perception, or stereopsis, which requires that individual lines of sight from each eye overlap. The area where these two monocular fields of view converge is called the binocular field of view. It's in this range that the eyes can see in depth. They can because each eye sees an object from a slightly different angle and projects a slightly different image onto each retina. This provides the brain enough information to estimate depth.
Not all animals have this. Some have widespread eyes whose fields of view don't overlap but offer a panoramic scope of the world. But it's depth perception that allows us humans to thread a needle much more easily with two eyes than one. When people lose sight in one eye, their depth perception suffers, although the brain, over time, often compensates for the loss.
Without good depth perception, T. rex might as well have adopted the hunting strategies of a frog. The amphibian's primitive vision can pick out a fly zigzagging across a flat background but otherwise can't see in depth until the fly flits within tongue-lashing distance. Or the Jurassic Park T. rex could have tried a strategy to imitate depth perception. It could have bobbed its head from side to side to see one object from two slightly different angles. It's the same effect that binocular vision creates at a standstill. But what does a bobbing head do? "It gives away the predator," Stevens says.
Around the same time Stevens saw the movie, he was in Toronto giving a talk on stereopsis. On a whim, he visited the Royal Ontario Museum. There he found paleo-artist Garfield Minott carving a T. rex. Stevens' intrigue dilated. Talking to Minott and seeing his sculpture sent Stevens on an archaic vision quest. Perhaps he could conduct some research to prove T. rex had the binocular vision he believed it did. He commissioned Minott to sculpt a few heads for him.
Back in Oregon, the computer scientist focused on a project to map the dinosaurs' visual fields. Meanwhile, Minott fined the scientist's lab with a menagerie of model heads -- Carcharadontosaurus, Tyrannosaurus, Daspletosaurus, Allosaurus, Nanotyrannus, Velociraptor, and Troodon -- all of varying size and scale but each with the paired glint of glass eyes. And a few little fellas without taxidermic eyes keep Stevens company in the office he calls "the comfy room."
Minott bases his creations on actual skulls, research about their eye sockets and facial structures, and his imagination. "I've been sculpting since I was about three," he says. He's been making dinosaur models for about nine years, giving each head a week or two and each eye about three hours. Sometimes he assiduously studies and photographs the bones he sees in museums, then goes home to fill the empty orbits. Other times, he's luckier. "With Tyrannosaurus rex, I got a chance to play with the actual skull."
Placing the eyes is natural, Minott says. He gets a good feel for where they should go by looking at the eyes of modem animals such as lizards and birds, which are related to dinosaurs. He also considers the location of the empty sockets in the skulls, being careful not to place the eyes in bulging, buggy positions that could throw the study off kilter. "Hardly Kermit the Frog," says Stevens.
One at a time, Stevens fixed each specimen in a laboratory device to keep the head steady and level. He placed a vertical glass plate a few inches in front of the dinosaur's snout. He then positioned a laser pen off to the side of the dinosaur's head, but close enough to illuminate one of the beast's eyes -- let's say the left eye, for example. Next, Stevens stood behind the glass and observed the creature as though he were looking at it through a window. To begin, he marked an "origin" on the window at the midpoint of the two eyes; then he set about mapping the dinosaur's visual field.
It's easy to map the human visual field because people can tell the scientist what they see. "But how do you do that on a dinosaur?" Stevens says. "You can't get on the inside and look out." So he used a little ingenuity and a copy of Douglas Adams' The Hitchhiker's Guide to the Galaxy. With one of his own eyes covered, Stevens viewed the dinosaur's left eye from a location on the animal's far right. Sticking to the "Bugblatter Principle" from the book, "If you can't see it, it can't see you," he marked areas on the plate where he could just make out the dinosaur's left eye over its snout. In short, he was marking the farthest locations the dinosaur could see to the right with its left eye, without its nose getting in the way. And because dinosaurs don't see on one horizontal plane, Stevens marked the locations up and down the glass, using the same technique.
He did this several times for the creature's right eye as well. When he was through, his marks on the glass plate looked something like a set of parentheses. Mathematically, those marks indicate the shape and extent of the area both eyes could see together -- the binocular field of view. He noted with interest the unusual shape of the plots: The widest line of sight was higher than the position from which the dinosaur would have looked straight ahead. This meant the animal's binocular vision was slightly elevated. Ours, by comparison, is wider in the middle and becomes more narrow as we look up or down without moving our heads. Stevens surmised that the dinosaur's wide binocular vision on top meant the creature might have hung its head a bit to get the best view straight ahead.
Stevens scanned his results into a computer and used some self-made software to convert the plots into a diagram he could use to more accurately measure degree angles. He found that Tyrannosaurus, Velociraptor, and Troodon had wide binocular fields of view of fifty degrees or more -- about the same as today's predatory hawks. Allosaurus and Carcharadontosaurus had binocular fields of view half as wide, comparable to alligators, which are known as "ambush predators" because they wait for prey to draw near and then suddenly attack. By comparison, a human has one hundred ten degrees, and that illustrious feline predator, the household cat, has one of the widest binocular fields at one hundred thirty degrees.
Stevens says he could be off a few degrees, depending on unknown variables such as the quality of the cornea, which bends light waves to form an image on the retina. But the margin of error isn't enough to alter his conclusions. And he believes dinosaurs other than the few he's tested probably had binocular vision as well. "I just haven't studied them." Yet.
The visual field is, of course, only one factor in the vision equation. A scientist also must consider how clearly an animal can see, a factor known as resolution. "That brings up questions about how specialized the vision is," Stevens says, because stereoscopic vision can be geared for particular tasks, working well only over a small range of distance. For example, a pigeon has resolution similar to ours but a narrow binocular field, using its depth perception for little else than pecking grains. Humans can see clear images at various distances because we have high resolution and a much wider binocular field. We can clearly see the pigeon at our feet, the grains it's pecking, and a hawk circling high above. Because of its better resolution, a raptor such as the hawk can see the pigeon farther away than we could, but its binocular vision doesn't kick in until it swoops down for the kill and needs to distinguish depth.
Some hunters are really limited by their vision. Ambush predators such as alligators have narrow binocular vision and less than a tenth the resolution of humans. Their best feeding strategy is to he in wait until their prey gets in close sight. Then they strike with precise timing.
Stevens suspects the overall vision of tyrannosaurs, velociraptors and troodons fell between that of modern ambush predators and modern raptors. Those with the widest binocular fields could have used their stereopsis for nocturnal vision, aiding their ability to see at dusk. Or they might have used their stereopsis to make fine spatial distinctions, the way a bird uses it to flit in and out of bushes without bumping into branches. Those with less stereopsis probably used their vision to ambush prey, with narrow binocular fields that adequately could have judged distances directly ahead.
Michael Parrish, a paleontologist at Northern Illinois University thinks Stevens is on track. "There really has never been anyone with a vision science background who's looked at the question of how dinosaurs used their eyes," he says. "It's actually pretty important for dinosaur biology. How they used their eyes is key to how they hunted."
And how they hunted has been a matter of controversy in paleontology. Not surprisingly, the controversy swirls around the massive, fearsome form of T. rex. Its razor teeth and giant jaws originally led paleontologists to say T. rex was one of the greatest predators ever. But others have suggested the mighty dinosaur was just an overgrown scavenger.
One of them is paleontologist John R. Horner at the Museum of the Rockies in Bozeman, Montana. He thinks people simply want to believe the vicious image of Tyrannosaurus. "I think those who cast T. rex as a predator are letting some common prejudices cloud their thinking," he writes in The Complete T. rex, co-authored with Don Lessem. "A predator is more the kind of movie monster that satisfies our heed to be scared out of our minds. I think that is why T. rex is usually depicted as hunting, running full speed, chasing down an animal, flashing huge teeth, and making bloody kills." But he thinks there's even stronger evidence on the scavenger side.
For one thing, predators don't usually chase prey as big as they are. Yet, paleontologists find T. rex teeth marks on the bones of Triceratops, animals nearly as large as T. rex. The locations of these bite marks also lend some evidence to the scavenger theory. Many appear on the butt and calf areas -- bones too wide for T. rex to get its mouth around easily if the Triceratops were alive and kicking. And if T. rex were chasing down prey, how would it attack? It's arms were too short for lashing out, and it might have lost its balance and fallen over if it tried to use its head. A toppled T. rex would take some time getting back up, only to find its prey long gone.
Stevens, however, comes down on the predator side. It's not because he spends too much time at the movies. Nor is it soley because Stevens thinks T. rex had stereoscopic vision. Good depth perception always comes in handy -- for scavengers, too. "It's always nice to know how far away something is as you approach it with your nose, even if it's dead meat. Keeps you from bonking your head into it," he says.
Stevens thinks the big tyrannosaur was a predator because dinosaurs with the most binocular vision also evolved facial traits, such as lower snouts and fewer bony obstructions around the eyes, that enhanced their binocularity. And he thinks creatures generally use their eyes to their fullest extent.
He doubts the dinosaur's heads would have evolved in ways that specifically increased their binocular fields if the beasts weren't using their vision to its greatest capacity. "A major revision of the head to push the snout down and visually out of the way... is not particularly necessary for scavenging. There'd be little evolutionary advantage or pressure to make those adaptations if not for using the binocular field of view for some advantage," Stevens says. If better smell or bite were the goal, he'd expect to see a larger snout. "I'm simply noting that it has a face well suited for binocular vision, and all the restructuring of the face to support that seems to require some explanation."
Horner isn't convinced. "I'd suspect vultures and hyenas and all the scavenging animals alive today all have stereoscopic vision," he says. "Stereoscopic vision is a primitive characteristic. That means that animals before T. rex had it. It's unlikely that stereo vision is a detriment, so it's unlikely he's going to lose it." If someone could prove T. rex were the first creature to see in stereo, "I would concede," Homer says.
Parrish, playing devil's advocate, acknowledges this. But he counters, saying the dinosaur's binocularity seems too wide for it not to have served some greater purpose. "It seems less important to an animal that's just scavenging to have well-developed binocularity," he says.
Stevens and Homer know that no two scientists will ever share identical images of a dinosaur. But they both agree that working on an ancient puzzle has its rewards. For Stevens, the allure lies in his imagination and the menagerie he brings to life. There, he sees not some crude Hollywood interpretation of a prehistoric world, but a sophisticated one with creatures highly adapted to their milieu and deserving the utmost regard.
"It's obvious to my eyes that these dinosaurs were superbly engineered to their lifestyles," he says. "There's no reason to believe these animals were primitive."
Source Citation:Coates, Karen J. "Through dinosaur eyes." Earth 7.n3 (June 1998): 24(8). General OneFile. Gale. Alachua County Library District. 7 Oct. 2009
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