Natural Science Forum / Biology / Paleontology / August 2006

http://www.sciencenews.org/articles/20060701/fob2.asp

Eric Jaffe

In the 1993 movie Jurassic Park, one human character tells another that a
Tyrannosaurus rex can't see them if they don't move, even though the beast
is right in front of them. Now, a scientist reports that T. rex had some of
the best vision in animal history. This sensory prowess strengthens
arguments for T. rex's role as predator instead of scavenger.

Scientists had some evidence from measurements of T. rex skulls that the
animal could see well. Recently, Kent A. Stevens of the University of
Oregon in Eugene went further.

He used facial models of seven types of dinosaurs to reconstruct their
binocular range, the area viewed simultaneously by both eyes. The wider an
animal's binocular range, the better its depth perception and capacity to
distinguish objects-even those that are motionless or camouflaged.

T. rex had a binocular range of 55°, which is wider than that of modern
hawks, Stevens reports in the summer Journal of Vertebrate Paleontology.
Moreover, over the millennia, T. rex evolved features that improved its
vision: Its snout grew lower and narrower, cheek grooves cleared its sight
lines, and its eyeballs enlarged.

"It was a selective advantage for this animal to see three-dimensionally
ahead of it," Stevens says.

Stevens also considered visual acuity and limiting far point-the greatest
distance at which objects remain distinct. For these vision tests, he took
the known optics of reptiles and birds, ranging from the poor-sighted
crocodile to the exceptional eagle, and adjusted them to see how they would
perform inside an eye as large as that of T. rex. "With the size of its
eyeballs, it couldn't help but have excellent vision," Stevens says.

He found that T. rex might have had visual acuity as much as 13 times that
of people. By comparison, an eagle's acuity is 3.6 times that of a person.

T. rex might also have had a limiting far point of 6 kilometers, compared
with the human far point of 1.6 km. These are best-case estimates, Stevens
says, but even toward the cautious end of the scale, T. rex still displays
better vision than what's needed for scavenging.

The vision argument takes the scavenger-versus-predator debate in a new
direction. The debate had focused on whether T. rex's legs and teeth made
it better suited for either lifestyle (SN: 3/2/02, p. 131:
http://www.sciencenews.org/articles/20020302/fob1.asp).

Stevens notes that visual ranges in hunting birds and snapping turtles
typically are 20° wider than those in grain-eating birds and herbivorous
turtles.

In modern animals, predators have better binocular vision than scavengers
do, agrees Thomas R. Holtz Jr. of the University of Maryland at College
Park. Binocular vision "almost certainly was a predatory adaptation," he
says.

But a scavenging T. rex could have inherited its vision from predatory
ancestors, says Jack Horner, curator of paleontology at the Museum of the
Rockies in Bozeman, Mont. "It isn't a characteristic that was likely to
hinder the scavenging abilities of T. rex and therefore wasn't selected out
of the population," Horner says.

Stevens says the unconvincing scene in Jurassic Park inspired him to
examine T. rex's vision because, with its "very sophisticated visual
apparatus," the dinosaur couldn't possibly miss people so close by. Sight
aside, says Stevens, "if you're sweating in fear 1 inch from the nostrils
of the T. rex, it would figure out you were there anyway."