Article: Tex Had "some of the best vision in animal history"

["Richard W. Travsky" <rtravsky@uwyo.edu>]

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

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 objectseven 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.
...
Stevens also considered visual acuity and limiting far pointthe 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.

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."

Stevens, K.A. 2006. Binocular vision in theropod dinosaurs. Journal of 
Vertebrate Paleontology 26(June):321-330.