July 2010

 

IN OTHER NEWS

 

Through the eyes of dinosaurs


by Enette Ngoei Senior EyeWorld Staff Writer

 

 

A computer science professor looks at how the prehistoric creatures saw the world

In June 1993 after a visit to the Royal Ontario Museum in Toronto, Canada, Kent A. Stevens, a computer science professor at the University of Oregon, Eugene, Ore., had just presented a talk on binocular vision at the International Conference and NATO Advanced Research Workshop on Binocular and Optic Flow when he met paleoartist Garfield Minott.

Minott was working on a life reconstruction of the Tyrannosaurus rex in collaboration with Thomas Carr, and Stevens was surprised to find the T. rex not looking like a reptile. “[The T. rex] looked almost mammalian in the way its head was built,” Stevens said.

Observing the differences in the eye position of certain types of dinosaurs, the professor became interested in studying the binocular vision of these prehistoric creatures and had Minott create a number of life reconstructions of seven therapod dinosaur heads for him. These were grouped into two categories: the first were allosauroids, which had eyes pointing laterally, and the second were coelurosaurs, which had eyes facing forward.

Stevens then used what he calls “inverse perimetry” to map the binocular fields of view of the dinosaur models and built software to compile and analyze the data. He explained his method of measurement on his website: “While conventional perimetry involves a human patient or subject reporting on the visibility of a probe that is presented at varying eccentricities and elevations, my technique of inverse perimetry permits estimation of whether a given probe would be visible, based on whether there is a clear, unobstructed view of the pupil along a line of sight at a given eccentricity and elevation.”

For visibility’s sake, the dinosaur pupil was illuminated by a laser, and Stevens positioned himself on one side of a glass plate with the model on the other side. The plate was used to record the locus of visibility of the laser’s glint at different elevations. Stevens said his inverse perimetry method is based on the Bugblatter Principle, named after the Ravenous Bugblatter Beast of Traal from The Hitchhiker’s Guide to the Galaxy by Douglas Adams. The creature was so stupid that it assumed if it put a towel over its head and couldn’t see you, then you couldn’t see it.

Reproducing dinosaur heads

The sculptures were based on a very good reproduction of the skull, and though it is not certain how much flesh covered them, it’s sure to be the case that the flesh was shrink wrapped, Stevens said. Some other issues involved with the reproduction were how big the eyeballs were and how far sunken in they would have been, Stevens said.

“We didn’t systematically study that but with the exception of birds, all reptiles and most mammalians protect the eye with the boney socket around the eye and only animals like birds and frogs have protruding eyes, so we felt we were pretty accurate about the position of the eye and the amount of flesh around it,” he said.

Stevens then put everything into context by compiling known data about the binocular vision of living animals, namely birds and reptiles. “The thing with dinosaurs is they fit evolutionarily some place between birds and reptiles so I had to use some sort of bracketing technique to try to get an estimate bracket on the capability resolution of binocular vision,” Stevens said.

Getting that information was difficult because very few people have studied the vision of reptiles, he added.

How dinosaurs saw the world

Stevens’ research culminated in a paper published in the Journal of Vertebrate Paleontology in 2006. He titled it, “Binocular Vision in Therapod Dinosaurs.” Stevens wrote in the paper, “The tall, narrow snout and laterally facing eyes of the allosauroids Allosaurus and Carcharodontosaurus restricted binocular vision to a region only approximately 20° wide, comparable to that of modern crocodiles.

In contrast, the coelurosaurs Daspletosaurus, Tyrannosaurus, Nanotyrannus, Velociraptor, and Troodon had cranial designs that afforded binocular fields between 45–60° in width, similar to those of modern raptorial birds.”

The paper also examined the binocular field width and predatory style of ambush versus pursuit living animals and discussed cranial adaptations that enhance binocular vision. Stevens said that according to his research, there is an argument to be made that the T. rex used binocular vision in its lifestyle because there seems so much specialization in the skull in support of it. “That’s kind of cool because if you look at modern animals that have a lot of binocular vision, they tend to be pretty mean pursuit predators. If you look at ambush predators like crocodiles, they don’t need as much binocular vision because they’re much more interested in being still and seeing what’s approaching them,” he explained.

Having found a large disparity in the amount of binocular vision between the two categories of dinosaurs he studied, Stevens said it’s tantalizing to think that they reflect different predatory styles.

Another interesting conclusion the paper made was the size of the T. rex’s head and eyeball, producing an amazing visual capability. The fearsome prehistoric creature had eyeballs separated far enough apart to get really good distance vision binocularly, and probably had binocular vision down the snout and detailed vision off to the side like a modern hawk, he said.

Contact information

Stevens: kent@cs.uoregon.edu

Through the eyes of dinosaurs Through the eyes of dinosaurs
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