Family feud - genetic evidence seems to show that guinea pigs are not rodents although they look like rodents - Brief Article

Science World, Oct 4, 1996 by Chana Freeiman Stiefel

We told you so! Four years ago, SW reported that some scientists had evidence that guinea pigs were not rodents (see SW 4/3/92, p. 5). Now a new study backs up that claim.

Taxonomists--scientists who classify living things--have traditionally grouped guinea pigs with rodents like mice and rats because of their morphology, or physical appearance. Mice, rats, guinea pigs, and some 2,000 other species have similar teeth, skulls, and jawbones.

But, as we reported in 1992, researchers found something surprising when they began to study the animals' proteins. These long, chainlike molecules, which make up the structure of an animal's body, are most similar among close relatives, explains Winston Hide, one of the researchers. Guinea pigs and mice, he found, have very different proteins. So they can't possibly belong to the same group, he says.

Last spring's study dug even deeper, into the animals' genes--the inherited instructions cells use to make proteins. The genes showed that mice and rats are indeed close cousins. But according to that study, guinea pigs don't belong on the same branch of the mammal family tree, and shouldn't called rodents anymore.

Some scientists still question whether this genetic evidence holds more weight than the morphological evidence. But Hide argues that animals that look alike don't always come from the same ancestor.

For example, sharks and dolphins look similar. But sharks are fish; dolphins are mammals. Comparing physical traits often involves guesswork, Hide adds, but genes don't lie. What kind of evidence do you think is most important?

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DIG THAT DINOSAUR!

This dinosaur skull is so big that paleontologist (fossil scientist) Paul Sereno could probably fit inside its mouth. Sereno found the skull during an expedition in Morocco last year. It belonged to a Carcharodontosaurus saharicus ("shark-toothed reptile from the Sahara Desert").

Based on the size of the skull (1.6 meters from front to back), Sereno estimates the dinosaur was about 13 to 15 meters (45 to 50 feet) long. That makes it slightly bigger than the largest known Tyrannosaurus rex, and "maybe a little bit heavier," Sereno says. In fact, says Sereno, C. saharicus was probably about the same size as Giganotosaurus, the largest meat-eating dino ever discovered (see SW 12/8/95). So who's "king of the dinosaurs" now?