NATURE'S LITTLE EXPERIMENTS. Horses with claws. Elephants with shovels for lower jaws. Camels with giraffe necks. Dog-bears.

The Natural History Museum's refurbished hall, "Mammals in the Limelight," shows the animals of North America during the 50-odd-million years between dinosaurs and ice age.

There were Tillodonts and Creodonts and Uintatheres.

"The problem with some of these," says Robert Emry, curator of fossil mammals, "is that they don't have any modern representative. The only names we have for them are the scientific names."

Not so the horse, trotted out to center stage here. Eighteen inches tall, with four toes on each foot, "dawn horse" can be found at the end of a trail 55 million years long. Poised at the exhibit's entrance, its skeleton resembles that of a large jackrabbit.

More than racetracks and rodeos separate it from modern descendants. The differences are as basic as hoof and mouth -- which changed as grassland replaced forests, and horses began grazing, instead of browsing in trees and shrubs for food. In the exhibit, horse skulls in perpetual smiles show how horse teeth came to have roots to the bottom of the jaw. And the evolution of the foreleg is traced, toes becoming more vestigial until the horse stands on a single tiptoe.

In the rest of the mammal hall, recent evolutions include more open exhibit areas. It's a third more filling than the old hall of mammals: Invited in now are the plants and fish, birds and bugs, lizards and turtles that shared the North American continent. And four new murals representing different eras present the mammals as they might have looked had they decided to powwow -- playtime in the Oligocene epoch.

In addition, many Green River shales are displayed, a superb collection of sculpture from the workshop of Mother Nature. Formed in sediments of an ancient lake system in parts of Utah, Colorado and Wyoming, the fossils make master brushwork look like fingerpainting -- deftly carved leaves of sumac and fern, a delicately pressed school of herring, bird footprints. Fly larvae never looked so good.

MAMMALS IN THE LIMELIGHT -- At the Museum of Natural History.