LOS ANGELES » Part of the oldest museum in Los Angeles County has been turned into a home for the aged — and the ages.

With specimens that date back 4 billion years, the "Age of Mammals" exhibit opens today in the north wing of the Natural History Museum of Los Angeles County.

It’s the first permanent museum exhibition in the world to trace 65 million years of evolution based on geology and climate, said curator John Harris.

"We are trying to figure out why the change took place, not just describe how the change took place," he said.

"Normally, mammals evolve to adapt to their surroundings. If that were the simplest part of it, after the dust cleared, after the asteroid exploded and the dinosaurs had been wiped out, then mammals would have filled the habitats left vacant by the demise of the dinosaurs, and that would have been the end of the story. But it wasn’t."

That’s because a restless Earth wouldn’t allow it.

"We’ve seen that humans themselves are a product of climate change. Now we are at the point where we are causing and contributing to climate change," Harris said.

The centerpiece of the exhibit is a 20,000-year-old mastodon found in Simi Valley, just 40 miles northwest of Los Angeles. A relative of the elephant, it is also the exhibit’s largest complete specimen, standing 8 feet 9 inches tall.

There is a paleoparadoxiid, an extinct relative of elephants and sea cows, that lived on the California coast about 11 million years ago. And there is an ancient species of sperm whale whose fossilized bones have been assembled for the first time by any museum and seems to float above the exhibit.

"Mammalian history is full of remarkable creatures on land and in the sea, and it is terrific that the museum chose to emphasize the role of climate change and shifting continents in the history of mammals," said Blaire Van Valkenburgh, a professor of ecology and evolutionary biology at the University of California, Los Angeles, and a member of the exhibit’s steering committee.

"It should help people understand the connections between the rocks beneath their feet and the history of life on the planet."

"The Age of Mammals" uses a mounted saber-toothed cat, a giant jaguar and a brontothere, or thunder beast, which resembles a dinosaur more than a mammal, to help tell its evolution story. The largest animal represented in the collection is a Columbian mammoth. A jaw from the 14-foot tall mammal will be on display and stands 2.5 feet high.

The first ever display of a bone-crushing dog’s complete skeleton is part of the show. The 8 million-year-old species represents the largest dog family ever evolved. With its powerful jaws and teeth, the bone-crushing dog could crack the bones of its prey, like a modern hyena.

Big makes bold statements in the exhibit, but little is there, too. The smallest mammals represented are an extinct mouse and the shell of a fossil land snail.

The museum, with its trademark rotunda and three wings, opened on Nov. 6, 1913, as the Los Angeles Museum of History, Science and Art. Through the years it has collected more than 35 million objects, some as old as 4.5 billion years.

Now called the Natural History Museum of Los Angeles County, it includes the 1913 building, the Page Museum at the La Brea Tar Pits and the William S. Hart Park and Museum in Newhall. In the early 1960s the art portion of the museum moved out and became the Los Angeles County Museum of Art.

Renovation of the 1913 building is part of a $107 million project that will include two more exhibitions (including "Dinosaur Mysteries"), a demonstration center, a nature lab, pedestrian bridge, a park and nature space, more gallery space, new stores and a cafe.