KATIE BURNS

Staff Writer

Long ago, northern San Diego County and Southwest Riverside County were home to camels, rhinos, horses, sloths, bison -- as well as mammoths and other extinct animals.

Over millions of years, the climate changed until literally only bits and pieces of these creatures remain in the form of fossils or the features of familiar fauna.

"Most of the species just have become something else by now," said Scott Rugh, collections manager of invertebrate fossils at the San Diego Natural History Museum.

Paleontology is the study of the natural history of the world via fossils. Fossils come in many shapes and sizes, but all of them provide information about the past in a given region.

"A fossil by definition is simply remains like bones or shells or teeth -- or remains like footprints or burrows -- an indication of prehistoric life," said Tom Demere, curator of the paleontology department at the San Diego Natural History Museum. "The primary characteristic of all fossils is that they're old. A fossil is a remnant of an organism that is typically older than 10,000 years."

The local fossil record dates back only about 100 million years. Dinosaurs never roamed the region because it was probably a string of volcanic islands at the time. The magma also destroyed any existing fossils. And scientists must grapple with more recent gaps in the fossil record, too, for a variety of reasons.

Reading the record

Fossils form only under certain conditions. The remains of plants and animals cannot last long in igneous rock, which comes from molten material, or in metamorphic rock, which comes from inside the Earth. Fossils reside mostly in sedimentary rock, which results from the slow settling of sediment.

"What we have is records of time when sediment accumulated," said Hugh Wagner, collections manager of vertebrate fossils at the San Diego Natural History Museum. "You need these sediments to have a record of the animals that lived."

Other materials such as ice or amber can encompass a plant or animal, but scientists usually find fossils along waterways that transport sediment.

In the case of bone, sediment preserves a skeleton in several ways. The bone lasts for a while -- as does the cavity in which it resides -- but it lasts longest if replaced partially or completely by minerals carried in water.

"Most times, there is some kind of mineralization going on to preserve the bone," said Michael Woodburne, a paleontologist at UC Riverside. "That would be the best way to do it."

A few fossils in the area linger from the age of fishes in the Paleozoic Era. A handful of fossils also dates to the Jurassic Period of the Mesozoic Era, the age of reptiles. Most of the fossils remain from the Cretaceous Period and the epochs of the modern era. To fill in the remaining holes, paleontologists study fossils from neighboring regions.

"You look around at other places that were geologically more feasible," Woodburne said.

Other areas often have the right rocks for a time period to preserve fossils. But even then, fitting together the puzzle is still difficult during the time periods when flora and fauna underwent rapid changes, said Philip De Barros, an anthropology professor at Palomar Community College in San Marcos.

Ancient animals

The Natural History Museum has clams from the Jurassic Period, but the collection from the Cretaceous Period is more extensive. The museum has dinosaurs that washed out to sea, along with more clams, snails, oysters and sea urchins. It also has shells from ammonites, extinct relatives of the spiral-shelled chambered nautilus, which could reach 3 feet in diameter.

The fossil record of the Paleocene Epoch, the first of the age of mammals, is missing. But the following epoch shows an astonishing array of mammals.

"The rocks here preserve a portion of the Eocene Epoch," Demere said. "That was an important time in mammalian history, when a lot of our modern families of mammals were evolving. It's a really amazing menagerie of what you would think of as exotic mammalian species."

The Eocene Epoch included varieties of camels, rhinos, whales, horses and primates as well as creatures that resembled pigs, sheep, deer, dogs, cats and shrews. In the Oligocene Epoch, mammals grew more advanced. They included saber-toothed cats, rabbits, beavers and jumping mice. At the same time, birds became common.

In the Miocene and Pliocene epochs, marine life diversified. Walruses, sea cows, fur seals, baleen whales, toothed whales, sea otters and giant sharks abounded, along with water fowl.

Throughout most of the past 100 million years, the region was warmer and wetter than at present.

"We have tropical species up till 120,000 years ago," Rugh said. "We're still almost tropical. We're still considered subtropical."

In the Pleistocene Epoch, the ice age, mammoths and mastadons ruled the land along with bison and bears and even sloths. The epoch also contained many animals common to the Holocene Epoch, which began about 10,000 years ago.

"This contains a lot of the fauna similar to the ones that are living today," Wagner said.

But in the next 10,000 years, the world could change enough to make pelicans, rattlesnakes, cougars, lizards and other modern species seem exotic to the future inhabitants of the region.

Contact staff writer Katie Burns at (760) 740-5442 or kburns@nctimes.com.

Cenozoic Era (age of mammals)

Quaternary Period

Holocene Epoch

began 0.01 million years ago

Pleistocene Epoch

began 1.6 million years ago

Tertiary Period

(Neogene)

Pliocene Epoch

began 5 million years ago

Miocene Epoch

began 24 million years ago

(Paleogene)

Oligocene Epoch

began 38 million years ago

Eocene Epoch

began 55 million years ago

Paleocene Epoch

began 66 million years ago

Mesozoic Era (age of reptiles)

Cretaceous Period

began 138 million years ago

Jurassic Period

began 205 million years ago

Triassic Period

began 240 million years ago

Paleozoic Era

(age of amphibians)

Permian Period

began 290 million years ago

Carboniferous Period

Pennsylvanian began 330 million years ago

Mississipian began 360 million years ago

(age of fishes)

Devonian Period

began 410 million years ago

Silurian Period

began 435 million years ago

(age of marine invertebrates)

Ordovician Period

began 500 million years ago

Cambrian Period

began 570 million years ago

Precambrian Era (origin of Earth)

began 4.5 billion years ago

9/16/01