North County Fossils Reveal a Lively History

North County history began about 5 billion years ago. But certain knowledge about prehistoric North County begins around 76 million years ago. From these fossils, assumptions about what life was like in North County through the ages can be made. A close look at that fossil record can be seen in an exhibit of San Diego County fossils on display at the San Diego Natural History Museum in Balboa Park until Nov. 24.

This brief history of North County’s earliest days was compiled with the assistance of Tom Demere, chairman of the paleontology department of the San Diego Natural History Museum, and Matt Colbert, also a paleontologist at the museum. CRETACEOUS PERIOD

About 76 million years ago Most of North County is under water. Waves are breaking over San Marcos; the ocean shore is just east of El Camino Real.

The seas, which have risen and fallen dramatically through the eons, are at a high point.

The air is humid and what land there is looks more like Costa Rica than California. In fact, most of the Earth is subtropical. Crocodiles live in Alaska.

Broad-leafed flowering trees pollinated by insects dominate the landscape.

There is life, especially in the waters of the North County coast. Large crabs, clams, oysters and ammonites, an extinct cousin of the modern chambered nautilus, feed on smaller animals.

Probably the most fearsome sea creature is the mosasaur, a giant sea lizard that prowls the coast.

The dinosaur population is beginning to decline. In another 10 million years, dinosaurs will be extinct. But, at this point, at least two are living along the shore of North County--a hadrosaur and a nodosaur.

The hadrosaur, also called the duckbilled dinosaur, is about 30 feet long and walks on four hoofed legs. It feeds on shrubs and trees, probably standing on its hind legs to reach tasty morsels. The nodosaur is part of a group of armored dinosaurs. It’s about 14 feet in length, walks on four legs and also feeds on plants.

THE PALEOCENE

About 66 million years ago The scene is about to change dramatically--some say with a whimper, some say with a bang.

According to one theory, an asteroid about 6 miles across slams into the Earth. A worldwide dust cloud of heavy metals obscures the sun. Plants die. Herbivores like the hadrosaur and nodosaur, which had evolved into new creatures over the past 10 million years, die. Many carnivores die, too. The seas become almost devoid of life.

Another theory holds that the Earth’s climate is changing rapidly on its own during this period. Sea currents are moving in different directions, shallow seas are drying up. Volcanoes are erupting and the Earth’s plates are sliding and subducting. The climate changes so much that animals cannot adapt, and they die.

Whatever the cause, most life around the world dies out, leaving fledging mammals, some seeds and a few carnivores like birds, crocodiles and turtles to regenerate life on Earth.

At this point, the North County fossil record blanks out.

Scientists believe a gap of about 15-20 million years exists. But scientists do know that the Paleocene epoch saw the erosion of the Cretaceous mountain range that soared along the coast. They also know that very thick exotic soils were developing, examples of which have been discovered in an abandoned quarry on the old Kelly Ranch around Palomar Airport.

These soils tell paleontologists that the North County climate was very humid and that the seas were much lower than their Cretaceous levels.

Researchers suspect that larger mammals about the size of wolves were living in the area. In the Rose Canyon area of San Diego, paleontologists have uncovered fossils of small rodents from the late Paleocene period. Presumably, these animals also roamed North County.

THE EOCENE

About 40-50 million years ago

Fossil making in North County really begins in earnest, leaving a detailed picture of the flora and fauna.

The environment is very diverse. Shallow lagoons reach much further inland than they do now. Sandy barrier islands like those off the Carolinas dot the coastline. The coast itself, situated near the base of Black Mountain, is a rugged place with sheer cliffs plunging into the surf.

Again, North County is a subtropical garden. Fossilized leaves found in Del Mar indicate that the plants alive in the Eocene bear a remarkable resemblance to those found in modern subtropical areas.

Mammalian life is exploding and flowering into more diverse groupings. Protoreodonts and tillidonts are common. Both eat plants. Protoreodonts, animals about the size of a dog, will eventually evolve into precursors of deer. The tillidont, a large rodent-like creature, stands on its hind legs to reach leaves in Encinitas.

North County is populated with primates, too.

Lemur-like, tree-dwelling animals are relatively common. Larger mammals also evolve and roam the area, mainly feeding on plant life.

Archaic versions of horses, rhinos and tapirs populate a wide swath of the county from the sea to the far interior. The brontothere, a gigantic early cousin of the modern rhino, browses on the tops of shrubs and the bases of trees.

At first, these animals dominate. But they are eventually replaced by early cattle, sheep, pigs and deer.

THE MIOCENE

About 6-7 million years ago

Plate tectonics really begin to cut loose. North County began edging north from its spot around Sonora a few million years earlier. Now things speed up with the formation of the San Andreas fault between the Pacific and North American plates. One million years later, the continent goes into overdrive.

According to Demere: “All hell broke loose around 5 million years ago when the San Andreas began to do its thing.”

Once again, much of the modern North County coastal strip is under water. In the Camp Pendleton area, the sea is at the foot of the rugged coastal mountains and evidence of its presence can still be seen today. Fossilized fish remains have been found in Oceanside dating from this era.

Because of the northward movement and climactic changes, the environment is cooling. Globally, Antarctica reaches its present latitude. The ice caps are formed. Masses of cold water sink in the oceans and affect weather.

Inland, Anza-Borrego teems with sea life such as clams, oysters and fish. In the south end of the park, near Coyote Mountain, an arm of the Gulf of California supports coral reefs and a wide array of mollusks and urchins. Larger animals like the baleen whale, sharks and barracuda also live in Anza-Borrego.

THE PLIOCENE

About 3.5 million years ago The greatest impact on the North County environment is taking shape in Africa. Hominids are standing erect and surveying their tiny valley domains for the first time.

But they will not arrive here for at least 3.4 million more years. Meanwhile, San Diego has become a large basin. In the southeastern portion of the county, the sea reaches the slopes of Mt. Helix. In Oceanside, though, the sea is about where it stands today.

The waters are filled with whales and dolphins. Some have unusual adaptations. One whale species has a long, extended lower jaw that it probably uses to mine the sea floor for small animals. Many of these early whales and dolphins are becoming big-brained and developing their modern look.

Other sea mammals live in the waters. One, a kind of sea cow, is an early ancestor to the modern dugong that lives on the west coast of Africa and the Great Barrier Reef.

THE PLEISTOCENE

About 2 million years ago

The Earth’s climate again begins to change rapidly. Glaciers advance and retreat, forcing sea levels up and down. North County’s mesas are formed by these fluctuations in sea levels. (Essentially, mesas are sea floors with about 2 feet of veneer of modern sediments on top.)

The sea eventually reaches far inland. At one point, Miramar Naval Air Station is under about 5 feet of water. In North County, the sea returns to the edge of Black Mountain. At Camp Pendleton, the slopes of the mountain are sea cliffs.

The major river valleys in North County, like the Santa Marguerita and the San Dieguito, are forming through the work of northern glaciers. When ice is forming, the seas retreat. The V-shaped space between hills or mountains is empty. When the glaciers melt, the sea level rises and fills the Vs with water and sediments.

In all there are four major glacial periods, and, although the ice does not reach as far south as North County, the weather here varies from cool to warm. The land is largely forested.

This is also the time of the familiar ice-age mammals like giant ground sloths, early camels, horses, mammoths and saber-toothed cats. Many of these animals roam the now grassy savannas of Anza-Borrego.

With the fluctuation of the ice ages, sea floors of barrier seas like the Bering and the North Sea were exposed, making a land bridge or corridor.

The ancestors of the first humans in North County walk across the land bridge between Asia and North America. They arrive in North County between 20,000 and 14,000 years before the first freeway is proposed.

These early North County settlers establish the commuting pattern for later arrivals. They are hunters, gatherers and fishermen, so they go where the food is. Some eventually try farming.

THE HOLOCENE

About 10,000 years ago and counting The automobile is invented, Interstate 5 is built and real estate appreciates.

FOSSIL DISPLAY

The San Diego Natural History Museum’s current exhibit on the county’s fossils, many of which came from North County, continues until Nov. 24.

The museum, in Balboa Park, is open from 10 a.m. to 4:30 p.m. seven days a week.

Admission is $5 for adults, $1 for children ages 6 to 18. Children under 5 are admitted free. There is no admission charge on the first Tuesday of each month.

FOSSIL GROUP

Those looking for a group of fellow fossil hunters should call the San Diego Mineral and Gem Society’s fossil division.

The division meets at 7:30 p.m. on the last Friday of every month in the society’s building in Balboa Park. For more information, call 239-8812. For a recorded message, dial 239-0925.