NORTH COUNTY -- The recent discovery of mastodon bones at a Carlsbad construction site wowed the region's residents but didn't surprise paleontologists.

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That's because Carlsbad, as well as the neighboring community of Oceanside, has long been a prehistoric fossil hot spot in San Diego County, unlike inland regions.

If the discovery of the roughly elephant-sized creature's tusks, teeth and ribs had happened in Poway, or the hillsides of Escondido, paleontologists say they would have been really startled.

Fossil discoveries are based on geologic conditions -- the types of rocks that form the land in San Diego County, said Tom Demere, curator of paleontology at the San Diego Natural History Museum.

San Diego's land surface can pretty well be divided into two categories. There's the fossil-free, formerly molten material that makes up the inland, granite rock mountain ranges. And, there is layer upon layer of rock and sand that have settled in the coastal valleys and other lowland regions over millions of years. Those sediments covered dying creatures and left fossils for people to find.

Roughly two-thirds of the exposed areas in San Diego County fall in the category of likely to be fossil-free, Demere said. Even in the inland valleys where some layers of sediment have collected, they are more recent deposits, he said.

Because of this, paleontologists tend to look more toward the coastal communities for their fossil finds.

Under the sea

The latest mastodon discovered in Carlsbad walked the Earth relatively recently -- some 120,000 to 220,000 years ago, Demere said. Other Carlsbad finds have even included dinosaurs from 75 million years ago -- something that's rare anywhere in California.

Dinosaur bone discoveries are incredibly unusual because California was underwater 75 million years ago when big reptiles roamed the Earth, paleontologists note. That means if any land-based dinosaur fossils were found, they would have probably died elsewhere, say Utah or Wyoming, with the bodies then washing out to sea.

"When you're walking in San Diego and Orange County, you're walking on the bottom of (an old) ocean," said Howell Thomas, a paleontologist who runs a fossil lab for the Natural History Museum of Los Angeles County, the largest natural history museum in the Western U.S.

The Los Angeles museum has a massive fossil collection in part because it operates the famed La Brea Tar Pits research area -- where crude oil has been seeping out of the ground for some 40,000 years, creating a sticky mass that entraps animals. But the tar pits only date to the Ice Age, when mastodons roamed the Earth.

"Unfortunately, there are no dinosaurs in the La Brea Tar Pits," Thomas said.

Dinosaur bones could have landed on the sea floor of what is now the Los Angeles basin region, but they've since been buried so deep in sediment that they're unlikely to be uncovered, Thomas added.

Carlsbad's geology is different. Its sediment layers from the days of the dinosaurs are closer to the surface. Paleontologists have gotten lucky twice when it comes to dinosaur discoveries, said Demere, the San Diego museum curator. Both the bones of a duck-billed hadrosaur and a bony-armored ankylosaur have been found in Carlsbad.

"For California, it's a remarkable record," Demere said. "Except for some (dinosaur fossils) near Fresno, there are virtually no other dinosaurs known from California."

The first fossil hunters

Fossil hunting in San Diego County informally dates to at least the early 1900s.

"These (geologists) were hearty individuals -- these geologists were mapping by horseback," Demere said.

A big burst of activity came in the late 1960s and the early 1970s, said Demere, who has worked at the museum for 28 years. It's during that period that a crew of paleontologists from UC Berkeley worked near Agua Hedionda Lagoon.

That's also the same period when San Diego State University hired its first paleontologist, said Jason Lillegraven, the man hired.

Now a retired University of Wyoming professor, Lillegraven worked for San Diego State until 1975 when he moved to Wyoming. Most of his local research occurred on Camp Pendleton, he said.

"We were allowed at that time amazing access to the base," he added.

Working along the coast near San Clemente, he and his students found the bones of ancient rodents, lizards, snakes, primates and even the small precursors of what became the modern rhino, Lillegraven said, adding that the bones dated from about 45 million years ago -- some 20 million years after the dinosaurs died out.

He was the first in a series of paleontologists hired at San Diego State.

These days, the university retains a small teaching collection of fossils but has transferred most of its collection to the San Diego Natural History Museum. The university tends to focus more now on researching genetic links between various fossils rather than digging up bones, said paleontologist Dave Archibald, a professor in the university's biology department.

"We don't want to develop collections when there are perfectly good repositories (such as the museum)," he said.

Fossil spotting requirements

These days, the folks digging up the bones are construction companies.

"Cities incorporated, freeways have enlarged and housing developments have spread … and the geologic strata have been laid bare for us to look at, essentially, and it's been exciting," said Demere, the museum curator.

Carlsbad has such a significant history of recent fossil discoveries that contractors regularly are required to have paleontologists present during grading operations.

"It's really triggered by the (city and state) environmental review process," Planning Department Director Don Neu said as he discussed the city's requirements.

Carlsbad planners know what areas are likely to turn up fossils -- the valleys along El Camino Real, rather than the mountains along Rancho Santa Fe Road, for example -- and developers are required to take that into account, Neu said.

The place where the mastodon was found -- the old Robertson Ranch along El Camino Real -- was considered to have a high potential for fossil finds and its developer was required to have a paleontologist present. That paleontologist, Brad Riney of the San Diego Natural History Museum, discovered the mastodon as construction equipment scraped away layers of earth. He's the man who discovered the city's dinosaur bones.

Oceanside also regularly has paleontologists at its construction sites, but some inland cities don't. Poway doesn't require it because it hasn't had any significant fossil finds, said Niall Fritz, the city's director of development services.

"It's a function of the geology and the history of the area," he said, adding that the city's lowland areas -- the areas most likely to produce fossils -- were developed many years ago before the state fossil requirements went into effect.

For his part, Demere wishes that all cities would require the monitors, saying he wonders what he's missing seeing. He would love to get on Camp Pendleton to look around, he added.

"I'm sure we'd find all sorts of great stuff," he said.

Contact staff writer Barbara Henry at (760) 901-4072 or bhenry@nctimes.com.

Noted fossil finds in North County

- Ammonites -- About 35 of these large prehistoric creatures, which are related to modern nautilus, were discovered at a single Carlsbad construction site. They're about 75 million years old -- the era of dinosaurs, which is the oldest period that people are likely to find good fossils in North County, the San Diego Natural History Museum reports.

- Ankylosaur -- Parts of an armored dinosaur (the back legs, part of the front legs, some teeth and some of its bony plates) were discovered during the College Boulevard extension project in 1987. The San Diego Museum of Natural History calls it the "most complete dinosaur skeleton ever found in California." Roughly 75 million years old, it's encrusted with fossilized shells and even has a few shark teeth embedded in it.

- Brontothere -- Several of these distant cousins of the rhinoceros were found in Oceanside near Rancho del Oro and in Carlsbad near El Camino Real. These animals would have stood up to 6 feet tall at the shoulder.

- Capybara -- The ancient precursors of these giant rodents weren't known to have lived in California until a 1995 discovery of part of a skull in Oceanside, the museum reports.

- Hadrosaur -- Parts of this duck-billed dinosaur (leg bones and a tailbone) were discovered at the Carlsbad Research Center construction site in the 1980s. The bones are about 75 million years old.

- Mastodons -- Discoveries have been made in Carlsbad and Oceanside. Smaller than mammoths, the mastodons weighed as much as 6 tons.

A few spots to view fossils:

- Buena Vista Lagoon Nature Center, Oceanside along Coast Highway -- home to a huge ammonite fossil discovered during construction at McClellan-Palomar Airport nearly two decades ago.

- San Diego Natural History Museum -- permanent exhibit "Fossil Mysteries" explores the discoveries made in Southern California and Baja California. Many Oceanside and Carlsbad discoveries are highlighted.

Web links:

Museum Web site -- http://www.sdnhm.org

Recent Carlsbad mastodon discovery:

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