The answer is satisfyingly full of toothy prehistoric beasts. But before we get to those, we need to explain a bit about the Bay Area during the Mesozoic era — the geologic time of the dinosaurs, ending with their sudden extinction 66 million years ago.
The California coastline was 100 miles farther east, toward the Sierra Foothills, putting the Bay Area thousands of feet underwater. And much of our ancient seafloor is long gone.
Over millions of years, the Farallon Plate subducted into an underwater trench, melting under the continent. This, along with frequent earthquakes and underwater landslides, meant bad news for ancient bones we’d like to find today.
“This would shake a skeleton apart, and the sediments being dragged down into a zone where heat and pressure would metamorphose them—all of this argues you wouldn’t get much preserved,” Sloan says.
So you won't find many fossils in the Bay Area -- though a few have survived.
In the Marin Headlands, you can find reddish chert made up of tiny prehistoric shells of radiolarians — think of plankton. A few larger spiral shells from extinct mollusks have also turned up; these belonged to ammonites, which swam through the water kind of like the modern-day nautilus.
But to find evidence of the predators that ate those ammonites, you have to look closer to the prehistoric shoreline to the east.
Our Aquatic Beasts
Northeast of Sacramento, in Rocklin, Dick Hilton is co-chair of the Sierra College Natural History Museum. Hilton authored the 3.4-pound "Dinosaurs and Other Mesozoic Reptiles of California," and is careful to say that many of the superb beasts above what’s now San Francisco were not exactly dinosaurs per se.
Dinosaurs, he explains, are terrestrial -- land dwelling. So a Mesozoic creature swimming through the modern-day bay’s waters or flying overhead wouldn’t qualify. There are also taxonomic distinctions to do with skulls and hip structure.
Still, Hilton says, all kinds of gnarly reptiles that weren’t dinosaurs swam along the coast, like one that resembled a dolphin -- if dolphins were terrifying reptiles. Known as ichthyosaurus, the name literally means fish lizard.
Another toothy carnivore upwards of 30 feet long was mosasaurus.
“If you saw the latest Jurassic Park movie, one of these comes out of the water and scares the heck out of you, doesn’t it?” Hilton remarks. Indeed. Mosasaur fossils have been found just a few miles from Sierra College.
Among the favorites of Kolten, our young question asker, is another sometimes found in California called plesiosaurus.
“And everybody says, well, what’s that? And I always refer them to the Loch Ness monster,” Hilton jokes.
Plesiosaurs had four flippers, long necks and sharp teeth. Their fossils sometimes turn up with rocks inside the rib cage. Hilton believes they swallowed the rocks to attain neutral buoyancy.
“You don’t wanna sink to the bottom, you don’t wanna float to the top, so if you can swallow just enough rocks and keep them in your body, then you’re neutrally buoyant and you just glide through the ocean,” he says.
There were also pterosaurs, the flying family of the iconic pterodactyl and its cousin, the pteranodon.
“The largest ones may have had a wingspan of around 40 feet, so we’re talking of a flying reptile here the size of a jet fighter,” Hilton smiles, “which is pretty cool.” (Fact check: accurate.)
While the bay’s jumbled, messy geology makes it difficult to find definitive fossil evidence, Hilton is convinced they were around.
“It’s like today. You go along the coastline and you’re liable to see whales. And they stay usually fairly close to the coastline. But you can go out on a boat 500 miles from the shore and still see whales and still see birds, just probably not as numerous,” he says.
As for actual dinosaurs in California, much of what’s found are fragments — a finger or two here, a scrap of leg there. The only California fossil suggesting a meat-eating dinosaur once lived here is on display in Hilton’s museum: a dark gray chunk of theropod leg small enough to hold in your palm.
Of the few largely intact dinosaur skeletons found in the state, one belonged to a duck-billed plant eater known as Augustynolophus morrisi. State lawmakers are considering a bill this year to make it California’s state dinosaur.
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