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A beachcomber on Ocean Beach. Dendraster excentricus can be found from Baja all the way up to Alaska. Pic: Ida Pap

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This illustration of sand dollars and heart urchins shows the diversity of echinoids, a class of marine animals.

How to see millions of years of history on a day at the beach

7 April 22

Sand dollars are striking - delicate in a way that the wayward rocks and driftwood are not - and so ubiquitous that it's easy to overlook the obvious question: What exactly are they?

From National Geographic: When walking along San Francisco’s Ocean Beach, it’s nearly impossible to miss the sand dollars that dot the shoreline. At almost any time of day and tide, any season of the year, these white discs stretch as far as the eye can see: some cracked underfoot or broken in the Pacific waves, others still full and flawless with a flower-like pattern on top.

In such a densely populated, thoroughly developed city, there aren’t many places left that feel truly wild. But this salty Bay Area stretch known for roiling surf, epic vistas, and mercurial weather still manages to conjure edge-of-the-world vibes. 

Like lots of people seeking fresh air and a restorative escape during the pandemic, I’ve been drawn to Ocean Beach. But as a lifelong beachcomber, I’m there looking down, not out, searching for treasures on the sand. Such a quest makes every visit to this living lab an exciting opportunity for an all-ages family adventure.

While sand dollars can be found along coasts around the United States, the seaside beaches near San Francisco are perfect places to take a deep dive into the fascinating history of one odd creature.

An eccentric species

 So what are they?

They’re a type of sea urchin, or echinoid - a class of living things that thrive in marine environments around the world. When submerged in their natural habitat, sand dollars are covered in short spines that look like a thick, fuzzy, purple coat; in death, that soft tissue decomposes and what remains is bleached by the sun. This means that the white ones that wash up are actually sand dollar skeletons (“tests,” in science-speak). Examine one closely and you’ll see a remarkable geometry of interlocking plates that make up the brittle form—but no backbone, because they’re invertebrates.

Preeminent expert Rich Mooi at the California Academy of Sciencesexplains that the distinctive five-petal pattern, or petaloid, is something all sand dollars have in common; it allows the animal to breathe underwater. “They’re gills, basically,” he says.

The sand dollars scattered over Ocean Beach are a species called Dendraster excentricus, and a few things set them apart from the 150-or-so other sand dollar species currently living across the globe. “They’re eccentric in behavior and eccentric in shape. But they’re from San Francisco,” Mooi says with a laugh. “Of course they have to be different.”

Most sand dollars lay prone on the ocean floor, feeding on what’s beneath them. “The sand grains are full of juicy particles—foraminifera, nematodes, diatoms—and they pass these sand grains into grooves that run into their mouth, almost like a conveyor belt,” Mooi says.

Dendraster excentricus, however, has these grooves on bottom and top. They stick themselves upright in the sand in groups of hundreds and thousands—“like coins in plasticine”—which lets them do double duty at mealtime. “They’re able to use both surfaces to pick food right out of the passing currents,” Mooi says. The petaloid of Dendraster excentricus is irregular—just off-center—so it stays free and clear of obstruction when the sand dollar is burrowed in on end.

Walking back in time

Sand dollar tests aren’t the only buried treasures here: sand dollar fossilsare just as ubiquitous. To spot them, look for grainy grey sandstones rubbed away in places to reveal hints of a sand dollar’s familiar petal design in shades from creamy white to rich brown. But where do they come from?

About seven miles south of Ocean Beach, an imposing bluff called the Merced Formation is bisected by the San Andreas Fault. “It dates back to the upper Pliocene to Pleistocene era—probably two-and-a-half to a half-million years old—but it was continuously deposited over this long period and it’s almost a mile thick,” says Ashley Dineen, from the UC Museum of Paleontology. “They’ve found, I think, over 50 different species [there]. And there are these intervals where you get very abundant echinoids.”

This was the source of the fossils on Ocean Beach—preserved, then exposed, then released back into the Pacific currents. As it turns out, they’re not Dendraster excentricus but rather a distant relative called Scutellaster interlineata—another eccentric sand dollar, extinct for one to two million years.

As long as you check the tide charts first and go at a low point, it’s possible to wander all the way from Ocean Beach to the Merced Formation, passing Fort Funston and exiting at Mussel Rock Park in Pacifica—a stunning day trip. As crowds thin out and civilization falls away, the dunes transform into a steep cliff that continues to rise, revealing “fossiliferous” strata. The sand beneath your toes takes on a coarser texture, with myriad loose fossils and specimens to marvel over.

Sand dollars exist here in such extraordinary numbers—“untold millions,” says Mooi—that the sheer volume of tests on the shore is, generally, no cause for concern. They’re not souvenirs, though: Ocean Beach is protected by the Golden Gate National Recreation Area, so the only thing you should be taking home with you is litter. I bring a small bag with me and collect what I can carry, and toss it in the garbage cans on Great Highway.

To see living Dendraster excentricus up close, head to the California Academy of Sciences, an incredible museum in San Francisco’s Golden Gate Park that also includes an aquarium and a planetarium.

There’s something soothing about finding, and holding, these tactile representations of deep time—history manifested in a physical form—especially in fast-paced San Francisco, but they’re just one of the wonders that the Ocean Beach area has to offer.

Everything that emerges from the depths gives a glimpse of a vibrant, evolving ecosystem: from molted Dungeness crab shells (and sometimes the crabs themselves) to tangled kelp in shades of glistening green and brown, to translucent jellyfish the size of Frisbees, to sand dollar skeletons and fossils. Go with a cloudy head, and leave with a clear picture of something that just might inspire your own journey of discovery.




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