Researcher discovers a rare fossil clam thought to be dead some 40,000 years ago

One afternoon in November 2018, Jeff Goddard, a research associate at UC Santa Barbara’s Marine Science Institute, was poking at rocks and turning them over, in an attempt to find nudibranch sea slugs at Naples point, when he spotted a pair of small, translucent bivalves. 

"Their shells were only 10 millimeters long," he said in a statement. "But when they extended and started waving about a bright white-striped foot longer than their shell, I realized I had never seen this species before."

The likeliest possibility was that Goddard had discovered a new species.

This surprised Goddard, who has spent decades in California’s intertidal habitats, including many years specifically at Naples Point. He immediately stopped what he was doing to take close-up photos of the intriguing animals. Little did he know that it wasn't a new species, but a clam that was assumed dead for more than 40,000 years - one that was only known from fossils.

"It's not all that common to find alive a species first known from the fossil record, especially in a region as well-studied as Southern California," said Goddard. "Ours doesn't go back anywhere near as far as the famous Coelacanth or the deep-water mollusk Neopilina galatheae — representing an entire class of animals thought to have disappeared 400 million years ago — but it does go back to the time of all those wondrous animals captured by the La Brea Tar Pits."

The discovery is detailed in the journal Zookeys.

A living fossil that kept to itself

Goddard figured that the animals were rare, so he took some close-up photos and sent the images to Paul Valentich-Scott, curator emeritus of malacology at the Santa Barbara Museum of Natural History, after noting down their taxonomic family.

Valentich-Scott was surprised and intrigued. "I know this family of bivalves (Galeommatidae) very well along the coast of the Americas. This was something I'd never seen before," he said.

He told Goddard that he needed to see the animal in person to make a solid assessment. But the creature eluded Goddard when he returned to find it. This happened several times until he found the clam in March 2019, some nine trips later.

The specimen was next to a couple of small white nudibranchs and a large chiton. Valentich-Scott and Goddard began their identification process immediately. The former knew that the specimen belonged to a member in the Santa Barbara region, but the clam's shell didn't match any of them.

How the Cymatioa cooki manage to elude experts for this long is a mystery

The two researchers looked up an "intriguing" reference to a fossil species and tracked down illustrations of the bivalve Bornia cooki from the paper describing the species in 1937. It seemed to match the modern specimen. 

Now, a scientist named George Willett who had described the species, had excavated and examined perhaps one million fossil specimens from the same location, the Baldwin Hills in Los Angeles, but he never found B. cooki himself. He named it after Edna Cook, a Baldwin Hills collector who had found the only two specimens known.

Valentich-Scott requested Willett’s original specimen (now classified as Cymatioa cooki) from the Natural History Museum of Los Angeles County. "This object, called the “type specimen,” serves to define the species, so it’s the ultimate arbiter of the clam’s identification," said the release.

But how did the species stay hidden for this long?

Goddard suspects the clams may have arrived on currents as planktonic larvae, carried up from the south during marine heatwaves from 2014 through 2016. 

"The Pacific coast of Baja California has broad intertidal boulder fields that stretch literally for miles and I suspect that down there Cymatioa cooki is probably living in close association with animals burrowing beneath those boulders," added Goddard.

Study Abstract:

A small bivalve mollusk previously only known from the Pleistocene of Los Angeles County has recently been found living intertidally near Santa Barbara, California. The bivalve has been determined to be Cymatioa cooki (Willett, 1937), a member of the Galeommatoidea J.E. Gray, 1840. We document the habitat for the newly discovered C. cooki, and compare it to C. electilis (Berry, 1963), the other extant member of this genus recorded from the region. Cymatioa cooki is rare, and while many galeommatoid species have been shown to be commensal with other invertebrates, we have been unable to determine any specific commensal relationships for it.