The unique underwater kelp forests that line the Pacific Coast support a varied ecosystem that was thought to have evolved along with the kelp over the past 14 million years. But a new study shows that kelp flourished off the Northwest Coast more than 32 million years ago, long before the appearance of modern groups of marine mammals, sea urchins, birds, and bivalves that today call the forests home.
“People initially weren’t sure if kelps existed earlier than 14 million years ago because the organisms associated with the modern kelp forest were not there yet,” said paleobotanist Cindy Looy, professor of integrative biology at the University of California, Berkeley. “Now, we show the kelps were there, it’s just that all the organisms that you expect to be associated with them were not. Which is not that strange, because you first need the foundation for the whole system before everything else can show up.”
Evidence for the greater antiquity of kelp forests comes from newly discovered fossils of the kelp’s holdfast—the root-like part of the kelp that anchors it to rocks or rock-bound organisms on the seafloor. The stipe, or stem, attaches to the holdfast and supports the blades, which typically float in the water. The fossilized holdfasts were dated to 32.1 million years ago, based on strontium isotopic ratios.
At the Advanced Light Source (ALS), the researchers used x-ray tomography at Beamline 8.3.2 to scan one of the holdfast fossils. The x-ray slices through the fossil not only provided structural information about the ancient kelp, it also revealed the presence of a barnacle, a snail, a mussel and tiny, single-celled foraminifera hidden within the holdfast, in addition to the bivalve on which it sat. Looy noted, however, that the diversity of invertebrates found within the 32-million-year-old fossilized holdfast was not as high as would be found inside a kelp holdfast today.
“The holdfasts are definitely not as rich as they would be if you would go to a kelp ecosystem right now,” Looy said. “The diversifying of organisms living in these ecosystems hadn’t started yet.”
S. Kiel, J.L. Goedert, T.L. Huynh, M. Krings, D. Parkinson, R. Romero, and C.V. Looy, “Early Oligocene kelp holdfasts and stepwise evolution of the kelp ecosystem in the North Pacific,” PNAS 121, e2317054121 (2024), doi:10.1073/pnas.2317054121.
Adapted from the UC Berkeley press release, “Pacific kelp forests are far older than we thought.”