Fossil holdfasts show animals that far predated the kelp animals we see in kelp forests today.

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X-ray reconstruction of a 32-million-year-old fossil kelp, colored to show the base (orange), holdfast (yellow) and bivalve shell (blue) to which it is attached. Credit: Tula Parkinson/By Advanced Light, Lawrence Berkeley National Laboratory

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X-ray reconstruction of a 32-million-year-old fossil kelp, colored to show the base (orange), holdfast (yellow) and bivalve shell (blue) to which it is attached. Credit: Tula Parkinson/By Advanced Light, Lawrence Berkeley National Laboratory

The unique underwater kelp forests that line the Pacific coast support a diverse ecosystem that is thought to have co-evolved with kelp over the past 14 million years.

But a new study shows that kelp flourished along the Northwest Coast 32 million years ago, long before the modern groups of marine mammals, sea urchins, birds and bivalves that call the forests home today appeared.

The enormous age of these coastal kelp forests is a rich ecosystem today that supports otters, sea lions, seals, and many birds, fish, and crustaceans that may have been an important source of food for ancient, now extinct mammals. called Desmostylian. The hippopotamus-sized herding species is thought to be related to today's sea cows, manatees and their terrestrial relatives, elephants.

„People initially said, 'We don't think kelp existed 14 million years ago because the species associated with modern kelp forests didn't exist yet,'” said Cindy Lui, a professor of integrative biology at the University of California, Berkeley.

„Now, we're showing that there were kelps, and not all the organisms you'd expect to associate with them. It's not strange, because you need a foundation for the whole system first before you can show everything.”

Evidence for the greatest antiquity of kelp forests, reported in the journal Proceedings of the National Academy of Sciences, comes from newly discovered fossils of a kelp's holdfast — the root-like part of the kelp that anchors it to rocks or reef-bound organisms on the seafloor. The stipe, or stem, attaches to the holdfast and supports the blades, which normally float in water, thanks to air bladders.

Lew's colleague Steffen Keel dated these fossilized holdfasts, 32.1 million years ago, to 32.1 million years ago, in the middle of the Cenozoic Era, which stretches from 66 million years ago to the present. The oldest known kelp fossil, with an air bladder and a blade similar to today's bull kelp, dates back 14 million years and is in the collection of the University of California Museum of Paleontology (UCMP).


A slice through a 32-million-year-old fossil shows a finger-like haptera growing on top of a shed. Credit: Steffen Kiel

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A slice through a 32-million-year-old fossil shows a finger-like haptera growing on top of a shed. Credit: Steffen Kiel

„Our holdfasts provide good evidence that kelp is a food source for an enigmatic group of marine mammals called Desmostylia,” said Kiel, lead author and senior curator at the Swedish Museum of Natural History in Stockholm.

„This is the only order of Cenozoic mammals that actually went extinct during the Cenozoic. Kelp has long been suggested as a food source for these hippo-sized marine mammals, but there is no real evidence. Our holdfasts indicate that kelp is a likely candidate.”

According to Kiel and Lew, senior author of the paper and UCMP curator of paleobotany, these early kelp forests were not as complex as those that formed about 14 million years ago. Fossils from the late Cenozoic along the Pacific coast indicate an abundance of bivalves—clams, oysters, and mussels—birds and marine mammals, including manatees and extinct bear-like ancestors of sea otter-related sirenians called Colphonomos. Such diversity is not seen in the fossil record dating back 32 million years.

„Another implication is that the fossil record shows, once again, that the evolution of life — in this case, of kelp forests — was more complex than estimated from biological data alone,” Kiel said. „The fossil record shows that numerous animals appeared and disappeared in kelp forests over the past 32 million years, and that the kelp forest ecosystem as we know it today evolved only in the last few million years.”

The value of fossil hunting amateurs

The fossils were discovered by James Goedert, an amateur fossil collector who had worked with Kiel in the past. When Godert broke apart four stone nodules found along the beach near Johnson Creek on Washington's Olympic Peninsula, he found what looked like clutches of kelp and other macroalgae commonly found on beaches today.

Kiel, who specializes in invertebrate evolution, identified rocks based on the ratio of strontium isotopes. He also analyzed oxygen isotope levels in bivalve shells to determine that Holdfasts lived in waters slightly warmer than today, at the upper range of temperatures found in modern kelp forests.


The timeline depicts the evolution of kelp forests and associated organisms along the Pacific coast over the past 32 million years, along with variations in water temperature. Black bars represent members of modern, complex kelp ecosystems – sea otters, abalone, sea urchins and, until recently, sea cows. Green bars indicate now-extinct members of the early kelp beds, including desmostylians and penguin-like pletopterids. Credit: Steffen Kiel and Cindy Lui

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The timeline depicts the evolution of kelp forests and associated organisms along the Pacific coast over the past 32 million years, along with variations in water temperature. Black bars represent members of modern, complex kelp ecosystems – sea otters, abalone, sea urchins and, until recently, sea cows. Green bars indicate now-extinct members of the early kelp beds, including desmostylians and penguin-like pletopterids. Credit: Steffen Kiel and Cindy Lui

For help in obtaining 3D X-ray scans using Synchrotron Radiation X-ray Tomographic Microscopy (SRXTM), Lui approached Tula Parkinson, a scientist working with the Advanced Light Source at Lawrence Berkeley National Laboratory. . When she reviewed the detailed X-ray slices through the fossil, she was surprised to find a cicada, a snail, a clam and small, single-celled foraminifera hidden inside the holdfast, in addition to the bivalve it sat on.

Lui noted, however, that the invertebrate diversity found within a 32-million-year-old fossil holdfast was not as high as that found within a kelp holdfast today.

„If you go to a kelp ecosystem now, the holdfasts are definitely not going to be rich,” Lui said. „The diversification of species living in these ecosystems has not yet begun.”

Kiel and Lui plan further studies of the fossils to reveal what they reveal about the evolution of kelp ecosystems in the North Pacific and how they relate to changes in the ocean-climate system.

Other co-authors of the paper are Rosemary Romero, an algae specialist who received her Ph.D. from UC Berkeley in 2018 and is now an environmental scientist with the California Department of Fish and Wildlife; paleontologist Michael Krings at the Ludwig-Maximilians-Universität in Munich, Germany; and former UC Berkeley undergraduate Tony Huynh. Goedert is a research associate at the Burke Museum of Natural History and Culture at the University of Washington in Seattle.

More information:
Kiel, Steffen et al., Early Oligocene kelp holdfasts and hierarchical evolution of kelp ecosystems in the North Pacific, Proceedings of the National Academy of Sciences (2024) DOI: 10.1073/pnas.2317054121. doi.org/10.1073/pnas.2317054121

Press Information:
Proceedings of the National Academy of Sciences


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