Fossils show that kelp predates the animals we see in kelp forests today

The unique underwater kelp forests that line the Pacific coast are home to a diverse ecosystem believed to have evolved with kelp over the past 14 million years.

But a new study shows that kelp thrived off the northwest coast more than 32 million years ago, long before modern groups of marine mammals, sea urchins, birds and fish appeared. bivalves that live in forests today.

The much older age of these coastal kelp forests, which today constitute a rich ecosystem home to otters, sea lions, seals and many birds, fish and shellfish, means that they were probably the main source of food for an ancient mammal that is now extinct. called desmostylian. The hippopotamus-sized grazer is thought to be related to today’s sea cows, manatees and their land relatives, elephants.

“People initially said, ‘We don’t think kelp was there before 14 million years ago because the organisms associated with the modern kelp forest weren’t there yet,'” said paleobotanist Cindy Looy, professor of integrative biology at the University of California. , Berkeley.

“Now we show that the kelps were there, it’s just that all the organisms you expect to be associated with them weren’t. Which isn’t that strange, because you first need of the foundations of the entire system before anything else can appear.”

Evidence for the greater antiquity of kelp forests, reported in the journal Proceedings of the National Academy of Sciences, comes from recently discovered fossils of the kelp tether, 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 wedge and supports the blades, which generally float in the water, thanks to air pockets.

Looy’s colleague Steffen Kiel dated these fossilized attachments, which still grip clams and envelop barnacles and snails, to 32.1 million years ago, to the middle of the Cenozoic era, which spans from 66 million years ago to the present. The oldest known kelp fossil, consisting of an air bladder and a blade similar to that of modern kelp, dates from 14 million years ago and is in the collection of the Paleontology Museum of the University of California (UCMP).

“Our materials provide good evidence that kelp is the food source for an enigmatic group of marine mammals, the desmostylia,” said Kiel, lead author of the paper and senior curator at the Swedish Museum of Natural History in Stockholm. .

“This is the only order of Cenozoic mammals that actually became extinct during the Cenozoic. Kelp has long been suggested as a food source for these hippo-sized marine mammals, but actual evidence has been lacking. Our data indicate that kelp is a likely candidate.”

According to Kiel and Looy, lead author of the paper and curator of paleobotany at UCMP, these early kelp forests were probably not as complex as the forests that evolved about 14 million years ago. Late Cenozoic fossils along the Pacific coast indicate an abundance of bivalves (clams, oysters, and mussels), birds, and marine mammals, including sirenians related to manatees and extinct bear-like predecessors of the sea otter, called Kolponomos. Such diversity is not found in the fossil record from 32 million years ago.

“Another implication is that the fossil record has, once again, shown that the evolution of life – in this case, kelp forests – was more complex than was estimated from biological data alone,” Kiel said. “The fossil record shows that many animals appeared and disappeared from kelp forests over the past 32 million years, and that the kelp forest ecosystems we know today only evolved over the past 32 million years. last millions of years.”

The value of fossil hunting enthusiasts

The fossils were discovered by James Goedert, an amateur fossil collector who previously worked with Kiel. When Goedert opened four stone nodules he found along the beach near Jansen Creek on Washington’s Olympic Peninsula, he saw what looked like reserves of kelp and other macroalgae common today along the coast.

Kiel, who specialized in invertebrate evolution, agreed and subsequently dated the rocks based on the ratio of strontium isotopes. He also analyzed oxygen isotope levels in the bivalves’ shells to determine that the holdfasts lived in water slightly warmer than today, within the higher temperature range found in modern kelp forests.

Looy contacted co-author Dula Parkinson, a scientist in the Advanced Light Source at Lawrence Berkeley National Laboratory, for help obtaining a 3D X-ray scan of one of the Holdfast fossils at the using synchrotron radiation X-ray tomographic microscopy (SRXTM). . When she examined the detailed X-ray slices of the fossil, she was amazed to see a barnacle, a snail, a mussel and tiny single-celled foraminifera hidden in the hold, in addition to the bivalve it rested on.

Looy noted, however, that the diversity of invertebrates found in the 32-million-year-old fossilized hold was not as high as would be found in a kelp hold today.

“The holds are definitely not as rich as they would be if you were looking at a kelp ecosystem right now,” Looy said. “The diversification of organisms living in these ecosystems had not yet begun.”

Kiel and Looy plan further studies of the fossils to see what they reveal about the evolution of the kelp ecosystem in the North Pacific and how that relates to changes in the ocean-climate system.

Other co-authors of the paper are Rosemary Romero, an algae specialist who earned her Ph.D. She graduated from UC Berkeley in 2018 and is now an environmental scientist at the California Department of Fish and Wildlife; paleobotanist Michael Krings of 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.