Kelp forests are older than thought

Fossil of a root-like structure that was used to attach a seaweed to a hard surface about 32 million years ago. © Steffen Kiel

Since when have the famous kelp forests swayed in the waters of the North Pacific coast? They apparently did not emerge together with their typical inhabitants today 14 million years ago, as previously assumed. Instead, new fossil discoveries show that the large brown algae existed over 32 million years ago. The hippopotamus-like Desmostylia probably initially fed on the early kelp forests. It was only much later that modern inhabitants such as sea urchins, sea otters and the like found their way into the lush underwater world, say the scientists.

They are considered the aquatic counterparts to the virgin forests on land: colonies of large brown algae (Laminariales) form the basis of species-rich ecosystems, especially on the Pacific coasts of temperate climate zones. These organisms are so-called macroalgae, which form complex structures similar to higher land plants. They anchor themselves to the bottom with root-like structures and then grow lushly branched and supported by buoyancy organs, many meters high up to the water surface. The kelp forests off the west coast of North America are particularly famous and important, providing a basis for life for many species of invertebrates, fish, marine mammals and seabirds.

But since when have these “marine jungles” existed? Despite their importance, their evolutionary origin has remained unclear due to a lack of fossil remains. The oldest known evidence was a kelp fossil that was dated to be 14 million years old. Fossils of animals associated with the kelp forests, such as kelp-eating manatees, also seemed to fit in with this. “Because the organisms associated with the modern kelp forest were not yet present, the dominant view was that the large brown algae did not exist before 14 million years ago,” says senior author Cindy Looy of the University of California, Berkeley . But now new fossil discoveries are changing the picture of the evolutionary history of the kelp forests.

Discovery of an amateur fossil collector

They are thanks to the American amateur fossil collector James Goedert. While walking on a beach on the Olympic Peninsula in Washington State, he discovered stone nodules that piqued his interest. When he then opened it, he discovered structures that reminded him of the seaweed species in the neighboring coastal waters. He then turned to experts and so began the scientific study of kelp fossils.

Initially, the investigations and analysis of the structures and the analyzes using, among other things, 3D X-ray scans confirmed that these are actually the fossil remains of the root-like structures that the large seaweed species still use to attach themselves to hard surfaces today. Specifically, it was shown that these plants had once anchored themselves to shell structures that sat in the rocky subsoil. The results of dating based on strontium isotope ratios then made it clear when that was. Accordingly, the kelp fossils date from 32.1 million years ago - the middle of the Cenozoic, which extends from 66 million years ago to the present.

As the team further reports, the fossils also included a kind of sample of some living creatures from the former seaweed environment: The 3D X-ray scans revealed structures of mussels, snails, barnacles and tiny shells of so-called foraminifera, which were from the former root organs of the macroalgae were clasped. The classifications of these organisms showed that the species diversity in the ecosystem 32 million years ago was not as complex as today's kelp forests. “The structure is definitely not as rich as it would be if you were to examine a sample from a kelp ecosystem today. The strong diversification had apparently not yet begun in the ecosystem at that time,” says Looy. “It therefore stands to reason that although the seaweed was already there, the organisms that are associated with it today were not.” Many of the special species of mussels, birds and marine mammals probably only entered the ecosystem later, as the fossil records show.

Step-by-step development history

However, a creature previously considered mysterious may have already adapted to the early kelp forests, the scientists say. “The seaweed could have been the food source for the hippopotamus-sized Desmostylia,” says lead author Steffen Kiel from the Swedish Natural History Museum in Stockholm. “Seaweed has been suggested as a possible food source for these extinct marine mammals, but concrete evidence has been lacking. They now provide our findings,” says the scientist.

Ultimately, the study now shows that the development history of kelp forest ecosystems was more complex than previously thought. “The fossil record suggests that certain animals appeared and then disappeared in the kelp forests over the last 32 million years. The living environment as we know it today probably only developed in the last few million years,” summarizes Kiel.

Source: University of California – Berkeley, specialist article: Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2317054121

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