Kelp forests are older than thought

How long have the famous kelp forests been bobbing in the waters of the North Pacific coast? They clearly did not appear with their typical inhabitants today 14 million years ago, as previously assumed. Instead, new fossil discoveries show that large brown algae existed more than 32 million years ago. The hippopotamus-like Desmostilia likely initially fed on early kelp forests. Scientists say that only much later did modern inhabitants, such as sea urchins, sea otters and the like, find their way into the lush underwater world.

They are considered the aquatic counterpart to pristine forests on Earth: colonies of large brown algae (Laminariales) form the basis of species-rich ecosystems, especially on the Pacific coasts in temperate climates. These organisms, called macroalgae, form complex structures resembling higher ground plants. They anchor themselves to the bottom with root-like structures, then grow densely branched and supported by buoyant organs, several meters up to the surface of the water. Kelp forests off the west coast of North America are particularly famous and important, as they provide the basis for the life of many species of invertebrates, fish, marine mammals and seabirds.

But how long have these “marine forests” been around? Despite their importance, their evolutionary origin has remained unclear due to a lack of fossil remains. The oldest known evidence was a kelp fossil dating back 14 million years. Animal fossils associated with kelp forests, such as kelp-eating manatees, also seem to fit this. “Because organisms associated with modern kelp forests did not yet exist, the prevailing view was that brown macroalgae did not exist 14 million years ago,” says lead author Cindy Lowe of the University of California, Berkeley. But new fossil discoveries are now changing the picture of the evolutionary history of kelp forests.

Discovery by amateur fossil collector

The credit goes to 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 opened it, he discovered structures that reminded him of the types of seaweed found in nearby coastal waters. He then turned to experts and thus began the scientific study of kelp fossils.

Initially, investigations and analyzes of the structures, using, among other things, 3D X-ray scans, confirmed that these are in fact the fossil remains of the root-like structures that large seaweed species still use to attach themselves to solid surfaces today. Specifically, it has been shown that these plants once anchored themselves to shell structures that existed in the rocky subsurface. I then explained the dating results based on strontium isotope ratios when this occurred. Accordingly, the kelp fossils date back to 32.1 million years ago – the middle of the Cenozoic, which extends from 66 million years ago to the present.

As the team also reported, the fossils also included a sample of some creatures from the past seaweed environment: 3D X-ray scans revealed the skeletons of mussels, snails, barnacles and the tiny shells of so-called foraminifera, from which the former root organs of the large algae were clasped. These taxonomies showed that the species diversity in the ecosystem 32 million years ago was not as complex as kelp forests today. “The structure is certainly not as rich as if you were to examine a sample of a kelp ecosystem today. “Clearly, strong diversification had not yet begun in the ecosystem at that time,” Lowe says. “So it makes sense that although seagrasses were… “Already existed, but the organisms associated with it today did not exist.” Many special species of mussels, birds, and marine mammals likely entered the ecosystem only later, fossil records show.

Development history step by step

However, scientists say a creature previously considered mysterious may have actually adapted to early kelp forests. “It is possible that seaweeds were the food source for hippopotamus-sized Desmostylia,” says lead author Steven Kiel of the Swedish Natural History Museum in Stockholm. “Seagrasses have been suggested as a possible food source for these extinct marine mammals, but there is no concrete evidence. They now present our findings,” says the scientist.

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

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

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