Doing Fin: Ocean Fish Thrived During Last Global Warming, Say Experts

During a brief period of elevated global temperatures 56 million years, some fish still flourished, according to researchers.

“Despite extreme conditions, the Ras Gharib A fauna … shows that diverse fish communities thrived in the paleotropics during the PETM [Paleocene-Eocene Thermal Maximum],” states an article in Geology, published on May 17.

The paper states that the PETM was a short interval of highly elevated global temperatures some 56 million years ago. The period in which temperatures across the world rose is frequently described as the best ancient analog for present-day climate warming.

Fish are among the organisms thought to be most sensitive to warming climates. Tropical sea surface temperatures during the PETM likely approached temperatures believed to be lethal to some modern marine fish species.

However, newly discovered fish fossils show that in that ancient period, certain fish seemed to be thriving.

“The impact of the PETM event on life at the time is of wide interest. But a major gap in our understanding is how life in the tropics responded, because this region is not well-sampled for many fossil groups,” said Matt Friedman, co-author of the study and director of the University of Michigan’s Museum of Paleontology.

The University of Michigan team worked with Egyptian scientists to build a snapshot of the ecosystem during the period.

“On the basis of the scant evidence we have for fishes — remembering that this Egyptian site provides our first peek from the tropics — they seem to have weathered the PETM surprisingly well. There are even hints that important diversification in the group might have happened around or just after this time.”

The newly discovered fossil site at Ras Gharib A was excavated from a site in Egypt’s Eastern Desert, near the coast of the Gulf of Suez.

The fossils provide the first clear picture of marine bony fish diversity in the tropics during the PETM. Previous studies estimated that sea surface temperatures in some parts of the tropics likely surpassed 95 degrees Fahrenheit (35 C) at that time, suggesting dire consequences for low-latitude ocean fish.

But the Egyptian fossils reveal an intact ecosystem with diverse fish lineages and a variety of ecologies. The composition of the Ras Gharib A fish community is similar to PETM-aged fish fossils from sites at higher latitudes.

“While the broader evolutionary consequences of the PETM for marine fishes remain little explored, the available paleontological evidence does not suggest a widespread crisis among marine fishes at that time,” said Sanaa El-Sayed of Egypt’s Mansoura University, a paleontologist and lead author of the article.

“In fact, the available records reveal this time might have been a significant episode of evolutionary diversification among key modern fish groups, similar to patterns reported for land-living mammals.”

Several factors might explain why the Ras Gharib A fish seem to have weathered the PETM.

First, previous estimates of sea surface temperatures exceeding 95 degrees apply broadly to tropical regions, but temperature data specific to the Ras Gharib A site is not yet available.

Researchers know it was possible the northern coast of Africa experienced an upwelling of cool water from deeper in the ocean. Or perhaps fish moved to deeper, cooler waters to avoid the warmest temperatures.

Another possibility: Marine fish at that time were more resilient than researchers thought. They evolved early in the Cenozoic Era when the climate was several degrees warmer than today.

“A more detailed picture of the setting in which these fishes lived is a key part of the puzzle,” Friedman said. “There’s much more to do when it comes to studying the fossils and their broader environmental context.”

Through collaborations, paleontologists can flesh out the fossil record in important regions, such as the tropics, and fill gaps in the story of life on Earth, Friedman said.

The PETM-aged fish fossils from Ras Gharib A were found in a layer of dark gray shale and include examples of more than a dozen groups of bony fish typical of the Eocene, the geological epoch that began with the PETM. Whole fish are relatively abundant, but many are small, measuring an inch or less in total length.

A group called percomorph acanthomorphs — which includes familiar Michigan fish such as walleye, bass and bluegills — are the most diverse kind of fish from Ras Gharib A. Other fish at the site include deep-sea hatchetfish and predatory species called bony tongues, whose relatives live in freshwater today.

The single most abundant fish type found is a moonfish from the genus Mene, represented by more than 60 specimens. Today, Mene is restricted to tropical and subtropical regions of the Indian and Pacific oceans. But during the PETM, these fish inhabited the tropics and were also found as far north as Denmark, showing how the warm period allowed some creatures to expand their range.

While the fish from Ras Gharib A survived and may even have thrived during the PETM, coral reef ecosystems were practically wiped out at low latitudes. Clams and snails showed a muted response, and some types of plankton diversified, according to Friedman.

“Impacts on ecosystems involve the interplay of multiple groups. The survival of one group in isolation shouldn’t be taken as evidence that changing climates are something to brush off,” he said.

“Also, it’s important to keep in mind that while the PETM is the best ancient analog for modern climate change, it’s still an imperfect comparison.

“By some estimates, humans are now releasing carbon dioxide into the atmosphere at more than 10 times the rate that led to the PETM,” he said. “During the PETM, global climate responded to the added carbon by warming 9 to 14 degrees Fahrenheit (5 to 8 C) over thousands of years. Today, realistic emissions scenarios put us on track for around half of that warming over just a few centuries.

“It’s really a sign of how unprecedented the current situation is.”

(Edited by Judith Isacoff and Fern Siegel)