Marine Fish Thrived During Last Period Of Global Warming Say Fossil Hunters

A brief period in which there were elevated global temperatures over 50 million years ago is showing that fish nevertheless managed to flourish.

Researchers made the claims in a paper published in Geology as confirmed in a statement by the University of Michigan into the Paleocene-Eocene Thermal Maximum, or PETM, that was a short interval of highly elevated global temperatures some 56 million years ago.

The period where temperatures across the world rose is frequently described as the best ancient analogue for present-day climate warming.

The moonfish Mene, which is still alive today in the Indian and Pacific oceans, is a common fossil at Ras Gharib A. Scalebar equals 10 mm. (Sanaa El-Sayed, The University of Michigan/Newsflash)

The paper is particularly significant because fish are among the organisms thought to be most sensitive to warming climates, and tropical sea-surface temperatures during the PETM likely approached temperatures that are believed to be lethal to some modern marine fish species.

However, the newly discovered fish fossils show that in this period marine fish at least seem to be thriving during the PETM.

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

U-M palaeontologist Matt Friedman, co-author of the study and director of U-M’s Museum of Paleontology as well as being an associate professor in the Department of Earth and Environmental Sciences said: “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.

“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, and there are even hints that important diversification in the group might have happened around or just after this time.”

A team of paleontologists from Mansoura University Vertebrate Paleontology Center excavates fish-bearing rock layers at Ras Gharib A in Egypt. (Sanaa El-Sayed, The University of Michigan/Newsflash)

The newly discovered fossil assemblage, known as Ras Gharib A, was excavated from a site in Egypt’s Eastern Desert, roughly 200 miles southeast of Cairo and west of the Gulf of Suez and the Sinai Peninsula.

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 fishes.

But the Egyptian fossils capture 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.

The lead author of the paper is Sanaa El-Sayed of Egypt’s Mansoura University, an Egyptian palaeontologist who will be an incoming University of Michigan doctoral student this fall.

He said: “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.

“In fact, the available records reveal that 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 help explain why the Ras Gharib A fishes seem to have weathered the PETM.

First off, 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.

A percomorph, a member of the same group that includes familiar Michigan species such as walleye and perch, from Ras Gharib A. Scalebar equals 10 mm. (Sanaa El-Sayed, The University of Michigan/Newsflash)

The experts know that it was possible that the northern coast of Africa experienced an upwelling of cool water from deeper in the ocean, for example. Or perhaps fishes moved to deeper, cooler waters to avoid the warmest temperatures.

Another possibility was that marine fishes at that time were simply more resilient than researchers had thought. After all, they evolved early in the Cenozoic Era when climates were already 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. “This report really marks the beginning of a research project, and there’s much more to do when it comes to studying the fossils themselves and their broader environmental context.”

Through international collaborations like this Egyptian project, palaeontologists can flesh out the fossil record in important regions like the tropics, helping to 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-grey shale and include examples of more than a dozen groups of bony fishes typical of the Eocene, the geological epoch that began with the PETM. Whole fishes are relatively abundant, but many individuals are small, measuring an inch or less in total length.

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

The single most abundant fish type in the assemblage is a moonfish from the genus Mene, represented by more than 60 specimens. Still alive today, Mene is now 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 ranges.

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

A percomorph, a member of the same group that includes familiar Michigan species such as walleye and perch, from Ras Gharib A. Scalebar equals 10 mm. (Sanaa El-Sayed, The University of Michigan/Newsflash)

He said: “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.

“Also, it’s important to keep in mind that while the PETM is the best ancient analogue 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. 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.”

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