When we talk about climate change, we think of ocean levels rising, warming oceans, and extinct fish species. It is widely documented that sudden spikes in temperature could cause devastating loss of life across the globe in many different ways. Flooding, mass extinction, and possible natural disasters are just some of the long-term effects we are already seeing worldwide. But a new finding in this field shows that fish are very hardy and withstood much higher ocean temperatures in the past.

Marine organisms exist on a very primitive and well-established chain. Change in water temperature affects the smallest plankton and coral which has a knock-on effect on larger fish, marine mammals, and reptiles. The analog used to measure modern-day global warming is a period in Earth’s history called the Paleocene-Eocene Thermal Maximum, or PETM. It was a short duration where the ocean temperatures were highly elevated around 56 million years ago. 

But a new discovery in Egypt of fish fossils found in an eastern Egyptian desert site shows that fishes during the PETM period lived in very hot and tropical waters, a fact that was previously unknown. The study, from a team of Egyptian scientists and a University of Michigan scientists, using these fossils, has gleaned some understanding of the marine ecosystem during the extreme warming event which could provide insights for the future.

The findings of the study of these fossils, published in the Journal Geology, show the impact of this warming event on life in the ocean. How did marine life cope? Can the warming now be a change that marine creatures adapt to? 

“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,” said Friedman, director of U-M’s Museum of Paleontology and an associate professor in the Department of Earth and Environmental Sciences in a press release by University of Michigan, Michigan News.

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 bony fish that inhabited the oceans during this period have often been a source of many questions and mysteries. The fossil findings allowed the team to examine their skeletal system closely. Previous information of the PETM estimated that sea surface temperatures in some parts of the tropics likely surpassed 95 degrees Fahrenheit (35 C) at that time, a temperature capable of wiping out most cean species today.

But the fossils show a diverse range of fish across a range of shapes and sizes, showing thriving diversity, 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,” El-Sayed said. “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.”

But how did these fish survive and potentially thrive in temperatures that none of the fish today could survive? Was the change in temperature gradual that these fish adapted or are marine species more robust than we previously thought? 

The study states that the temperatures of the prehistoric waters around the Ras Gharib A site is not yet available. The study says that  it is possible that pockets of isolated oceans, placed inland, could have avoided the rapid change, allowing small pockets of fish to adapt and survive. Also, it is possible that deep ocean water in the northern coast of Africa cooled down because of water from deeper in the ocean. 

Another possibility is 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, paleontologists 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.

“Impacts on ecosystems involve the interplay of multiple groups,” he said. “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 analog for modern climate change, it’s still an imperfect comparison. It is known that global warming today is  caused by human interference and we do not know how this differs from teh natural events that preceded the PETM temperature change. 

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, Friedman said.

But, it provides us an interesting point of comparison and could allow researchers to explore the more robust ocean species in today’s oceans and see what sets them apart from the vulnerable species.