ANN ARBOR—Fish have a missing chapter in their history: after an asteroid slammed into Earth 66 million years ago, there is very little evidence of fish in the fossil record.
When whole fossil fish skeletons become common again about 10 million years after the asteroid impact, they include many species that look different than those that came before, leaving scientists to wonder what evolutionary paths fishes took between about 66 and 56 million years ago. Now, a research team including University of Michigan graduate student Sanaa El-Sayed has discovered the earliest known examples of six modern fish groups that still swim in Earth's seas today.
The finding, published in Science Advances, describes marine fishes dated to 62.2 million years ago, helping to fill a 10 million year interval with sparse fossil information about modern fish evolution. Among the findings include the earliest known fossil skeletons of jack, a type of sportfish, moonfish and pipefish, the family to which seahorses belong.
The first clues of the new fish site in the Eastern Desert of Egypt came from a geologist who provided dates for the deposit. El-Sayed realized that this earlier study mentioned a fish fossil bed, dated to almost the middle of the 10 million year fossil record gap. She, along with the lab team of Hesham Sallam , founder of the Mansoura University Vertebrate Paleontology Center (MUVP) in Egypt, began excavating the site.
"We have this 10 million year gap with a very limited fossil record. We know the asteroid impacted the marine environment, but it was unclear how the oceans came to have these modern fishes," said El-Sayed, a doctoral candidate in the Department of Earth and Environmental Sciences and Museum of Paleontology, who led the work. "It was mindblowing that this site is now helping us answer the questions of when and where and what was present in the modern ocean just a few million years after the dinosaurs went extinct."
In total, the researchers discovered 21 kinds of fishes across nine orders of fish. Most of the fishes are percomorphs, a major group in today's oceans but which were relatively uncommon during the age of dinosaurs, according to Matt Friedman , co-author of the study and director and curator of the U-M Museum of Paleontology.
The findings also reinforce the idea that the biological crisis event linked to the asteroid impact, called the Cretaceous-Paleogene (K-Pg) extinction, led to the demise of certain kinds of fishes, followed by the rapid establishment of other groups of fishes that look distinctly modern.
The research was supported by Mansoura University in Egypt, National Geographic, the Ministry of Higher Education of the Arab Republic of Egypt, the American Association of University Women, the Paleontological Scientific Trust, the U.S. National Science Foundation and U-M.
Patterson's Gap
Friedman has long been vexed by a fossil-poor part of the record around the K-Pg, a gap he and colleagues called Patterson's Gap after a paleontologist who had previously noted it. Because of its timing, the gap muddies our picture of how fishes were impacted by the extinction.
"This gap early in the Cenozoic record leads to two interrelated questions," said Friedman, also a professor in the Department of Earth and Environmental Sciences. "First, did the fish that we generally assume went extinct at the end of the Cretaceous Period really not limp into the next interval, called the Paleogene, and we've just missed them because the record is poor? Second, when did the more familiar modern groups appear? The gap represents a long span of time during which we have poor grasp of what happened, and it's frustratingly coincident with one of the most interesting intervals of Earth's more recent history."
Perhaps, researchers thought, that the gap in the fossil record meant the conditions did not exist during that time period to optimally preserve fish as fossils. That meant uncertainty about which groups the extinction impacted and how recovery proceeded.
"Here we have this remarkable deposit that opens a new window on this critical time. There are plenty of skeletons preserved, but none of the kinds of fishes we thought went extinct were there," Friedman said. "Our findings suggest that those fish likely did go extinct at or around that major cataclysm at the end of the Cretaceous, rather than their absence just reflecting a lousy record. At the same time, the site provides direct evidence that a lot of these modern-looking fish groups were established pretty early on."
The researchers also wondered what their findings meant in the broader context of the fossil record early after the K-Pg extinction. Comparing their findings to information from other fossil deposits, the researchers found that most of the percomorphs found just after the extinction event were mostly in the tropics. There appeared to be fewer percomorphs in higher latitudes. Only long after the extinction did percomorphs seem to become common everywhere.
"There is a coarse but intriguing geographic pattern to how these modern-looking faunas arose. Maybe they developed in the tropics, for instance, and then spread to higher latitudes as climates changed or as these groups dispersed. That will be something to test more critically as we continue to improve the record," Friedman said.
Widening the fossil search
The researchers say there could be many reasons why the 10-million year gap has occurred. One reason could be that the majority of paleontological work has been concentrated in Europe and North America, missing valuable deposits that might be present elsewhere—something El-Sayed hopes will change.
"It's always good to look at other places for finding fossils. We can't keep focusing on Europe and North America," she said. "New discoveries like this site in Egypt are showing us some of the oldest examples of this group, and this, in the long run, will change how we understand modern fish evolution. We are examining a site that's very well dated—62.2 million years old. It's hard to get more precise than that."
El-Sayed is also a senior student researcher at MUVP and assistant lecturer at Mansoura University, where she studied vertebrate paleontology as an undergraduate. There, she was the first student to work in the first lab in Egypt studying vertebrate fossils, led by Sallam, the first Egyptian scientist to earn a doctorate in vertebrate paleontology.
"What we are seeing now is only a small light illuminating a long and previously dark corridor in the early history of modern marine fish evolution. This Egyptian site shows that many important answers are still waiting to be discovered," said Sallam. "Exploring older and new fossil localities in Egypt will be a major focus of our ongoing collaboration between our center and the University of Michigan."
The study's coauthors include the geologist who first identified the site, Robert Speijer, of KU Leuven in Belgium, as well as Belal Salem, Abdullah Gohar, Shorouq Al Ashqar, Mohamed Amin and Hossam El-Saka of MUVP and U-M's Hadeel Saad.
