Ancient mass extinction of fish may have paved way for modern species
A report looks at a 360-million-year-old gap in the fossil record and finds that marine vertebrates were recovering from an extinction event on par with the one that killed the dinosaurs. What happened is unclear.
The report, by University of Chicago researchers, focused on events at the end of what is commonly called the Age of Fishes, which lasted from 416 million years ago to 359 million years ago. That age was followed by a 15-million-year period of relative silence in the fossil record.
Paleontologists had tended to ignore the rarity of fossils from that period, which is known as Romer's gap — assuming that the fossils just had not been found, or shrugging it off as an unusual period of low diversity. But in a paper published online Monday in Proceedings of the National Academy of Sciences, the authors proposed that Romer's gap is a sign that the world's marine vertebrates were recovering from a global-scale extinction event.
That gap left ocean niches bereft of weird, now-extinct fishes like the giant armor-plated Dunkleosteus that had ruled the seas up till then, permitting then-marginal species such as sharks to gain ascendency, scientists said. If they had not, the forms of vertebrates existing today may have been very different.
That event was one of the most devastating in Earth's history, on a par with the one that killed off the dinosaurs 65 million years ago, said Lauren Sallan, a paleontologist at the University of Chicago and lead author of the paper.
And just as the fall of the dinosaurs made room for mammals to rise, she said, this extinction made way for modern marine life such as sharks and the ancestors of modern fish — as well as for tetrapods, ancestors of terrestrial vertebrate life.
The Age of Fishes earned its nickname from the diversity of marine life at the time.
During this period, lobe-finned fishes — descendants of which include the lungfish — ruled the oceans, as did armored fish called placoderms and many other forms of fish that no longer exist. Placoderms, which were a dominant life form on Earth for 70 million years, included a diverse array of species, including the 8-meter-long Dunkleosteus and the much smaller Bothriolepsis, which had arm-like spines near the front of its body.
According to data analyzed by Sallan and her colleague Michael Coates, about 345 million years ago — after the extinction and subsequent 15-million-year silence in the fossil record — the ecological balance of power had shifted. Sharks had risen to the top. Ray-finned fishes, of which there had been a mere dozen or so species that mostly looked like salmon, diversified extensively, leading to the appearance of such creatures as angel fish and eels in the fossil record, Sallan said.
Today, Sallan added, there are about 30,000 different species of ray-finned fish.
Researchers are still unsure what precipitated the extinction. There is some evidence of a period of low oxygen levels in the oceans at that time. Sallan said it is also possible that a miles-deep drop in the ocean's water level caused the die-off. But the causes of this major change will have to be studied further, she said.
Sallan said the popular view of evolution was to think of it as a steady improvement — as evidenced by the idea that tetrapods, or four-footed life forms, "conquered the land," somehow defeating now-extinct species.
"But by finding this extinction event," Sallan said, "basically, [we show that] those kinds of progressive stories can't be used anymore." Luck — bad and good — plays a huge part in what species come out on top, she said. "There's no reason for dinosaurs to die 65 million years ago except that they got killed by an asteroid."