Scientists at UCSD’s Scripps Institution of Oceanography have found that the major form of fishes began their dominance 66 million years ago, aided by the dinosaurs’ extinction.
Scripps graduate student Elizabeth Sibert and Professor Richard Norris analyzed the microscopic teeth of fishes found in sediment cores around the world and found that the abundance of ray-finned fish teeth began to explode in the aftermath of the mass dieoff of species, which was triggered by an asteroid strike in the Yucatan Peninsula.
Paleobiologists refer to this episode as the Cretaceous-Paleogene extinction event.
Ninety-nine percent of all fish species in the world – from goldfish to
tuna and salmon – are classified as ray-finned fishes.
They are defined as species with bony skeletal structures and have teeth that are well preserved in deep ocean mud.
Sharks, in contrast, have cartilaginous skeletons and are represented by both teeth and mineralized scales, also known as denticles, in marine sediments.
“We find that the extinction event marked an ecological turning point for the pelagic marine vertebrates,” write the authors in the study. “The K/Pg extinction appears to have been a major driver in the rise of ray-finned fishes and the reason that they are dominant in the open oceans today.”
The breakthrough for the researchers in reaching their conclusion came
through their focus on fossilized teeth and shark scales. In cores from
numerous ocean basins, they found that while the numbers of sharks
remained steady before and after the extinction event, the ratio of ray-finned fish teeth to shark teeth and scales gradually rose, first doubling then becoming eight times more abundant 24 million years after the extinction event.
Now there are 30,000 ray-finned fish species in the ocean, making this class the most numerically diverse and ecologically dominant among all vertebrates on land or in the ocean.
Scientists had known that the main diversification of ray-finned fishes
had happened generally between 100 million and 50 million years ago.
“The diversification of fish had never been tied to any particular event. What we found is that the mass extinction is actually where fish really took off in abundance and variety,” said Sibert, who is the recipient of an NSF Graduate Research Fellowship.
“What’s neat about what we found is that when the asteroid hit, it completely flipped how the oceans worked. The extinction changed who the major players were.”
Sibert and Norris think that some key changes in the oceans might
have helped ray-finned fishes along. Large marine reptiles disappeared
during the mass extinction, as did the ammonites, an ancient
cephalopod group similar to the chambered nautilus.
Those species, the researchers think, had been either predators of rayfinned fishes or competitors with them for resources.
“What’s amazing,” said Norris, “is how quickly fish double, then triple
in relative abundance to sharks after the extinction, suggesting that fish were ‘released’ from predation or competition by the extinction of other groups of marine life.”
Sibert noted that before the extinction event, ray-finned fishes existed in a state of relative ecological insignificance, just like mammals on land.
“Mammals evolved 250 million years ago but didn’t become really
important until after the mass extinction. Ray-finned fishes have the
same kind of story,” Sibert said. “The lineage has been around for
hundreds of millions of years, but without the mass extinction event 66
million years ago, it is very likely that the oceans wouldn’t be dominated by the fish we see today.”
The paper, “New Age of Fishes initiated by the Cretaceous-Paleogene
mass extinction,” appears June 29 in the early edition version of the
journal Proceedings of the National Academy of Sciences.
