For mammals, the mass extinction event at the end of the Cretaceous was the crisis that opened the door to evolutionary success.With so many species gone, like the dominant dinosaurs (minus the ancestors of birds), the chances are plentiful.Our small, sneaky ancestors took full advantage of these opportunities, leading to the diversity of mammals today.
Perhaps finfish—which include nearly every fish except sharks and rays, and make up almost half of all modern vertebrate species—have found a similar opportunity.Researchers know that this group of fish only took off in the last 100 million years (so from the mid-Cretaceous), but early details are murky.Elizabeth Seabert and Richard Norris of the Scripps Institution of Oceanography set out to reinforce this history by searching for tiny fish teeth in the mud on the seafloor.
This seafloor mud comes from deep drilling at multiple locations in the Atlantic and Pacific.A sample of Italian limestone long enclosed in the ocean by plate tectonics completes the series.In all samples from the late Cretaceous (about 75 million years ago) to the mid-Eocene (about 45 million years ago), the researchers screened for teeth that had been lost from the fins and scales of sharks or rays.Both types of fossils are abundant because they do not dissolve on the ocean floor.
The researchers mainly looked at the relative abundance of teeth and scales.The number of scales was remarkably consistent, as was the number of teeth before the extinction event, but it wasn’t long before finfish teeth became more common.
By the end of the Cretaceous, less than a third of all sharks and rays went extinct, and looking at the scale counts in these cores, there is no sign of a decline in the overall population of the entire event.But after death, the finfish appeared to be much more numerous.Some niches will definitely open up for them, as the effects of losing more than 90% of plankton species with calcium carbonate shells ripple through the food web.For example, ammonites were important players before disappearing.
There are also signs of changes in the size of the finfish teeth.Just after the extinction, the larger teeth especially increased, suggesting that some changes were taking place in the fish community.At least in the core of the South Pacific, there has been a massive increase in the number of the largest teeth that lasted only a few million years after the mass extinction — possibly the result of some evolutionary experiments that didn’t last long-term.
The researchers also noted that the change at the end of the Cretaceous was abrupt, consistent with the Chicxulub asteroid’s impact, rather than a more gradual change during the giant volcanic eruption that began earlier.This is also what we see on land, but it’s still interesting given that recent research has timed all of these events more accurately.
Overall, this reads a lot like the story of our mammal family.The mass extinction at the end of the Cretaceous “appears to have been the main driver of the rise of ray-finned fishes and the reason they dominate the high seas today,” the researchers wrote.
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Post time: May-17-2022