Extinctions that obliterate wide swathes of life have
reshaped Earth’s history. Typically triggered by an enormous environmental
change — like an asteroid impact or a significant shift in climate — these
events give scientists an intimate look at how life recovers after a cataclysm.
Researchers believe they’ve now pinpointed a previously unknown planetary-scale
reset that occurred about 19 million years ago.
اضافة اعلان
This extinction event transpired in the world’s oceans and
decimated shark populations. The sharks still have not recovered from the
damage, the team suggests in a paper published last week in Science.
Scales cover the bodies — and even the eyeballs — of sharks.
Known as “dermal denticles,” the scales function like protective armor and
their ridges also reduce drag as the animals swim, said Elizabeth C. Sibert, an
oceanographer and paleontologist at Yale University. The scales are microscopic
— each one is only about the width of a human hair — but sharks slough off
about 100 denticles for each tooth they lose, making them common in the fossil
record.
This abundance makes them valuable to scientists seeking to
understand the past, said Paul Harnik, a paleobiologist at Colgate University
who was not involved in the research.
“It’s a sheer numbers game,” Harnik said.
In 2015, Sibert received a box of mud spanning about 40
million years of history. The reddish clay, extracted from two sediment cores
that had been drilled deep into the Pacific Ocean seafloor, contained fish
teeth, shark denticles and other marine microfossils. Using a microscope and a
fine paintbrush, Sibert picked through the sediment and counted the number of
fossils in samples separated in time by several hundred thousand years.
About halfway through her data set, Sibert spotted an abrupt
change in the fossil record. Nineteen million years ago, the ratio of shark
denticles to fish teeth changed drastically: Samples older than that tended to
contain roughly one denticle for every five fish teeth (a ratio of about 20
percent), but more recent samples had ratios closer to 1 percent. That meant
that sharks suddenly became much less common, relative to fish, during an era
known as the early Miocene, Sibert concluded.
Sibert and her collaborators, in an earlier study using the
same data set, had also found that sharks declined in abundance by roughly 90%
about 19 million years ago.
“We had a lot of them, and then we had almost none of them,”
she said. “Basically the sharks almost completely disappear.”
These declines in relative and absolute shark abundance
suggest that something happened to shark populations about 19 million years
ago, Sibert concluded.
But there was still the question of whether a true
extinction occurred, she said.
“We wanted to know if the sharks went extinct or if they
just became less prominent,” Sibert said.
To test the idea of an extinction, she recruited Leah D.
Rubin, a marine scientist then at the College of the Atlantic in Maine.
Together,
they developed a framework to identify distinct groups of denticles.
The researchers settled on 19 denticle traits — such as
their shape and the orientation of their ridges. Sibert and Rubin sorted
roughly 1,300 denticles into 88 groups. These groups don’t correspond exactly
to shark species, but seeing more groups is an indicator that a shark
population is more diverse, the researchers proposed.
Of the 88 denticle groups present before 19 million years
ago, only nine persisted afterward. The reduction in shark diversity suggests
that they experienced an extinction around that time, Sibert and Rubin
concluded. In fact, this event was probably even more cataclysmic to sharks
than the dinosaur-killing asteroid impact that occurred 66 million years ago,
they said.
“There were just a small fraction that survived into this
post-extinction world,” Sibert said.
The effects of this extinction were probably felt around the
world. The consistent results from the two sediment cores — separated by
thousands of miles — suggest that this was truly a “global event,” two
paleontologists, Catalina Pimiento of the University of Zurich and Nicholas D.
Pyenson of the Smithsonian Institution, wrote in a perspective article that
accompanied the study in Science.
So far, the cause of this die-off remains unknown. There
were no significant climatic changes in the early Miocene, and there’s no
evidence of an asteroid impact around that time.
“We have no idea,” Sibert said.
“It’s a fascinating mystery,” Rubin added.
Sharks never fully recovered from this incident, and they
have been declining in abundance in recent decades because of overfishing and
other human-caused pressures, said Nicholas Dulvy, a marine conservation
biologist at Simon Fraser University, who was not involved in the research.
Scientists in the far future who pull up sediment cores dating to the 20th and
21st centuries might see an even bigger extinction signal than this one 19
million years ago, he said.
“There’s a very real risk," he said, "that it is
going to look worse than any of the mass extinctions that have ever happened on
Earth.”
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