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A mélange
of shark teeth found in Alberta from the Cretaceous period,
when dinosaurs roamed the Earth.
Image courtesy of Dr.
Mark V. H. Wilson, University of Alberta. |
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Sharks: Then and Now
Dinosaurs may have come and gone but sharks are here to stay
Story by Sarah Everts
Older than dinosaurs, sharks survived two major extinctions and are still swimming today.
When this aquatic predator first evolved some 455 million years ago, the only land animals
were millipedes and scorpions, and there was little vegetation to speak of.
During that era, the ocean was teeming with life — life that would be mostly wiped
out in the Paleozoic extinction, which took place just before dinosaurs evolved. It all happened
close to the equator in the Iapetus Ocean, which was bordered by the Laurentia land mass,
what was to become North America and Europe. Impressively, sharks survived the Paleozoic
extinction and the K-T event (Cretaceous-Tertiary mass extinction) that wiped out the dinosaurs.
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| The oldest shark fossil (Doliotus problematicus)
in the world, found near Atholville, New Brunswick. (Image courtesy of the New Brunswick
Museum) |
What made sharks so resilient?
Paleontologists say this is a tough question to answer, but most agree the persistence
of this predator is probably due to a sound initial design. Sharks got off on the right foot … or
fin, so to speak.
Since sharks evolved, they have had skeletons made of cartilage rather than bone and are thus
part of the Chondrichthyes class. Ancient sharks, much like their modern predecessors, have
replaceable teeth — a feature that makes them excellent predators. The oldest intact
shark fossil in the world, Doliotus problematicus, found near Atholville, New Brunswick
by Canadian paleontologists Richard Cloutier, Randall Miller and Jeff McGovern, confirmed that
ancient sharks replaced their many rows of teeth much like their modern counterparts.
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A 15-centimetre fossilized tooth of Carcharodon megalodon,
an extinct giant shark of the Miocene age (18 million years ago).
(Image courtesy of Dr. Mark V. H. Wilson, University of Alberta) |
Mark Wilson, a paleontologist at the University of Alberta in Edmonton, says ancient sharks
were also probably able to detect the electrical muscle activity of their prey — a
characteristic that helps modern sharks hone in on their dinner when murky waters prevent
accurate visual positioning.
Some basic features did evolve over time. For example, the shape of shark teeth changed.
Early forms of sharks had smooth, pointy teeth with many side cusps — like the bumps
on human molars. Modern shark teeth are serrated and have few, if any, cusps.
Other features, like how shark jaws attach to the braincase and the shape of shark vertebrae
and pectoral fins, also evolved.
But most of the changes were just “fine tuning,” says Cloutier, a paleontologist
at the University of Quebec at Rimouski. Cloutier points out the design of early Chondrichthyes
species suited their successful predatory lifestyle and led to an evolutionary longevity.
Sharks persisted through the seas of time because they started off being compatible with
their surroundings.
“Sharks have no danger of going extinct by their own activities. They are well-adapted
to the marine environment,” says Wilson. “But human activity does pose a real
danger to shark survival.”
This ancient predator’s numbers have been dropping globally. In the last fifteen
years most shark populations in the northwest Atlantic Ocean declined by 50 percent, and
the hammerhead, thresher and white by more than 75 percent. After surviving mass extinctions,
sharks may be succumbing to human interference, such as shark-finning, by-catches in the
fishing industry, over hunting, and pollution in our oceans.
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