Sharks have existed in one form or another for millions of years.
Just when you thought it was safe to go back in the water, it was never safe.
It has never been safe!
Hey there street sharks, HOST here for DNews.
Sharks are truly ancient.
There’s some disagreement on when they actually appeared on the scene, but any way you slice
it, they were here almost 200 million years before the first dinosaurs.
It’s pretty impressive we can trace their ancestry back that far, when you consider
that sharks have skeletons made of cartilage, not bone, which inherently doesn’t fossilize
as often because it’s easier to break down.
In fact the earliest evidence of sharks is from just a handful of scales found in Colorado
that date back 455 million years.
These scales have a characteristic tooth-like structure to them that makes us reasonably
certain they were once stuck on something we would classify as a shark.
The earliest undisputed shark scales are 420 million years old and were found in Siberia.
But aside from their scales, we have no idea what early sharks looked like.
When you hear “shark” your first thought is probably of a torpedo shaped superpredator
with rows of teeth like steak knives, but millions of years ago their shape could have
been something we wouldn’t recognize.
It’s the definition of what makes a shark a shark that allows for a lot of flexibility.
Sharks must have replaceable teeth, internal fertilization, paired fins, the aforementioned
cartilaginous skeletons and distinct scales, and of course, Jaws.
So when the fossil record starts becoming richer around the devonian period 400 million
years ago, sharks of all shapes and sizes start popping up.
We’ve found shark teeth that are two pronged like the “rock on” symbol, suggesting
that they had to eat their prey whole instead of ripping chunks out of them like they do
today.
Early shark bodies may have been more eel-shaped, with long tapering tails.
Even when they began looking more familiar, they were still quite different.
They tended to have long jaws that were at the front of their heads, and they couldn’t
extend them to catch prey like modern sharks.
There were actually two major adaptive radiations where shark diversity exploded.
The first was during the carboniferous period starting 360 million years ago.
Mass die offs of competitors meant sharks could thrive, and in particular one group,
the stethacanthids, really pushed the envelope of style.
One stethacanthus species had an anvil on its back, and nobody's quite sure what advantage
that gave it.
Maybe it was waiting for the hammerhead to evolve so they could fashion swords together
and create an unstoppable shark army.
Another shark, the Helicoprion (hell-icko-pryon), appears to have had a buzz saw shaped lower
jaw.
But of course, we don’t see these guys around today.
At the end of the Permian 250 million years ago, 99% of marine species died out, including
most sharks.
Some sharks made it through to the other side though, possibly by sticking to deeper seas
where the water was less acidic.
These sharks were minescule, only an inch long.
But they would be the start of something big.
By the jurassic period 42 million years later, they would cause the second adaptive radiation
of sharks, leading to modern sharks as we know them today.
Sharks have survived through 5 mass extinctions because they’ve taken on so many different
forms and filled so many niches.
But today their populations are dwindling.
The rate we catch sharks, either intentionally or as bycatch, is devastating for species
with a low reproductive rate.
And sometimes we intentionally slaughter sharks in the name of protecting swimmers near coast
lines.
In the last 450 million years, sharks have seen a lot, but never anything like us, and
many species may not last much longer.
If you want to learn more about what sharks have become tune into Shark Week which kicks
off Sunday, June 26th 8/7c
Want to know what you can do to stave off shark extinction?
Tara has a video on that here.
Would you rather swim with one 10 foot long shark, or 1200 1 inch long sharks?