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  • 500 million years ago, back in the Cambrian Period, a pioneering little mollusk floated

  • up off the ocean floor.

  • It had developed a way to use its defensive shell for a whole new purposebuoyancy.

  • It turned out that by filling its shell with gas, this mollusk could literally reach new

  • heights, gaining a key advantage over its relatives on the seafloor.

  • Scientists believe that this was the very first cephalopod — a group that now includes

  • squids, octopuses, cuttlefish, and nautiluses.

  • While we might think of the nautilus with its shell as an oddity today, the fact is

  • that the ancestors of modern, squishy cephalopods like the octopus and the squid all had shells.

  • And early cephalopods are actually defined by their shells -- or, more specifically,

  • by how their shells adapted to suit their needs.

  • Some cephalopods truncated their shell.

  • Others acquired a different shape.

  • Some of them internalized their shell like a backbone.

  • And, in some cases, they got rid of the thing altogether.

  • In ancient times, the shell was cephalopodsgreatest asset.

  • But it also proved to be their biggest weakness.

  • Mollusks were some of the first truly complex animals, probably appearing in the late Ediacaran

  • Period.

  • Although there is some evidence of hard mineralized shells from this time, shells as we know them

  • today become much more common after the Cambrian Explosion, which -- not coincidentally -- is

  • when the first evidence of predators appears.

  • So, early shells worked like shields, protecting the soft body, or mantle, of the animal from

  • predators lurking above.

  • But by the late Cambrian, one mollusc known as Plectronoceras had acquired a couple of

  • adaptations that marked the beginning of a brand new form of transportation -- and a

  • whole new kind of mollusk.

  • For one thing, its shell was divided into sealed-off chambers by thin walls called septa.

  • As the animal grew, it added new chambers to its shell.

  • This in itself wasn’t new, but it ended up being instrumental to another adaptation

  • that was.

  • As Plectronoceras added septa to its shell, it left behind a small, tube-like part of

  • its mantle in each chamber.

  • This little tube of tissue is known as a siphuncle, and as unassuming as it seemed, it helped

  • Plectronoceras perform a trick the world had never seen before.

  • By making the blood that flowed through the siphuncle super salty, Plectronoceras was

  • able to absorb all of the water from the chambers in its shell.

  • As water diffused out of the shell and into the salty blood, gas seeped in, and what was

  • once a suit of armor became a personal floatation device.

  • The very first true cephalopods had arrived, and they looked like tiny, adorable, upside-down

  • ice cream cones.

  • This development of a gas-filled, chambered shell, also known as a phragmocone, was a

  • triumphant, history-making adaptation.

  • By the time the Cambrian had segued into the Ordovician, cephalopods had entered a golden

  • age.

  • There were few predators to threaten them, and a rise in ocean oxygen levels caused life

  • to flourish, diversify, and occupy new habitats, providing an abundance of food.

  • This is known as the Great Ordovician Biodiversification Event.

  • And that’s when they got very big.

  • The Ordovician endoceratids cephalopods were the biggest animals of their time, reaching

  • an impressive 6 meters in length.

  • And as the Ordovician progressed, cephalopods began to leave the shallows to explore the

  • open ocean.

  • So they had to find ways to become more fast and agile.

  • Some species developed shells that coiled, forming a more compact and maneuverable form,

  • like the modern nautilus.

  • By the Silurian Period, a genus called Sphooceras tried a different tactic: Instead of coiling

  • its shell, it broke off the end of it.

  • Sphooceras periodically wrapped part of its soft mantle around the outside of its shell,

  • and then secreted enzymes that helped break off the chambers at the end.

  • This made the end blunter, shorter, and sturdier.

  • Which in turn made the shell less vulnerable to breaking and easier to maneuver.

  • Sphooceras might be the very first cephalopod that kept its shell inside its mantle for

  • any length of timeand this was an experiment that was about to be taken to a whole new

  • level.

  • That’s because a new evolutionary pressure was waiting for cephalopods in the Devonian

  • Period: fast, jawed fish.

  • While fish with jaws first appeared in the Silurian, they proliferated in the Devonian.

  • And that kicked off an evolutionary arms race between fish and cephalopods.

  • Up until this point, all cephalopods had been members of the slow and steady group known

  • as nautiloids, from the pioneering little Plectronoceras to the imposing Cameroceras.

  • This ancient lineage still survives today in the form of the modern nautilus.

  • But in the Devonian, a new branch of the cephalopod family tree appeared: ammonites.

  • And they coped with the rise of fish with a live-fast, die-young strategy.

  • Unlike nautiloids, which grew slowly and invested a lot of energy into making a few offspring,

  • ammonites grew quickly and had many offspring.

  • They ended up being so successful, diverse, and numerous that their shells are now used

  • as index fossils to define Periods in the Mesozoic.

  • And ammonites developed a huge variety of shell sizes and shapes, growing shells that

  • looked like hooks or knots or even paper clips.

  • Then, around the beginning of the Carboniferous, a new lineage appeared with an even more radical

  • strategy to deal with the fish problem.

  • They were the first coleoids

  • Like Sphooceras millions of years before them, coleoids wrapped their soft mantles around

  • their hard shells.

  • But unlike Sphooceras, they kept it there permanently.

  • Hematites, for example, was one of the earliest coleoids, and it had a cone-shaped shell inside

  • its soft body.

  • Then, over millions of years, the shells began to shrink, and what remained was built with

  • lighter-weight material.

  • After all, internal shells no longer offered protection, so there was no reason to keep

  • lugging around all that extra weight.

  • So they lost the gas-filled chambers that had kept them afloat, and developed new ways

  • to stay buoyant, and new, faster forms of jet propulsion to get around.

  • In time, the internal shell was streamlined down to a long, chitinous structure, kind

  • of like a backbone, called a gladius.

  • All squid alive today still have some kind of gladius, while octopuses have a pair of

  • similar structures called stylets.

  • Armed with these adaptations, coleoids began to take advantage of a new niche: the deep

  • sea.

  • While the old gas-filled phragmocone couldn’t withstand the pressure of the deep ocean,

  • the gladius had no such problem.

  • And their ability to live in the deep turned out to be what saved coleoids from extinction.

  • At the end of the Cretaceous Period, a fatal blow struck the ammonites and most of the

  • nautiloids: the Cretaceous-Paleogene extinction event -- the same event that killed the non-avian

  • dinosaurs.

  • Acid rain changed the pH of the oceans, compromising the integrity of the shells these animals

  • needed to survive.

  • This hit baby ammonites, which relied on their thin, fragile shells to passively float near

  • the ocean surface, especially hard.

  • At the same time, there was likely a massive die-off of ammonitesmain food source,

  • plankton.

  • The nautiloids were probably saved by their slow and steady lifestyles, and six species

  • in two genera have survived to the present day.

  • But the coleoids were able to take refuge in the deep sea, and were no longer dependent

  • on their shells.

  • So with the ammonites gone, when conditions improved, the coleoids rose up and took their

  • place.

  • Today, coleoids have colonized every marine ecosystem on the planet, and they play a vital

  • role in ocean food webs.

  • Instead of relying on a suit of protective armor, they now use intelligence, camouflage,

  • and agility to outsmart predators and prey alike.

  • Their journey from small, passive molluscs to sleek, voracious predators took hundreds

  • of millions of years of trial and error -- from developing shells to survive, to finally learning

  • to thrive without them.

  • And the squid still swims around with its gladius intact, and the octopus with its stylets

  • -- reminders of the history they share with the shelled creatures of the past.

  • Thanks for joining us today!

  • And as always, I want to know more of what you want to learn more about!

  • So leave me a comment below, and don’t forget to go to youtube.com/eons

  • and subscribe.

  • And please. Tell people about how cool

  • our channel is

  • And if you want to learn more about how

  • life on earth functions with a real life biologist

  • my friend Dr. Joe Hanson, you can check out

  • It's Okay To Be Smart. Also from PBS Digital Studios.

500 million years ago, back in the Cambrian Period, a pioneering little mollusk floated

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鱿鱼如何丢壳(How the Squid Lost Its Shell)

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    joey joey 發佈於 2021 年 05 月 04 日
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