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 purpose — buoyancy. 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 cephalopods’
greatest 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 time — and 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 ammonites’ main 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.
The Eons series is the best hing since Discovery channel's real documentaries on dinosaurs
Praise HELIX
squids with shells are OP pls nerf
I love this channel. It is one of those that are making youtube better than the discovery channel ever was.
Still a little uneasy with squids and so on after watching Gargantia years ago.
I'm confused. which ones were on the Ark?
Ammonites did not exist in the Devonian..