Monarchs of the Sea: The Extraordinary 500-Million-Year History of Cephalopods

by Danna Staaf

spherocones as vertical migrators.15 Vertical migration is a habit that lots of marine animals share today.

the heteromorphs, or “other shapes.” Among the multitude of heteromorph oddities are a number of helices shaped like soft-serve ice cream. They would have been challenged to swim in straight lines, but migrating up and down in slow spirals would have been an efficient way to extract every particle of food from the surrounding water.

“Imagine a Cretaceous evening scene a few miles offshore at dusk. At a depth of 100 meters or so, vast shoals of helical ammonites . . . are slowly corkscrewing their way upwards. . . . For tens of millions of years, these elegant, pirouetting predators must have been a very distinctive and beautiful part of the marine realm.”

In addition to helical ice-cream cones, heteromorphs grew in the shape of large aquatic paper clips and in bewilderingly recurved knots.

In recent years, evidence has mounted to suggest that the two most abundant groups of heteromorphs did not swim fast, nor yet migrate vertically—instead, they hovered near the seafloor, scavenging or filtering odds and ends from the water and from the ground.

The most abundant kind of heteromorphs were called baculites. These creatures didn’t live in a coil, a helix, or a knot. The shell of a baculite was quite simply straight—an unexpected throwback to the early days of cephalopods.

One of the largest shale formations in North America, the Pierre Shale, contains fossilized examples of entire ecosystems known as methane seeps. They formed yet another component of that broad Western Interior Seaway, and they probably functioned much like the ones in our modern oceans. Methane seeps as we know them today begin with methane and hydrogen sulfide gas bubbling up from underground. These chemicals attract gas-hungry bacteria that attract grazers, which then draw larger predators—octopuses today, ammonoids in the Cretaceous.

Called scaphites, these are the ones that grew their shells into paper-clip-type hooks, and except for baculites they were the most abundant heteromorphs.

Squid and octopus beaks are made of chitin, a stiff composite of linked sugars and nitrogen. Nautiluses, by contrast, have solid beaks made with calcium like our bones.

Ammonoids, meanwhile, extended their tremendous diversity of form and habit to their jaws. Some, like coleoids, built them with chitin; others, like nautiloids, built theirs with calcium. And some ammonoids expanded, flattened, and modified their lower jaw almost beyond recognition.

Snails have doors like this, called opercula, which you can see by turning your garden variety upside down. No living cephalopods have opercula, though the leathery hood of a nautilus serves a similar purpose.

Other scientists suggested that aptychi might protect specific organs, like gills or ovaries, the way our rib cage protects our lungs and heart. Someone even raised the possibility that aptychi were the shells of parasitic males living inside females.

It wasn’t until the 1970s that paleontologists had amassed enough evidence to be certain that aptychi were modified lower jaws.

An ammonoid might have used such an aptychus like a baleen whale uses its enormous jaws, scooping up a mouthful of ocean and then pressing out the water while retaining all the tiny edibles.

list of previous proposals: “lower mandible, protection of gonads of females, protective operculum, ballasting, flushing benthic prey, filtering microfauna and pump for jet propulsion.” Nothing daunted, they proposed an eighth: stabilizing ballast during swimming.

What Kruta found in the ammonoid fossils was an unfoldable radula covered with delicate, comblike teeth. Overall, in shape, it resembles the radulas of modern sea snails that feed on plankton.

Such evidence makes a strong case that these baculites—and quite possibly all ammonoids with aptychi—ate plankton.

Indeed, Mesozoic evolution spun the three threads of cephalopod history in wildly different directions: ammonoids followed an action adventure, packed with fast-paced speciation and extinction, death and survival in equal measure. Meanwhile, nautiloids puttered along without much obvious change, characters in a contemplative tale. And coleoids, as we’re about to see, displayed one blockbuster success while quietly cooking up another one in the background.

Named belemnites after the Greek word for “dart” because of their streamlined shape, these coleoids grew a straight internal chambered shell along with a solid shell guard to serve as a counterweight. On the outside, they looked rather like squid, with two fins and ten hook-covered arms.

But without the shell they were vulnerable, so a new defensive tool arose: ink. Never seen in nautiloids or ammonoids, ink is often preserved in coleoid fossils, thanks to the stability of the pigment melanin. Fossilized belemnite ink was first discovered by English paleontologist Mary Anning in 1826. Her friend and fellow fossil hunter Elizabeth Philpot reconstituted the ink to draw ichthyosaurs, beginning a trend of fossil ink illustration that continues today.

Vinther to wonder whether melanin could be found in other fossils—like dinosaur feathers. It could, and eventually Vinther published definitive evidence of dinosaur coloration, including a species with black-and-white-banded wings and reddish head feathers.

Squid modified their fourth pair of arms into tentacles; octopuses modified and eventually lost their second pair of arms. This is yet another case of convergent evolution, like nautiloids and ammonoids arriving separately at the coiled shell.

Suckers, on the other hand, are generally thought to have evolved only once, although they’ve developed to look quite different in modern squid and octopuses. Octopus suckers are flexible and versatile; they can grab and manipulate small objects in addition to suctioning onto larger ones. Squid suckers are more rigid but their suction is much stronger; they sit on stalks like umbrellas blown inside out, and they often contain little rings of teeth as hard as fingernails.

Some species of squid have no suckers at all, instead lining their arms and tentacular clubs with hooks. The most famous of these is the colossal s...

Squid hooks and sucker rings are made of the same tough material that coleoids use for their beaks: chitin. (Scientists and engineers have found an astonishing array of uses for squid chitin in recent years, from prosthetics for amputees to biothermoplastic for 3D printing.)

No suckers, rings, or hooks have been found on any fossil ammonoids or nautiloids, so these appendage accessories are considered one of the many exclusive coleoid inventions.7 The ink sac is another, of course, and so is the breathtaking ability to change skin color, pattern, and texture.

Another significant difference is that fish went on to evolve color vision by increasing the variety of light-sensitive proteins in their eyes; coleoids never did and are probably color-blind. I say “probably” because the idea of color blindness in such colorful animals has flummoxed generations of scientists, and a few have suggested that modern squid and octopuses have the potential to exhibit unconventional kinds of color vision. Perhaps light-sensitive pigments distributed throughout their bodies could send color signals back to the brain.9 Or maybe, by quickly changing the shape of their eyes, coleoids could scan through a series of wavelengths, comparing each new view to earlier views in order to see color over time.

Color-blind or not, coleoids can definitely see something we humans are blind to: the polarization of light.

The internal shell of the most thoroughly studied early coleoid, Hematites, still had a hard tubular “living chamber” that contained most of the animal’s meat. Its jet propulsion would have been of the less efficient shell-pumping type seen in modern Nautilus,

“But octopuses don’t have pens,” you may be thinking. Astute reader, you are correct. Their ancestors did, however, and the structure of the octopus pen can still be seen in one living species: the vampire squid, which really should be called a vampire octopus.

Vampyropoda, which is a weird word. It means “vampire feet,” and no, that doesn’t make any sense. It’s simply the marriage of the two group names Octopoda and Vampyromorpha, conducted by scientists when they realized that octopuses and vampire squid are more closely related to each other than either is to any other cephalopod.

Ancestral vampyropods diverged from the ancestors of squid way back in the Triassic.

Sometime in the Jurassic, as coleoids were reinventing themselves, the vampyropod lineage split—into vampire squid on one hand and octopuses proper on the other, both still bearing pens.

octopuses proceeded apace along the path of shell reduction. Around the end of the Jurassic or beginning of the Cretaceous, octopuses had another evolutionary split.

One lineage, the cirrate octopuses, kept the gladius as a single piece that eventually became shaped like a horseshoe. These are deep-sea octopuses with big floppy fins, the most noteworthy of which is named the Dumbo octopus

“Cirrate” refers to rows of short tendrils called cirri that grow along their arms. The cirri probably serve some feeding purpose, but observing a deep-sea octopus’s dinner is difficult. It’s almo...

Incirrate octopuses, or, as I like to think of them, real octopuses, have no cirr...

About 10 million years after the meteor impact, global temperatures rose enough to be given a name—the “Paleocene-Eocene Thermal Maximum”—and

This thermal maximum is considered the best model within our planet’s history for the sort of global warming that we’re creating today, as it was caused by a similarly excessive release of carbon dioxide.

Many animals responded to the heat spike by migrating. On land, they went north or south, toward the poles and away from the equator. Some marine animals did the same, but they also had the option of moving deeper. Few creatures were truly cold-adapted, so it didn’t matter that there was nowhere truly cold to live.

Then Earth’s climate began a lengthy switch from warm and melty to cool and icy, possibly thanks to a single energetic plant named Azolla. This fast-growing aquatic fern could have grown so quickly and abundantly that it slurped huge quantities of carbon dioxide

Normally such carbon drawdown would simply get recycled back into the air, as bacteria or animals consumed the plant material and breathed the carbon out. But with the right conditions, masses of Azolla could have sunk quickly to the ocean bottom and been buried before decomposing, constituting a one-way transfer of carbon from the atmosphere into the earth. As its blanket of carbon dioxide thinned, Earth rediscovered the joys of glaciers and ice caps.

South America began to head toward North America while Australia moved toward Asia, both of them leaving Antarctica behind at the South Pole. The ocean was now free to slosh around this southernmost continent in a continuous circle, creating a circum-Antarctic current that further isolated Antarctica.

Once Antarctica chilled enough for the seawater around it to start freezing, a strange chemical fact came into play: there is no such thing as saltwater ice. When seawater freezes, it leaves its salt behind. The water around the forming ice thus becomes even saltier—and saltier water can stay liquid at colder temperatures. So whenever a bit of seawater freezes, it creates two things: freshwater ice and extremely salty, extremely cold liquid water.

this frigid saltwater sinks to the bottom of the sea, following the dips and ridges of the seafloor to spread across ocean basins. That’s why the modern deep sea is so cold.

This was the beginning of a global “ocean conveyor belt” that still flows today, carrying profound implications for everything from weather patterns to fishing grounds. Deep cold water not only flows along the seafloor, but is drawn back up to the surface in “...

Behavior is certainly a key aspect of the convergence between cephalopods and fish. Both developed large brains relative to their forebears, and both are frequent subjects of fond behavioral anecdotes from hobby and professional aquarists alike.

Scientists have corroborated such tales with empirical studies of tool use in octopuses. In the wild, these animals collect and arrange rocks to modify their dens, likely improving defensibility against predators.

In one study, researchers gave octopuses an empty bottle and observed that, after bringing it to their mouths to answer the initial “Is this food?” query, the animals began to treat it like a toy. Several octopuses used their water jet to send the bottle circling around the tank, over and over, which reminded the researchers of a human bouncing a ball.

people often wonder if squid are equally intelligent. Unfortunately, squid intelligence is a difficult study, because these animals are accustomed to swimming freely in the open ocean and don’t take well to captivity.