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

by Danna Staaf

Pumping water over the gills becomes fueling up; expelling respiratory waste becomes propelling the body through water.

Endoceras giganteum, grew up to about 12 feet (3.5 m) long—taller than a basketball hoop and far bigger than any Anomalocaris, larger indeed than any living creature the world had yet seen.

The word “plankton” usually evokes microscopic representatives like single-celled plants, but it also refers to the great jellies and salps that can grow longer than a person—and possibly some of the largest fossil cephalopods.

Figure 2.4 The evolutionary history of cephalopods is here mapped onto the earth’s geologic timescale. Species of squid, cuttlefish, and all the rest that we see in the ocean today are merely the latest descendants of their storied lineages—they themselves were not around in the Triassic! Internal anatomy is drawn in coleoids to illustrate the steps of shell reduction, and it is omitted in externally shelled cephalopods to show shell ornament. Dotted lines indicate genera whose stories are told in the text; all other lines represent substantial taxonomic groupings. Danna Staaf and C. A. Clark

While cephalopods large and small spread throughout the Ordovician oceans, hunting, filtering, or scavenging (or perhaps all three), a rather tangential and unassuming lineage called vertebrates produced the first proper fish. They had fins and tails and gills; they even had skulls—but no jaws. They slurped up whatever food didn’t need biting or chewing and were therefore almost certainly no threat to cephalopods. Then, in the subsequent Silurian period, these odd bony creatures evolved something truly dangerous. Jaws were just as effective as beaks for catching and eating prey.

All these early jawed fish wore armor that looks rather clunky to us today—it certainly wasn’t the most hydrodynamic style. They were no sharks and they were no marlins. Still, they were nothing nearly as clunky as cephalopods, who had to truck around massive molluscan shells. Fish armor was at least articulated, able to bend and move. Cephalopod shells were less armor and more mobile bomb shelter.

better at eating. The first jawed fish had evolved back in the Silurian, but it wasn’t until the Devonian that they really went wild, producing updated versions of the awkwardly armored placoderms as well as a number of sharks and bony fish.

A coiled shell is harder to grab, harder to hold, harder to break. Cephalopods needed that defense.

A water column that once housed mostly small drifters and large but ponderously slow cephalopods was now a highly competitive racecourse, with the losers facing death by jaws.

bottom-dwelling benthos, drifting plankton, or actively swimming nekton

Most early animals back in the Cambrian had been benthos. Cephalopods were some of the first to move into plankton and nekton,

Now the Devonian was seeing an evolutionary shift in all animals, regardless of taxonomic affiliation, out of both bentho...

“The pressure on the benthic organism was rising,” says Klug. The Devonian saw “the evolution of spiny trilobites, spiny echinoderms [sea urchins]—evolution of defense mechanisms happened across all groups.”

“It could be that jaws evolved in cephalopods as a reaction to the evolution of jaws in fish,” speculates Klug.

It means that various prey species were evolving better armor and stronger defenses in response to fish jaws, and any cephalopods without comparably powerful chompers were going hungry.

the Devonian Nekton Revolution marked the beginning of a relationship that would last all the way through to the modern day. Fish and cephalopods, cephalopods and fish, both diversifying to fill the oceans, competing with each other and eating each other. “That seems to be the theme in cephalopod evolution,” says Vinther, “that they are constantly coevolving with fish.”

there’s some scientific hemming and hawing over whether interactions between the two groups are primarily competitive or predatory. Do cephalopods and fish jostle each other for the same prey—or are cephalopods just trying to escape fish jaws?

“Age of Ammonoids,” after the creatures that may well have taken up a central position in the marine food web. This could have been when cephalopods began to serve as ecological keystones, devouring the ocean’s smaller edibles and providing ample food for its largest denizens.

This is one arena in which cephalopods, both ancient and modern, are actually less alien than many animals—even other mollusks. Slugs, for instance, are hermaphroditic, and in the course of impregnating each other their penises sometimes get tangled, so they chew them off.

All living cephalopods have separate male and female sexes, so we assume that extinct cephalopods did too.

A male nautilus is known by his spadix, which frankly should just be called a penis since it’s erectile and serves to transfer sperm to the female.

Male coleoids are a little more reserved; they keep their penis equivalent inside their mantles and use a modified arm called a hectocotylus for sperm delivery.

Females of the octopus known as an argonaut are five times larger than males. (A killer whale is about five times larger than an average adult human,

Still, in a few fossil nautiloids, scientists have gone ahead and labeled the nautiloid macroconch “female” and the microconch “male.” However, the shells of modern nautiluses show the opposite pattern—males are somewhat larger than females, with a wider aperture to accommodate the spadix.

Carbon comes in a heavy version and a light version, both of which are readily available all over the planet. As Dieter Korn explains, “Organisms like to take from the global pool the lighter one to make their soft parts. If there is a lot of life on Earth, the pool is depleted in the lighter carbon.”19 When organisms die and decompose, their carbon is returned to the global pool.

Throughout the Permian, there was a lot of life on Earth. And so the available carbon was light on the light kind and heavy on the heavy kind. Then, about 250 million years ago, the rocks record a sudden influx of light carbon. “This can hardly be explained by natural processes,” says Korn. “Something was wrong with the carbon cycle, and the main hypothesis is that there were these big volcanoes in Siberia. An extremely big volcano burnt all the organic material in this area, and this was then the source of the light carbon.”

The Siberian eruptions are thought to have lasted 100,000 years, and their geologic impacts are easily visible today as around 772,000 square miles (...

It took a brutal toll on nearly every group, from wiping out 70 percent of vertebrates to exterminating substantial quantities of insects—the only time a mass extinction has affected this incredibly resilient group.

As the earth belched out massive amounts of carbon dioxide, the global temperature was cranked up by 11–18 degrees Fahrenheit (6–10°C). Equatorial ocean water might have reached or even exceeded safe hot-tub temperatures. Warm water holds less oxygen than cold water, so oxygen levels plummeted. At the same time, the ocean absorbed excess carbon dioxide from the atmosphere, which led to a chemical reaction that lowered oceanic pH.

the greatest death of the Great Dying occurred in the sea, where 96 percent of all marine species disappeared. Invertebrates were harder hit than vertebrates, but still a great many sharks and rays were lost. Ammonoids, of course, were devastated.

paleontologists describe a “Triassic Explosion” of animal diversity, comparable to the Cambrian in scale. Also like the Cambrian, it happened mostly in the ocean—the

Cephalopods seem to have been particularly well suited to take advantage of the dynamic environment. They bounced back after each aftershock of the Great Dying, experiencing a relatively low level of competition and predation from vertebrates.

Triassic oceans were a hotbed of evolutionary radiation. Warm water ringed the supercontinent and its outlying islands, and there were no ice caps at the poles. The first marine reptiles showed up in the early Triassic, and ammonoids recovered quickly from their near obliteration.

The pelagic realm, in short, was doing great. The seafloor took longer to recover, perhaps because of a persistent lack of oxygen.

Not only do modern nautiluses have the unusual ability to breathe easily when carbon dioxide levels are high, but they can also reduce their metabolism in low-oxygen water.

the Humboldt squid and the vampire squid, are well able to lower their metabolic needs when oxygen is hard to come by.

Marine animals often rely on chemical signals passing through the water in order to find mates, and when the water changes, these signals can go haywire.

Though its causes are uncertain, the impact of the end-Triassic extinction event was dramatic. Killing off most of the dominant predators on land, including Temnospondyli and crocodile cousins, it opened up space for the dinosaurs.

nautiloids were reduced to nautilids alone, the single narrow lineage that would lead to modern nautiluses. Nautilids puttered along from that day to this, growing slowly, building thick shells, and laying large yolky eggs.

for ammonoids, so many of them died off at the end of the Triassic that scientists have been challenged to find a single species that crossed the boundary into the Jurassic—though several must have done so, to become progenitors of subsequent generations. Whoever these survivors...

First came the ichthyosaurs, “fish lizards” with big propulsive tails, four smallish flippers, pointy heads, and no appreciable necks. They looked, and probably behaved, a lot like dolphins

plesiosaurs with long necks, small heads, and short slender tails that gave them a resemblance to the massive long-necked sauropods on land. Plesiosaurs propelled themselves with four large muscular flippers and probably went after small, slow prey. But then some plesiosaurs grew shorter necks and bigger heads

The reptiles in this short-necked subgroup of plesiosaurs are called the pliosaurs, in what is surely one of the most unnecessarily confusing bits of nomenclature ever.

Ichthyosaurs had begun to fade away by the Cretaceous, when the third group of reptiles showed up with a reprise of the large propeller tail that ichthyosaurs had sported. These were the mosasaurs.

One remarkable shell even bears bite marks from both a larger and a smaller mosasaur, which one scientist took as evidence of a parent instructing its offspring in proper hunting technique.5 That might seem a little far-fetched—it’s at least as likely that two unrelated mosasaurs of different sizes were squabbling over food,

Three groups of predatory marine reptiles would seem to be more than enough for anybody to contend with. But marine reptiles weren’t the only shell breakers, and cephalopods weren’t the only victims. The perpetrators of the Mesozoic’s widespread shell predation were fish and sharks, crabs and lobsters, and even snails, taking no pity on their kin. They crunched, cracked, drilled, and pried their way into virtually every mollusk shell in the sea.

Mollusks built thicker shells. They grew long spines as predator deterrents. They made smaller shell openings, sacrificing their own wiggle room in exchange for a more defensible front door.

most ammonoid shapes varied between three simple types: slim disks named oxycones (from the Greek root for “sharp”); loosely coiled serpenticones, which look like snakes; and fat globular shells called spherocones, which look like, you guessed it, spheres.

The quickest and nimblest were probably the oxycones, throwing themselves through the water like discuses. Paleontologists suspect they were active predators, jetting after their prey and capturing it alive.

Both the loose serpenticones and the globular spherocones are thought to have experienced too much drag to move quickly. Instead of hunting, they might have used their arms and jaws to sieve the water, consuming whatever tiny particles they happened across.