Chitons

A Chiton (Tonicella lokii) off the coast of California

Italo Calvino’s existential classic, Il cavaliere inesistente (“The Nonexistent Knight”) is a novel about Sir Agilulf, a medieval knight who follows the rules of chivalry with complete devotion. Unfortunately, Sir Agilulf is only an empty suit of armor: although the knight trains and stands vigils and fights in the manner of a person, there is actually nobody inside the shell. Calvino’s perplexing fable evocatively captures the workaday world where many (maybe most) people are empty “suits” who exist merely to keep a seat warm, however this predicament is nothing when compared to that of the chiton, a primitive marine mollusk covered by a row of aragonitic shells. Not only are the chiton’s rocky eyes a constituent part of its armor, the chiton itself is part of its armor!  Bands of dense muscles are interwoven through the plates and sensory cells are embedded within. When the chiton dies, the shell phalanx falls apart into perplexingly shaped hunks of calcium carbonate.   The remarkable segmented shells are composed of 8 plates and they afford substantial protection to the chitons which are capable of rolling themselves into armored balls. Chitons are classed as the Polyplacophora.  They take their common name “chiton” from the Latin word chitōn, which means “mollusc”. The Romans derived this word from the Greek word “khitōn”, meaning tunic.

Another Chiton (Tonicella marmoreal)

Cryptoconchus porosus

 

For locomotion the chiton relies on its rubbery “foot”, a large band of adhesive muscles with which it crawls along the sand and rocks of the ocean bed. A chiton’s foot can produce substantial adhesion, and the creatures are able to cling to rocks with amazing tenacity.  Most chitons are herbivorous grazers and eat algae, bryozoans, diatoms, and other microbes, all of which they scrape up with their sharp radula tongue–however, a few species of chiton have left the gentle lifestyle of herbivore behind and become predators. These hunting chitons trap their prey (usually small shrimp and fish) by making a box trap out of their enlarged, hood-like front end.  They hold this segment of their girdle above the ocean bottom and then clamp down on unsuspecting prey which thinks they are small cavern-like rocks.

The Gumboot Chiton (Cryptochiton stelleri), the world's largest chiton!

 

Chitons are ancient.  Fossils of stem-group chitons date back to the beginning of the Ordovician over 488 million years ago (and these lineages probably stretch back earlier into the Cambrian). Yet the Chiton’s rocky aragonite eye is comparatively recent, having evolved only ten million years ago. Chitons are hundreds of millions of years older than mammals but their eyes are much more contemporary than our own.

The Age of Cephalopods

A diorama portraying the seas of the Ordovician Period (from the Exhibit Museum, University of Michigan)

The Ordovician Period, the second period of the Paleozoic Era, took place from 488 to 444 million years ago.  During those 44 million years, the landmasses of earth were devoid of life except for hardy lichens, tiny algae, and a few lowly non-vascular plants. The oceans, however, teemed with diverse marine invertebrates and primitive vertebrates.  The ecosystems of these great shallow seas seem familiar–with colorful intermeshed filter feeders, grazing herbivores, and swift deadly predators. And yet the creatures are so alien as to make Ordovician reconstructions almost resemble another planet.  All of the creatures of a modern coral reef are replaced by strange analogs:  the dominant filter feeder were not corals but weird sponge-like animals—the archaeocyathids.  The grazers were conodonts and trilobites.  The predators were primitive sharks, huge scorpion-like eurypterids, and above all, the nautiloids–for the Ordovician was a time when cephalopods ruled the earth.

Clever mollusks with multiple tentacles, eyes, and a method of jet propulsion, cephalopods had evolved in the Late Cambrian from a snail-like ancestor.  Their taxonomy exploded in complexity during the Ordovician period.  They also left the shallow continental shelves to range across the pelagic ocean and to descend into the benthic depths.  The cephalopods of the Ordovician period were the nautiloids, animals which manipulated a bubble of air within a chambered shell to move up and down the water column.  They could grab prey with their many tentacles or retreat into their calcium-based shell. The family quickly exploded in complexity. To quote palaeos.com:

At least ten different orders flourished at this time, all but one appearing for the first time during the early or middle part of the Ordovician.  This astonishing diversity included straight, curved, loosely coiled, and tightly coiled shelled types, and even one group (the Ascocerids) that in order to become lighter and more streamlined lost the a large part of their shell altogether.  These intelligent carnivorous molluscs replaced the Cambrian Anomalocarids as the dominant life form and top predator of the world’s ocean.  The biggest, such as the endocerids, attained huge size; with shells of up to 10 meters in length they were the largest animal that, up until that time, had ever lived.

Some paleontologists have expressed doubts about this magnificent ten meter endocerid shell.  But, even so, it is worth remembering that this measurement did not include the tentacles and the head of the creature.  These giant orthocones must have been formidable predators, living on nautiloids, eurypterids, and jawless fish.  The great monsters are believed to have had weak eyes (and could probably be avoided by staying shell-side of the behemoths).  However an even bigger problem faced the tentacled masters of that world.

A giant orthocone

During Ordovician times, the land masses that are now South America, Africa, Madagascar, Antarctica, India, South Asia, and Australia were all joined together as a supercontinent, Gondwanaland.  Over tens of millions of years Gondwanaland gradually drifted into the Southern Polar regions of the globe.  This resulted in heavy glaciation, which in turn caused rapid deep freezes and sudden interglacial warm periods—in other words, an ice age.  This great ice age caused the depth of the ocean to fluctuate wildly which brought a crashing end to the Ordovician and its dominant cephalopods.   The mass extinction which ended the Ordovician period was the second worst in the history of the planet (eclipsed only by the mass extinction at the end of the Permian period).   More than 60% of marine invertebrates went extinct (including whole families of mollusks).   This was but one set back for cephalopods.  The family has burgeoned and then crashed many times, but it marked the end of their time as apex predator.

Various Ordovician cephalopods, including the infamous Cameroceras trentonese, shown feeding on a hapless Aphetoceras americanum, while a quartet of Cyclostomiceras cassinense swim by (illustration by Stanton F. Fink).