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Roger Weller, geology instructor

by Sean Fossgreen
Historical Geology
Spring 2010

                                               Prehistoric Cephalopods

         Animals of the Class Cephalopoda are of the oldest known animal groups on the planet.  They seem to appear in the fossil record in the Cambrian around 550 million years ago.  They evolved very quickly with the other complex life forms and have been very adaptive and hence successful through the many trials and tribulations life has been put through.

         There are three major groups to have evolved in the class:  the Nautiloids, the Ammonoids, and the Coleoids.

Photo Credit: Roger Weller


         Nautiloids are the first to show up on the fossil record, appearing en masse in the Ordovician and Silurian periods during the early Paleozoic Era.

In between 505 and 408 million years ago, these simple-shelled organisms begin to thrive.  Straight-shelled nautili like the one pictured were the only large animals to be able to freely swim above the sea floor.  At the time, many fish were small and bony, and sharks were still small and primitive.

         Nautiloids probably hunted, killed, and ate anything they were capable of, much like modern-day sharks.  On the other hand, some may have developed a more whale-like habit of eating smaller crustaceans like the whales eat krill today.

         In any case, modern-day nautili seem to solely content with fulfilling a carrion-like role, scavenging off of other creatures kills.

         Nautiloids did not all have coiled shells like their modern-day descendants, instead they had straight shells.  They did, however, possess the gas-filled chambers of modern-day nautili.  These gas-filled chambers controlled the nautiloid’s buoyancy in the water.  Coiled shells like the accompanying nautiloid pictured did not arrive in the line until later, after the ammonoids separated from the nautiloids.  They did develop a water jet-propulsion system before the ammonoids broke away.

          Nautiloids seem to be some of the most resilient creatures ever to evolve, having outlasted even their close cousins the ammonoids when they experienced decimation.  Today’s nautili fill a niche not filled by many large creatures:  they live in low-oxygen environments—something other Cephalopods are utterly incapable of doing due to blood that doesn’t hold on to oxygen quite as well as their fish counterparts.
Photo Credit:  Roger Weller


          The ammonoids seem to be virtually identical to their relatives the nautiloids.  They, however, developed something special about their shells.

For increased strength, the chambers in the shells of the ammonoids ceased to be smooth and rounded-off and developed a more textured appearance.  Some were as subtle a difference as the one pictured above, others as intricate as the following:
Photo Credit:  Roger Weller

          So what are the advantages of such a strange adaptation?  It turns out that with a crinkled pattern, the shell actually has more strength.  This can be good if adapting to live in rougher waters, or generally more dangerous habitats.

          Ammonoids are the most prevalent Cephalopod fossils found from 408 to 65 million years ago, playing major roles in most marine ecosystems around the globe from the Devonian to the latter part of the Cretaceous.

          Separately from the nautiloids, the ammonoids also developed a coiled shell.  Not only did the ammonoids stop there, however, they continued to specialize.  They also developed ribbed shells and spiny shells as well.  While for defense, this probably meant that those ammonoids that specialized in such a way fulfilled less of a fish-like role as a more of a crustacean-like role.  This is probable because the ribs and spines would hinder quick movement, but would be excellent for a slow bottom-dweller that feeds on smaller creatures on the sea floor.

          Many successes and catastrophic failures were experienced by the ammonoids.  Many ammonoid groups lasted only a million or so years before going extinct.  Others lasted and thrived for hundreds of millions of years, even becoming the ancestors to new ammonoids as other varieties died out.

The ammonoids’ demise is still fairly uncertain, but a number of different factors are now blamed for the destruction of the ammonoids.  Predators, for one, able to break even the ammonoid shell became more prevalent, leading to the slow ammonoids’ demise.  Also blamed are sea level and climate change, meaning that the ammonoids weren’t able to keep up with some kind of catastrophe that might have come as a result of the same thing that killed off the dinosaurs.

Photo Credit:  Roger Weller


          Coleoids seem to have developed at the same time as their ammonite cousins, though independently.  The most obvious difference that the coleoids have from the other members of Cephalopoda is that they either have an internal shell no longer used for defense, like the Belemnites above, or did away with their shells completely, like the squid and octopus that now dominate the Cephalopod Class.

          It is difficult to pin down a date when the Coleoids lost their shells, as no fossil record can be left behind if there are no hard parts to an animal.  It’s possible that octopus-like animals existed very distantly in the past, but there is no way to conclusively prove it due to a lack of fossils.

          Belemnites, however, had single, internal, chambered shells used entirely for buoyancy.  This group arrive during the Jurassic (231 million years ago) in abundance, and all but disappear in the Cretaceous (65 million years ago.)  Their internal shells were very well-developed, but were not strong enough to provide significant protection from predators.

          Octopi and squids seem to both have lost and reduced (respectively) their shells independently of each other, rather than one being a descendant of the other.

          But what would be the benefit of losing a protective structure like that?  For Octopi, it can be seen in their habits.  Many Octopi dig or go into small caves and holes.  A shell of any kind would most definitely hinder such practices.

          But in the squids, what is their reason to have such a drastically reduced, chamberless, shell?  Squids are very maneuverable, very much more so than they could be if they had a heavy, buoyant, outer shell.

          Shell-loss among the cephalopods of today could also be explained by predatory dangers from hunters such as dolphins who use echolocation techniques to find food in the water.  A buoyant, gas-filled shell would have been a hindrance and a dead-giveaway to the survival of a Cephalopod.

Whatever the reason, Cephalopods have proven themselves as some of the toughest life forms to evolve.  Hard to eradicate through any one means, the Cephalopods have enjoyed the domination, completion, and simple fight for survival of the oceans for hundreds of millions of years.