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Roger Weller, geology instructor

Armored Fish
Freddie Arreola

Historical Geology
Spring 2005
                    The Evolution of Armored Fish

The evolution of fish is an extraordinary subject.  Fish have been small, big, finned, un-finned, jawless, and with monstrous jaws.  The legacy of fish stretches all the way back to the beginning of the Paleozoic period.  The first of the armored fish appeared during the Ordovician.  From the Ordovician through the Devonian, fish ruled the world.   Bone is another characteristic which enhanced the structure of the fish.  Fish started out as cartilaginous and later developed a bone skeleton.   Throughout the evolution of fish we can see many key aspects take place such as the jaw and vertebrate structures.  Armored fish are the key to the successful evolution of fish.


The rise of the ostracoderms (shell-skinned) occurred 510 million years ago.  Most of the world if not all was covered with water during the Paleozoic.  Shallow seas covered continents especially in modern day Ohio, Kansas, and the rest of the Great Plain states in the U.S.  These warm shallow seas left well preserved fossil deposits for us to later excavate.  Most ostracoderms had a bony plate on the outside of their cartilaginous body.  Their armor covered their head and upper torso exposing their back half which was made up of cartilage.  Many ostracoderms had scratches and pieces knocked out of their armor from predators or rough terrain.  The major predator theorized for these scratches and dents are the Eurypterid (sea scorpion, as seen to the left courtesy of Philippe Janvier  Since the jaw was non-existent at this time the ostracoderms were filter feeders.  Their mouth was located either on the bottom or the front of their head.  Some ostracoderms such as the Sacabambapis (as seen above, courtesy of Philippe Janvier) had no front fins it only had a rear fin much like an eel.  These types of fish use the “S” motion in order to swim rather than fins to help flap their way through the water.  During the Silurian a bony vertebral column began to appear in fish.  The fish began to have complicated structures and had many spikes in their evolution.   The suspected cause for rapid evolution is the movement and placement of the continents.  In the beginning of the Devonian nearly all ostracoderms were wiped out.  Other fish began to show up in huge quantities after most of the ostracoderms were gone.  Fish popped up in huge numbers during the Devonian giving it the name of “The Age of Fish”.

            Placoderms (armor-skinned) arrived during the Silurian and Devonian time replacing the Ostracoderms.  The armor of the placoderm covered their entire head and some of their body.  Some placoderms even had armor surrounding their eyes and unlike the ostracoderms the placoderms had functional jaws.  The jaw is said to have derived from gills that took on a function like a jaw. Some ostracoderms had a jaw like structure, but had no teeth, so the blueprint for the jaw has been in effect for a long time. The monstrous Dunkleosteus (seen below courtesy of Rick Miller) did not have “teeth”, but extrusions of bone that connected directly to their skull.  The position of their fangs actually allowed them to sharpen themselves.  These fish reached up to five meters in length and had no known predators.  With no predators one would think that all the armor would be useless, but some theories suggest that it did have a purpose.  Scientists believe that a predator has not yet shown up in the fossil records or they were vicious creatures that ate each other.  The armor on the Dunkleosteus is over two inches thick.  Placoderms were a very successful group but their time on earth was brief and only lasted 50 million years.  Though their fossil records are short the placoderms separated into several different families across the world.  The Devonian contained vast periods of evolution among fish. These vast periods of change occurred due to climatic reasons, geological, competition, or all of the above.

            The evolution of a jaw, vertebrate, and fin revolutionized the anatomy of fish.  The jaw began as simple gill arches.


1.2. 3. Through the process of evolution (As seen above courtesy of Rick Miller) the gills began to take on a separate role that allowed the fish to open and close its mouth. No one knows if this function was for eating purposes or for respiratory purposes, but the jaws give fish an obvious feeding advantage.  The introduction of the jaw also changed the appearance of the fish’s head by extending it forward.  The vertebrate began as cartilage and later developed a bone structure to create a spinal column.  The early ostracoderms used their spinal column to contribute to their balance while they swam.  The ostracoderms did not have fins to help support their bodies in the water until late in their existence.  The first fins may have looked like one fin such as the one the modern day manta ray has.  This fin would increase stability and movement in the water.  Finned fish can be broken down in to two groups the lobe fins and the ray fins.  The lobe finned fish have two pairs of fins one in front and the other in the back which resemble legs.  The lobe finned fish are the ancestors to the first amphibians.  Ray finned fish have two fins in the front and are the ancestor of modern day fish today.  The diverse evolution that took place during the Paleozoic is unimaginable. 

            Ostracoderms and placoderms played a major role in the history of fish.  Fishes armored ancestors were first to have jaws, the ostracoderms developed a functioning vertebrate, and the placoderms are found in both ray fins and lobe fins.  Amphibians would eventually crawl out of the oceans and on to land as relatives to lobe finned fish which breaks down the barrier between land and sea animals.  Some people believe our ancestors are fish.  Many characteristics can be tied into the structure and overall anatomy of fish.  So one can only imagine their great, great, great, GREAT grandfather being a huge Dunkleosteus.


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