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