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Biology 103
Web Reports 1997
From Serendip


Rebecca Kagle
Fish are incredible diverse group, made up of three living classes that hold an important place both in modern ecology and in evolutionary history. Fish are incredibly successful, they are numerous and have a huge variety of adaptations. They are successful in every water filled habitat. Furthermore, fish are the first known vertebrates and also the stepping stone to all land-walking vertebrates (tetrapods). Fish are complicated, diverse, and have a long evolutionary history.

In order to understand the place of fish as both a transitory group and as a modern one it is important to understand the times in which and from which they evolved. Fish are the first known true chordates. The first vertebrate that has been found is the Upper Cambrian fossil Anaspis, which is more than 500 million years old (3). This fossil, while being fragmentary, is thought to be an armored, jawless fish (3). Fish did most of their evolving between five million and three and a half million years ago. These two periods were known as the Silurian and the Devonian periods. In yhe middle Silurian, the jawless fishes had diversified, but it was not until the Devonian that the true variety of fishes really flourished. In fact, the Devonian, is often referred to as the "Age of Fishes". Towards the end of the Devonian the first tetrapods (vertebrates which evolved true legs with which they could walk on land) had evolved from one specific branch of fish. Fish greatly specialized in their aquatic niche during both the Devonian and the Silurian and part of this evolution led to adaptations to land in the form of amphibians (9).

Fish are in no way simply a stepping stone to amphibians, they are a much more significant than this. They have evolved to be masters of their domain, the water. They come in many forms, have the ability to eat a huge variety of foods, and have populated almost every body of water. In fact, fish are the most common vertebrate, with there being approximately 24,000 species alive today. This number is mind boggling when put in perspective; the next most common vertebrate are birds with a mere 8,600 species (8). This multitude of species ranges in size, morphology, agility, and adaptations to environment (10). Fish have been broken down into a series of classes that separate them based on characteristics. These groups help to classify the wide variety of species that make up fish and help lead to understand of the current and evolutionary niche of fish.

The first fish to evolve were the Agnathans (Class agnatha) (7). These jawless fishes are the first vertebrates. These fish have round mouth parts that could be used for sucking or filter feeding (10). These rasping, sucking mouths are currently found on modern lampreys and hagfishes. These fish were often extremely armored in order to help them protect themselves. One group that evolved before the Silurian were the Ostracoderms which have been described as "small, blunt-headed forms...they fed on debris in the mud, bullet shaped swimmers, and some...with an astonishing array of spines and crests on their heads." (1). Most of these types of fish are currently extinct with the exceptions of the lampreys and the hagfish.

From these bottom feeding, jawless fish came the evolution of jawed fish. Jaws evolved only once (rather than evolving multiple times in different species through parallel evolution). Jaws evolved from gill arches which are the bony parts between gill slits. It is thought that a gill arch in an agnathan became fused to its skull (11). The upper part of the gill support became the top jaw and the bottom part of the gill support became the bottom jaw. Embryology points to this and the arrangement of nerves in shark heads and most simple fishes shows that jaws are in line with gill arches. While fish had the first bony jaws, they also have some of the most complicated. While the human head has only one moving part (the jaw), the head of a fish may have more than twenty-four bones that may move together in feeding (8). The evolution of the jaw is incredibly important because it led to fish to be able to ingest a much wider variety of foods and allowed them to be active hunters as opposed to passive filter feeder (1). This led to a wide variety of adaptations in morphology. Fish became more agile to be better predators, they were able to reduce their armor because they were less vulnerable, and their muscle density was able to decrease becuase they no longer led such a sluggish lifestyle (10).

The first jawless fishes include the acnathodians and placoderms. Even early in their development the jawless fishes showed great diversity. The acnathodians appeared around four and half million years ago. They were the first jawed fishes to evolve and they tended to be small, streamlined, and had huge eyes. Placoderms on the other hand appeared in the early Silurian and dominated the Devonian. These were some strange looking fish! As described by Peter Gore, "the Placoderms had bony armor that covered the head and forepart of the body. In many, a movable joint between the head and body armor let the head rock back to open the mouth wide. The primitive jaws had jagged bony edges that served as teeth. The tail end usually lacked protection"(11). At the beginning of the Devonian, Placoderms were small, but they soon increased in size. The Dunkleosteus, a placoderm, grew to be as large as 35 feet, had well developed jaws, with fang like teeth. The front of the trunk was heavily armored and the hind part was either bare or covered with small scales (11). The Dunkleosteus and all the other large placoderms are extinct, but in the Devonian they dominated both salt and freshwater.

While the huge, menacing placoderms have long since vanished from the earth, members of the Class Chondritchthyes, some of which are equally menacing, have not. This class includes the sharks, skates, and rays (along with some other fish) and these first evolved between 400 and 450 million years ago (1, 4). They probably shared a common ancestor with the placoderms (1). This class is commonly refer to as the cartilaginous fish because they lack true bone, instead they have cartilage and calcified cartilage for internal support. This type of skeleton is extremely light and flexible and helps these species be agile predators. Chondrithyes have evolved two kinds of jaws; sharks have extremely powerful biting and crushing jaws while skates and ray uses their jaws to be bottom feeders searching for mollusks (4). Sharks are often referred to as living fossils since they have not done much evolving since the Late Paleozoic. This is not to say that the sharks that exist now are they only sharks ever to exist. After the decline of the placoderms, there was a shark explosion that led to sharks with mouth's filled with coils of serrated teeth, or bony protuberances on their fins. These Paleozoic sharks could be a large as 14m, more than double the size of the current Great White Sharks.

In terms of currently living fish, Class Chondrichtyes make up only 900 species. The bulk of the fish are from Class Osteichtyes, the true bony fish. There are a reported 19,000 bony fish (this is a very conservative estimate) (5). This is an incredibly diverse class which includes everything from tuna to eels. These fish evolved more that 410 million years ago (Late Silurian), but did not rise until the middle Devonian as the placoderms and huge sharks began to recede in dominance (1). The bony fish, while being varied, all share an extremely important characteristic: a swim bladder. This probably evolved from lungs which had appeared in some freshwater species. The swim bladder is a internal structure which allows bony fish to float easily at any water level. The bony fish can be broken up into three major groups. The first are the ray-fins. These are most of the common bony fish such as tuna, bass, salmon (1). This group is very diverse and inhabits both fresh and salt water. The second group are the lungfish which are freshwater fish. The three genera which still exist are bottom feeders who use jaws to crush their prey. They can get up to 90% of their oxygen from the air if necessary (1). This can be very important when these fish live in areas prone to drought. The third group are the lobed-fin fish. This entire group was though to be extinct until the 1930s when a live one was recovered. It was called a coelacanth and is described as having a incredible swimming style which "consists of walking-like movement[s] of the various paired fins, with some wriggling of the body but no side to side beating of the tail as in most other fishes." (1). While this species is extremely important as a living fossil, the rhipidsistains, a group of lobe-fins, are also key sine they are the source of the first land vertebrates (1).

While there is still much controversy about the exact steps of evolution from lobe-finned fish to walking, land living, vertebrates (in the form of amphibians). There was originally much controversy over whether the limbs of tetrapods (limbed vertebrates) evolved in water or on land as an emergency feature. The original hypothesis was put out by Alfred Romer in the 1940s. It was that early lung fishes in times of drought had used their fleshy fins to pull themselves from a pond that was trying onto land to search for a more fruitful water source. The fish that made this journey successfully were able to reproduce and their offspring began to have modified limbs which were able to allow them to move from water source to water source and eventually these developed into true limbs (9). Romer relied on the idea that the end of the Devonian was very harsh and that fish would need to adapt rapidly in order to survive, but many believe that this is not the case. In fact, another theory is that the Devonian was so conducive to fish that the were able to have many offspring survive and that these were drawn to the land with its not yet exploited food sources (9). This theory still supports Romer's ideas that limbs evolved to aid in walking on land. A newer theory as developed by Jenny Clack in the early 1990s due to her studying of Ancathostega fossils. Ancathostega is one of the earliest tetrapods found, and by far the most complete. These creatures had four limbs, but its limbs were still proportioned like fins (very long radius in comparison to ulna) and had eight fingers and a weak wrist. They also breathed like fish, and had ribs that were too small to hold up its gut. In other words, these tetrapods were not adapted to walk on land (9). They had many adaptation to water, including a powerful tail that would lead to agility. Clack believes that Acanthostega evolved limbs to help maneuver through the Devonian habitat which consisted of very dense wetlands. These creatures could maneuver through these dense branches and plants by using their limbs rather than just having to "wiggle" through (9). They could also use the limbs to anchor themselves to wait silently for prey and then use their water adaptations to snag prey deftly. These huge tetrapods may have began to use land in a very gradual fashion, for breeding or to escape predators. The evolution of fish into land walking tetrapod is in no way close to being understood, but as more specimens are being found (a very early one was just found near the Poconos) the path of evolution may become more clear.

In conclusion, there is much to be learned from fish. They have evolved to fill every niche that includes water and have an incredible diversity of adaptations. Their evolutionary history is complicated and complex. They began as jawless, bottom feeders and evolved into sharks, rays, tunas, and many extinct species including the placoderms. Fish are also important as an evolutionary step. They are the first known true vertebrates and also provide a very key stepping stone to the evolution of all land living vertebrates. This evolutionary step from bony fish to amphibians is still not fully understood, and the research is very current. The multiple theories all have merit and a crucial debate is whether limbs evolved out of a push to be land dwellers or as an adaptation that made certain fish able to manipulate the water of the Devonian period. Many more specimens need to be found in order for this debate to lead to one side or the other. Fish are too often dismissed as either ruthless killers (sharks) or as a source of food, but in truth they are much more vital to our own history than most people care to think about.


1. Vertebrate Evolution, by Thomas Hebert, University of Miami

2. Vertebrates: Systematics, from Museum of Paleontology, University of California, Berkeley

3. Vertebrates: Fossil Record, from Museum of Paleontology, University of California, Berkeley

4. Introduction to the Chrondrichthyes, from Museum of Paleontology, University of California, Berkeley

5. Life of the Devonian, from Museum of Paleontology, University of California, Berkeley

6. A Devonian Time Line, from the Paleontological Research Institution, Ithaca, New York

7. Agnathans, from Royal Tyrrell Museum, Canada

8. When Fish Bite - Evolution of Fish Jaws, from the Florida State University Research Magazine

9. Coming Onto Land, from Discover Magazine

10. General Trends in Fish Evolution, from California State University, Chico

11. Evolution of Jaws, from DeKalb College


Comments made prior to 2007

"Fish did most of their evolving between five million and three and a half million years ago."

Can you see a problem with this? This would have been just before humans started eating them.

I think you mean "Fish did most of their evolving between five hundred million and three and a half hundred million years ago." ... Peter, 31 July 2007


penerbit buku's picture

thank you

thank you for the info, very interesting

matthew gilbreath's picture

how do they get lungs in the first place?

I get that they can flop on land, but how do they get lungs? wouldn't they suffocate and die? that is one of the many things that I don't understand.

Fluffy One's picture

Fish evolved lungs long

Fish evolved lungs long before leg like appendages. The swim bladders in modern fish evolved from lungs.

Serendip Visitor's picture

fish evolution

In this article above; the writer relates to the sharks and bony fishes having a common ancestor in the placoderms as the gnathostomes begin the evolutionary process. Question is if the placoderms had bony jaws in the evolutionary process then is Rasmussen correct when he suggested that the selachians evolved after the osteichthyes?

Serendip Visitor i doing pictures c;'s picture


Ahh i doing a report on evolution of fish by it hard to understand without picture c;?

Serendip Visitorblessed makotose's picture

The evolution of fish is

The evolution of fish is still a matter of controversy as all theorists are based on the evolution of jaws . this seems as if jaws are the only parts of the fish that have evolved

matthew gilbreath's picture

the lungs

they would suffocate and die before they could flop around some and maybe get the info to evolve, so how did they not suffocate on dry land if they had no lungs in the start?

YourTruely's picture

Geologic timelines quoted here ....


Fish did most of their evolving between five million and three and a half million years ago. These two periods were known as the Silurian and the Devonian periods.

This obviously should be five BILLION and 3 BILLION year ago .....

I have another query - please see if you could find an answer and update the content (which by the way I think is an excellent summary of a huge topic) -

Q. How did fish, especially very large catfish, get into upland freshwater habitats like mountain streams and plataeu lakes?

Thanks & Regards

Kelly Pomeroy's picture

Major period of fish evolution

Billion is not the correct time unit either. That would have fish evolving before the Earth was even created (four and a half billion years ago)! The intended time unit is obviously a hundred million years. She meant between 500 million and 350 million years ago. She just left out the word "hundred".

Quentin's picture

Swim Bladder from Lungs

You say that "The bony fish, while being varied, all share an extremely important characteristic: a swim bladder. This probably evolved from lungs which had appeared in some freshwater species."
My understanding was that what we now think of as lungs, actually developed from what were once swim bladders.
Out of interest, have you got any references to support the speculation that they developed from freshwater lungs?
Nice article by the way!

David Whitehurst's picture

Fish Interest

Do you watch Jeremy Wade's River Monsters? His show kind of shows that we don't realize the life of these fish in all of the world's waters. I truly think that the catfish species is probably one of the oldest. Sharks too but I have more interest in fish that hide in murky water.

Serendip Visitor's picture

fish evolution article

Excellent article. Very good explanation of complex topic.

Serendip Visitor's picture

Okay Article

This was helpful, but hard to understand. What about a diagram?

Eric's picture


Terrific post, exactly what I was looking for.

Some of the dates in the article seem to be off by a factor of 100, though. You talk about 4.5-5 million years ago, and I suspect you mean 450-500mya.

buzulak's picture

trans. species?

The coelacanths, which are related to lungfishes and tetrapods, were believed to have been extinct since the end of the Cretaceous period. They were considered the "missing link" between the fish and the tetrapods until the first Latimeria specimen was found off the east coast of South Africa, off the Chalumna River in 1938.

doesn't this just destroy the "geologic" column. Since lobe finned fish are still alive then they could be found in any layer, thereby destroying any validity in the "column".

Matthayichen's picture

The Creationist Art Of Not Thinking It Through

Sharks evolved very early. Nobody had to rediscover sharks. Sharks didn't invalidate the 'geologic column'.

p. s. There are no fossil coelacanths in the Pre-Cambrian layers.

Serendip Visitor's picture

Geologic column is non-existant

You seem to miss my point. Looks like the "column" takes another "hit". Of course, no such "column" exists-anywhere except the textbook. Right?
Coelacanths are still alive-YET thought to be extinct thousands of years ago. Problem is the "whoops" syndrome-guess what? We have coelacanths still living in the Indian Ocean.
Sharks evolved early?? What emperical proof have you? Were you there to annotate "early" evolution. If you weren't who was? Such nonsense. I can line up a plastic knife, spoon, spork and fork. Looks like they evolved but they didn't. Lining up things based upon appearance, size and shape mean absolutely nothing.
"Nobody had to rediscover sharks"-I guess not there still quite alive living among us.
Give my some proof of the "column". How deep would that "column" be-exactly? Who has checked all of the supposed "Pre-Cambrian" layers?

stephaniejean's picture


this artical was EXTREMLY helpfull! thank you!


Serendip Visitor's picture


Dear precious person!
Looking at the eyes of people it is so very hard to believe we are evolved from the monkey. Devil is real and he had been trying to deceive people for many years.
He became so good at it that we actually come to believe him. God loves us, God came to save us from our sins, from our disobedience to Him. God paid the price for our sins already.
God does not send anyone to hell. WE CHOOSE HELL ourselves by rejecting Jesus Christ and His gift of forgiveness. Accept His gift of forgiveness, you will see your life change forever.
God Bless you!

Serendip Visitor's picture


"God came to save us from our sins, from our disobedience to Him. God paid the price for our sins already.
God does not send anyone to hell. WE CHOOSE HELL ourselves by rejecting Jesus Christ and His gift of forgiveness"

Are you insane??
There is no such thing as sins, only morally pleasant or not, i.e. friendly or not!! So even if we are a good person, we are under threat for choosing not to believe that something is controlling us. Why would such a "divine" being threaten us.. oh yeah that's right, he wouldn't, because he/it doesn't exist. Only humans would choose to threaten others in this way, the higher intelligence controlling the weaker minded, by threat or duress, so that the one in control of your thoughts and behaviour can clearly have a submissive society, under control. Which worked for a while, but when people don't believe, and behave sociably, then the law of force and policing come into effect.
If there was a god, and he was so ultimately all mighty then surely he would not contradict so many words in "the book", nor would he allow for so many separations and divisions of religious beliefs.
Who said he was a MAN, or of the MALE sex, only a human would suggest such a notion, as 2014 years ago, man was generally more domineering than a woman.

YourTruely's picture

Actually it is a non-issue, time you forget it man

Hey Creationist:

Science and Religion are not at odds, so please dont bring in unnecessary standoffs into this or anything. All great scientists and all great philosophers have opined like this, it is only us lesser mortals who fight over small things using our small knowledge.

Whatever you said in terms of religion is true, in its own sociological and philosophical context and abstractions, and highlighting the eternal fight of good over evil, principles without which 'thinking beings' like mankind cannot stay together as a society and collaborate meaningfully.

That has nothing to do with how mankind has evolved biologically. Or the way the scientists have shown us as to how the universe works. More we learn more we appreciate the immense beauty and the inherent intelligence in its designs, Please call that God and find out, that it is not at all at odds with the traditional notions used in religion, barring of course some headstrong predujices and often fratricidal practices used in organized religions of the world.

Serendip Visitor's picture


cats are pretty cool, do you like cats?

Serendip Visitor's picture

I can't believe it took someone this long to respond

The human eye is virtually indistinguishable from most other primate eyes. The standard eye of jawed vertebrates evolved hundreds of millions of years ago. Know your facts before you go spouting off.

tyrone's picture

Why no pictures??? really,

Why no pictures???

really, with no good pictures, this article is hard to understand. Think Richard Meyer!!!