THE EVOLUTIONARY STEPS OF FISH
THE EVOLUTIONARY STEPS OF FISH
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