Introduction
This website will explain the evolution of Tetrapods and how their skeletal systems may have matched their environment. Firstly what is a Tetrapod? Tetrapods are a group of animals defined by having four limbs suggested by 'tetra', Greek prefix of four, and pod meaning ‘legs’. The tetrapod group consist of all descendants, living and extinct of the first lobed finned fish. These include humans, mammals, amphibians, reptiles and birds. Ultimately the descendants of these first lobed fish all adapted their own skeletal system to adjust and survive effectively in their environment.
There are four parts of this website that will be explained:
-Origins of the Tetrapods: The move from water to land
-Structure and function of the skeletal systems
-Natural selection and how the skeletal system of tetrapods changed with it
-Diversity of tetrapod skeletons today
There are four parts of this website that will be explained:
-Origins of the Tetrapods: The move from water to land
-Structure and function of the skeletal systems
-Natural selection and how the skeletal system of tetrapods changed with it
-Diversity of tetrapod skeletons today
Origins of the tetrapods : The move from water to land
At some time in history certain species started to evolve into all the species of animals we know today. At 385-380 million years ago there were two classes of known fish; Ray finned fish, and Lobe finned fish (Crossopterygians). Scientists believe that the tetrapod superclass can be traced back to the lobe finned fish. Fossils suggest the lobe finned fish had lungs (as in the present day lung fish). Lungs proved to be very beneficial to these lobe finned fish when not enough water was around to sustain gill breathers such as ray finned fish. When needed, these lobe finned fish could survive by breathing air if the water supply ran out, and natural selection would choose the survivors. (Zimmer, The origin of tetrapods)
The fins of Lobe finned fish are believed to be what the first legs of tetrapods formed from. The bones in these fishes fins were arranged in pairs and were supported by the skeleton, this allowed limbs to emerge. It is from here that the first land dwelling animals appeared however there are hypotheses’ as to why these fish left the water (Coates, 2002). The first hypothesis is thought to have been because of natural selection. It is thought that these lobe finned fish may have been caught in masses of water which dried out, this determined which of the fish died and allowing all that lived to evolve to suit the environment. The other hypothesis is that these fish were chased from the water by predators and took to the land; this acted both a refuge and a place to feed on insects and plants. However, to prove these statements and really understand the evolution of legs to enable the change from water to land, a very primitive tetrapod had to be found. (Zimmer, The origin of tetrapods)
Acanthostega dating from 360 million years ago is the most primitive tetrapod discovered as a complete skeleton and has given insights on how the fins of lobe finned fish evolved into legs. The first insight that shows Acanthostega’s part in the evolution of legs was found in its forearms. In Acanthostega's arms, it was found that the two bones, (found in most tetrapods, and known as the radius and the ulna), were found to be in different proportions to each other. The radius in Acanthostega was found to be almost a third longer than the ulna showing more resemblance to early lobe finned fish. The bones in the forearm of Acanthostega were then compared to those of later animals, which were much the same length as each other, showing that Acanthostega might have been, in evolutionary terms older. (Murphy, Acanthostega Gunneri, 2005)
Although it had limbs, Acanthostega was still unable to make the change to walk on land. This was because of the differing lengths in its forearm bones, which would have left the creature unstable when walking. Acanthostega also had the first fully structured hip bones, which can now be seen in all tetrapods today, but they weren’t fully fused to the spine and therefore, it could not support much weight. Due to its presumed inability to support itself on land Acanthostega was not the first tetrapod to leave the water . (Murphy, Acanthostega Gunneri, 2005)
Ichthyostega (see figure below) is a tetrapod discovered many years before Acanthostega. Ichthyostega, unlike Acanthostega, had evolved hip bones that were fully fixed to its spine. This provided the beginnings for a tetrapod to support its weight without water to hold it up. Unlike Acanthostega with its uneven, misshaped forearm bones, Ichthyostega evolved these bones roughly the same size and as a shape that could support itself. While no ‘hands’ have been found for the creature it is believed that it crawled around in the shallows, supporting it’s frontal body weight, while it’s back legs were thought to be functional but more useful for paddling. (Murphy, Ichthyostega Stenioei, 2006)
Descendants of Ichtyostega show the improvement in the complexity and efficiency of the legs, wrists and hands, e.g. the turlerpeton (see figure below), which enabled them to move far more efficiently. All of the different evolutionary stages of the legs eventually enabled more variety and diversity between the later species. The ability for weight bearing movement allowed species to emerge from a water based existence and develop skeletons to adapt to other environments. With the ability to now move, different species change from having fish like skeletons which only suited the environments that was mostly water based. This ability to move away from the water, many species’ skeletons evolved to suit and adapt to their environment. This therefore has led to all tetrapod (vertebrate) life on the land. (Murphy, Ichthyostega Stenioei, 2006)
The fins of Lobe finned fish are believed to be what the first legs of tetrapods formed from. The bones in these fishes fins were arranged in pairs and were supported by the skeleton, this allowed limbs to emerge. It is from here that the first land dwelling animals appeared however there are hypotheses’ as to why these fish left the water (Coates, 2002). The first hypothesis is thought to have been because of natural selection. It is thought that these lobe finned fish may have been caught in masses of water which dried out, this determined which of the fish died and allowing all that lived to evolve to suit the environment. The other hypothesis is that these fish were chased from the water by predators and took to the land; this acted both a refuge and a place to feed on insects and plants. However, to prove these statements and really understand the evolution of legs to enable the change from water to land, a very primitive tetrapod had to be found. (Zimmer, The origin of tetrapods)
Acanthostega dating from 360 million years ago is the most primitive tetrapod discovered as a complete skeleton and has given insights on how the fins of lobe finned fish evolved into legs. The first insight that shows Acanthostega’s part in the evolution of legs was found in its forearms. In Acanthostega's arms, it was found that the two bones, (found in most tetrapods, and known as the radius and the ulna), were found to be in different proportions to each other. The radius in Acanthostega was found to be almost a third longer than the ulna showing more resemblance to early lobe finned fish. The bones in the forearm of Acanthostega were then compared to those of later animals, which were much the same length as each other, showing that Acanthostega might have been, in evolutionary terms older. (Murphy, Acanthostega Gunneri, 2005)
Although it had limbs, Acanthostega was still unable to make the change to walk on land. This was because of the differing lengths in its forearm bones, which would have left the creature unstable when walking. Acanthostega also had the first fully structured hip bones, which can now be seen in all tetrapods today, but they weren’t fully fused to the spine and therefore, it could not support much weight. Due to its presumed inability to support itself on land Acanthostega was not the first tetrapod to leave the water . (Murphy, Acanthostega Gunneri, 2005)
Ichthyostega (see figure below) is a tetrapod discovered many years before Acanthostega. Ichthyostega, unlike Acanthostega, had evolved hip bones that were fully fixed to its spine. This provided the beginnings for a tetrapod to support its weight without water to hold it up. Unlike Acanthostega with its uneven, misshaped forearm bones, Ichthyostega evolved these bones roughly the same size and as a shape that could support itself. While no ‘hands’ have been found for the creature it is believed that it crawled around in the shallows, supporting it’s frontal body weight, while it’s back legs were thought to be functional but more useful for paddling. (Murphy, Ichthyostega Stenioei, 2006)
Descendants of Ichtyostega show the improvement in the complexity and efficiency of the legs, wrists and hands, e.g. the turlerpeton (see figure below), which enabled them to move far more efficiently. All of the different evolutionary stages of the legs eventually enabled more variety and diversity between the later species. The ability for weight bearing movement allowed species to emerge from a water based existence and develop skeletons to adapt to other environments. With the ability to now move, different species change from having fish like skeletons which only suited the environments that was mostly water based. This ability to move away from the water, many species’ skeletons evolved to suit and adapt to their environment. This therefore has led to all tetrapod (vertebrate) life on the land. (Murphy, Ichthyostega Stenioei, 2006)
( figure1 : Chart of the evolution of legs through species across millions of years )
Structure and function of skeletal systems
The skeletal system is a vital part of any animal. This system within the body of an animal (endoskeleton) or outside the animal (exoskeleton) is what supports the animal. The skeleton in all animals is firstly a structural system, helping the animal to move. To achieve this movement the skeleton must also be connected to a muscular system. The muscles in the body of an animal are what enable it to move the bones within its body. The skeleton also has many other uses than that of just support. (Ryan, 2012)
As most living animals have organs within their body a major function of the skeletal system is to provide protection to the organs from damage. As can be seen in the human skeleton, the rib cage protects our heart, lungs and other organs from physical damage. But there is one major part of the skeleton which protects the most sensitive organ, the brain. In most animals the brain is encased in a skull, consisting of mainly bone with only very small gaps, allowing for pressure buildup. All animals, whether with an endoskeleton or an exoskeleton, use the skeleton as protection for the sensitive organs inside. (Ryan, 2012)
The skeletal system of animals also has the ability to be a mineral storage system and in the larger bones, a blood cell production system. The bones, like in our bodies, store calcium, phosphates and fat cells to help maintain homeostasis, or balance, in the body. When levels of minerals decreases in the blood supply of the body, these are released from the bones to sustain homeostasis.
One reason as to why all animals’ skeletons look, and function almost the same way is because we all evolved from a common ancestor. This possible ancestor was explained previously which was thought to be a Lobe finned fish, and maybe even the first known complete tetrapod skeleton belonging to the Acanthostega. It is from the evolution from this ancestor and natural selection that occurred over time that has given us the wide range of diversity in skeletal systems today even with the many similarities.
As most living animals have organs within their body a major function of the skeletal system is to provide protection to the organs from damage. As can be seen in the human skeleton, the rib cage protects our heart, lungs and other organs from physical damage. But there is one major part of the skeleton which protects the most sensitive organ, the brain. In most animals the brain is encased in a skull, consisting of mainly bone with only very small gaps, allowing for pressure buildup. All animals, whether with an endoskeleton or an exoskeleton, use the skeleton as protection for the sensitive organs inside. (Ryan, 2012)
The skeletal system of animals also has the ability to be a mineral storage system and in the larger bones, a blood cell production system. The bones, like in our bodies, store calcium, phosphates and fat cells to help maintain homeostasis, or balance, in the body. When levels of minerals decreases in the blood supply of the body, these are released from the bones to sustain homeostasis.
One reason as to why all animals’ skeletons look, and function almost the same way is because we all evolved from a common ancestor. This possible ancestor was explained previously which was thought to be a Lobe finned fish, and maybe even the first known complete tetrapod skeleton belonging to the Acanthostega. It is from the evolution from this ancestor and natural selection that occurred over time that has given us the wide range of diversity in skeletal systems today even with the many similarities.
(Figure 2: Skeletal System Diagram)
Natural selection and how the skeletal system of tetrapods changed with it
Lobe finned fish (Crossopterygians) of about 360 – 408 million years ago are thought to have branched off into many major groups. The first major sight of natural selection is seen at around 245 – 286 million years ago when the great Permian extinction occurred. It was from this extinction that one group is thought to be the only one to survive, the Actinistia (coelacanths). It is from this group that tetrapod species continued to evolve. (Zimmer, The origin of tetrapods)
As the tetrapod superclass once moved onto the land, the skeleton of each generation would start to adapt and evolve to suit the environment that they were in. This evolutionary process would take many hundreds of thousands of years, but ultimately the skeleton of the animals changed to suit their environment and their way of living. This evolution became so diverse and spread out that it ultimately became all life on earth as we know it. Some species that evolved kept some or most of the same skeletal system as their ancestors, but as can be seen by life today, some species have almost completely changed the structure that took so long to form in the early Lobe finned fish.
As the tetrapod superclass once moved onto the land, the skeleton of each generation would start to adapt and evolve to suit the environment that they were in. This evolutionary process would take many hundreds of thousands of years, but ultimately the skeleton of the animals changed to suit their environment and their way of living. This evolution became so diverse and spread out that it ultimately became all life on earth as we know it. Some species that evolved kept some or most of the same skeletal system as their ancestors, but as can be seen by life today, some species have almost completely changed the structure that took so long to form in the early Lobe finned fish.
(Figure 3 : Lobe Finned Fish and Early Amphibian (Tetrapod))
Diversity of tetrapod skeletons today
The modern day horse is a good example of how the skeleton evolved to suit environments. As we all know, the modern horse has a single hoof on each leg, but why did it evolve a single hoof when other tetrapod skeletons kept adaptations of the fingers and toes of their common ancestor? Scientists examined the first horse species fossils to find out what caused the change. From this there could be three main possibilities: Adaptation, Mutation or Natural selection.
Scientists have traced the horse back to a dog sized animal that lived around 60 million years ago, Hyracotherium. This creature lived in forests and was apparently very successful in its time. This ancestor had very unusual feet for ‘normal’ mammals as it had only four toes on its front feet and on its back feet it had three normal toes and two vestigial (small/ functionless) toes. It is thought that the initial process was caused by a gene called the ‘Hox’ gene. This gene controls toe growth and is found in animals today. The reason as to why these toes changed on Hyracotherium was because of gene mutation. It is thought that over time this gene mutated and made the vestigial toes shorter, and so leading to the horse standing on their middle finger with the remnants of the other fingers further up the leg. (Evidence of Horse Evolution)
But of course as with most mutations, it did not affect the entire species. This therefore poses the question: why did horses with the mutated hox gene survive while the other types did not. This could be explained by natural selection. Epihippus, descendant of Hyracotherium, is the next stage in horse evolution that shows that longer legs may have been an advantage. Around the time of Epihippus, open grasslands were beginning to form and forests were shrinking, its legs gave it speed which allowed this animal to venture out to graze in the grasslands and retreat back into the forests for safety. Mesohippus was the next type of horse to evolve. This horse was even taller than its ancestors and had fused bones in the lower legs instead of two separate ones. This change reduced the possibility to twist the leg, and more power could be put into the forward and backward strides. As this horse was a grazer, it needed this extra power in its stride if it was going to escape from predators. This shows how and why the horse may have evolved into the way it is today, that this was an effective adaption as it has survived through natural selection. (Evidence of Horse Evolution)
Scientists have traced the horse back to a dog sized animal that lived around 60 million years ago, Hyracotherium. This creature lived in forests and was apparently very successful in its time. This ancestor had very unusual feet for ‘normal’ mammals as it had only four toes on its front feet and on its back feet it had three normal toes and two vestigial (small/ functionless) toes. It is thought that the initial process was caused by a gene called the ‘Hox’ gene. This gene controls toe growth and is found in animals today. The reason as to why these toes changed on Hyracotherium was because of gene mutation. It is thought that over time this gene mutated and made the vestigial toes shorter, and so leading to the horse standing on their middle finger with the remnants of the other fingers further up the leg. (Evidence of Horse Evolution)
But of course as with most mutations, it did not affect the entire species. This therefore poses the question: why did horses with the mutated hox gene survive while the other types did not. This could be explained by natural selection. Epihippus, descendant of Hyracotherium, is the next stage in horse evolution that shows that longer legs may have been an advantage. Around the time of Epihippus, open grasslands were beginning to form and forests were shrinking, its legs gave it speed which allowed this animal to venture out to graze in the grasslands and retreat back into the forests for safety. Mesohippus was the next type of horse to evolve. This horse was even taller than its ancestors and had fused bones in the lower legs instead of two separate ones. This change reduced the possibility to twist the leg, and more power could be put into the forward and backward strides. As this horse was a grazer, it needed this extra power in its stride if it was going to escape from predators. This shows how and why the horse may have evolved into the way it is today, that this was an effective adaption as it has survived through natural selection. (Evidence of Horse Evolution)
(Figure 4 : Evolution of the horses legs)
To continue to show the diversity of the tetrapods, another animal is the modern whale. As all tetrapods are thought to have originated from the water and moved to land, it is interesting why the ancestors of the whale turned back to the water.
The whales oldest direct ancestor was known as Pakicetus that lived around 52 million years ago. This creature looked similar to what we would recognise as a dog and was believed to have spent much of its life on land, but because it’s ears were a cross between that of an aquatic and land mammal it is thought that it hunted in the water. Two million years after, a new type of creature evolved. Its name was Ambulocetus, also known as the walking whale. This creature had adapted to both land and water, and moved between fresh and salt water. This creature had changed from its ancestor by adapting its hind legs to be more like paddles for better water propulsion. The whales ancestors continued to evolve, with their hind legs gradually shortening through all the fossils of Rodhocetus, Dorudon and Basilosaurus, until the hind legs eventually disappeared as the evolution got closer to the modern whale (about 40 million years ago). (Zimmer, The evolution of whales, 2009)
The whales oldest direct ancestor was known as Pakicetus that lived around 52 million years ago. This creature looked similar to what we would recognise as a dog and was believed to have spent much of its life on land, but because it’s ears were a cross between that of an aquatic and land mammal it is thought that it hunted in the water. Two million years after, a new type of creature evolved. Its name was Ambulocetus, also known as the walking whale. This creature had adapted to both land and water, and moved between fresh and salt water. This creature had changed from its ancestor by adapting its hind legs to be more like paddles for better water propulsion. The whales ancestors continued to evolve, with their hind legs gradually shortening through all the fossils of Rodhocetus, Dorudon and Basilosaurus, until the hind legs eventually disappeared as the evolution got closer to the modern whale (about 40 million years ago). (Zimmer, The evolution of whales, 2009)
( Figure 5 : Evolutionary ancestors of the whale )
These changes to the horse and whales skeleton are because of natural selection, mutation and adaptation to their environments. With these two very different examples of how tetrapods and their skeleton changes show the diversity of animals and their skeletal systems. It also shows that depending on the environment, mutations and natural selection, only the most successful skeletal systems have survived to the present day. But the most interesting thing of all is that this wide range of diverse tetrapods came from one single ancestor, the Lobe finned fish.
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Banner image obtained from : http://www.redorbit.com/news/science/1589243/walking_fish_helps_fill_evolutionary_gap/
Figure 1 obtained from : http://evolution.berkeley.edu/evolibrary/article/evograms_04
Figure 2 obtained from : http://biology.clc.uc.edu/courses/bio105/bone.htm
Figure 3 obtained from : http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookdiversity_9.html
Figure 4 obtained from : http://scienceblogs.com/laelaps/2009/03/16/the-horse-as-evolutionary-para/
Figure 5 obtained from : http://www.christs.cam.ac.uk/darwin200/pages/index.php?page_id=g3
Evidence of Horse Evolution. (n.d.). Retrieved November 3, 2012, from SaintJoe.edu: http://www.saintjoe.edu/~dept14/environment/rogero/Winter06/natural_selection_in_horse_evolution06.html
Gorr T., K. T. (2002 - 2012). Close tetrapod relationships of the coelacanth Latimeria indicated by haemoglobin sequences. Retrieved November 2, 2012, from UniProt: http://www.uniprot.org/citations/2034288
Lecture 8. (n.d.). Retrieved November 3, 2012, from http://rainbow.ldeo.columbia.edu/courses/v1001/8.html
Murphy, D. C. (2005, July 9). Acanthostega Gunneri. Retrieved October 27, 2012, from Devonian Times: http://devoniantimes.org/Order/re-acanthostega.html
Murphy, D. C. (2006, June 15). Ichthyostega Stenioei. Retrieved November 1, 2012, from Devonian Times: http://www.devoniantimes.org/Order/re-ichthyostega.html
Ryan, J. M. (2012). Skulls and Bones: Comparing Form and Function of Vertebrate Skeletal Systems. Retrieved November 1, 2012, from Yale-New Haven Teachers Institute: http://www.yale.edu/ynhti/curriculum/units/2012/3/12.03.07.x.html
The Evolution of Whales. (n.d.). Retrieved November 3, 2012, from Cochise College: http://skywalker.cochise.edu/wellerr/students/whales/whales.htm
The Tetrapods Transition to Land. Evansville.edu.
Zimmer, C. (1995, 06). Coming Onto The Land. Retrieved 10 25, 2012, from mesacc.edu: http://web.mesacc.edu/dept/d10/asb/origins/comingonto.html
Zimmer, C. (2009). The evolution of whales. Retrieved November 3, 2012, from Berkeley.edu: http://evolution.berkeley.edu/evolibrary/article/evograms_03
Zimmer, C. (n.d.). The origin of tetrapods. Retrieved October 24, 2012, from Understanding Evolution: http://evolution.berkeley.edu/evolibrary/article/evograms_04
Banner image obtained from : http://www.redorbit.com/news/science/1589243/walking_fish_helps_fill_evolutionary_gap/
Figure 1 obtained from : http://evolution.berkeley.edu/evolibrary/article/evograms_04
Figure 2 obtained from : http://biology.clc.uc.edu/courses/bio105/bone.htm
Figure 3 obtained from : http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookdiversity_9.html
Figure 4 obtained from : http://scienceblogs.com/laelaps/2009/03/16/the-horse-as-evolutionary-para/
Figure 5 obtained from : http://www.christs.cam.ac.uk/darwin200/pages/index.php?page_id=g3