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A shark that walks like a tetrapod
http://vimeo.com/72995710But the really interesting thing to me is the gait the fish is using – this is apparently typical of Hemiscyllium sharks, which prowl across coral looking for food. It is using the classic alternate movement of a tetrapod.
We’ve previously talked about the evolution of the tetrapod gait in lungfish. Now you are more closely related to Tiktaalik, the famous fish/tetrapod intermediate form, than Tiktaalik was to a shark (because you and I and Tiktaalik all have bones; sharks do not have bones, only cartilage).
So this suggests that the neuronal control of the way that you run (your right arm moves with your left leg, and your left arm moves with your right leg – try it) goes waaaayyyy back even beyond our fishy ancestors, to the time before the evolution of bone.
Another alternative is that this is convergent evolution – if you are going to ‘walk’, the alternate gait is the best way of doing it. Today’s question: How could we test between these two hypotheses?
http://whyevolutionistrue.wordpress.com/2013/08/27/bamboo-shark-walks-the-walk/
We’ve previously talked about the evolution of the tetrapod gait in lungfish. Now you are more closely related to Tiktaalik, the famous fish/tetrapod intermediate form, than Tiktaalik was to a shark (because you and I and Tiktaalik all have bones; sharks do not have bones, only cartilage).
So this suggests that the neuronal control of the way that you run (your right arm moves with your left leg, and your left arm moves with your right leg – try it) goes waaaayyyy back even beyond our fishy ancestors, to the time before the evolution of bone.
Another alternative is that this is convergent evolution – if you are going to ‘walk’, the alternate gait is the best way of doing it. Today’s question: How could we test between these two hypotheses?
http://whyevolutionistrue.wordpress.com/2013/08/27/bamboo-shark-walks-the-walk/
17 replies
= new reply since forum marked as read
= new reply since forum marked as read
The origin of tetrapods
And for a more detail discussion of skeletal changes and modeling of tetrapod locomation and its evolution, here's the abstract of journal article by a Russian scientist: Evolution of tetrapod locomotion:
The word "tetrapod" means "four feet" and includes all species alive today that have four feet — but this group also includes many animals that don't have four feet. That's because the group includes all the organisms (living and extinct) that descended from the last common ancestor of amphibians, reptiles, and mammals. So, for example, the ichthyosaur, an extinct swimming reptile, is a tetrapod even though it did not use its limbs to walk on land. So is the snake, even though it has no limbs. And birds and humans are tetrapods even though they only walk on two legs. All these animals are tetrapods because they descend from the tetrapod ancestor described above, even if they have secondarily lost their "four feet."
Tetrapods evolved from a finned organism that lived in the water. However, this ancestor was not like most of the fish we are familiar with today. Most animals we call fishes today are ray-finned fishes, the group nearest the root of this evogram. Ray-finned fishes comprise some 25,000 living species, far more than all the other vertebrates combined. They have fin rays — that is, a system of often branching bony rays (called lepidotrichia) that emanate from the base of the fin.
In contrast, the other animals in the evogram — coelacanths, lungfishes, all the other extinct animals, plus tetrapods (represented by Charles Darwin) — have what we call "fleshy fins" or "lobe fins." That is, their limbs are covered by muscle and skin. Some, such as coelacanths, retain lepidotrichia at the ends of these fleshy limbs, but in most fleshy-finned animals these have been lost.
The common ancestor of all those different organisms (ray-fins, coelacanths, lungfishes, tetrapods, etc.) was neither a lobe-fin nor a ray-fin. This ancient vertebrate lineage had fins (with lepidotrichia), scales, gills, and lived in the water. Yet they also had air bladders (air-filled sacs) connected to the back of their throats that could be used for breathing air (i.e., as lungs) or for buoyancy control. The air bladders of many ray-fins no longer connect to their throats, and so they are not able to breathe air. In these ray-fins, the air bladder is used mainly for buoyancy control and is known as a swim bladder. By contrast, tetrapods have taken an alternative route: they have lost the buoyancy control function of their air bladders, and instead this organ been elaborated to form the lungs that we all use to get around on land.
<lots more at the link>
In contrast, the other animals in the evogram — coelacanths, lungfishes, all the other extinct animals, plus tetrapods (represented by Charles Darwin) — have what we call "fleshy fins" or "lobe fins." That is, their limbs are covered by muscle and skin. Some, such as coelacanths, retain lepidotrichia at the ends of these fleshy limbs, but in most fleshy-finned animals these have been lost.
The common ancestor of all those different organisms (ray-fins, coelacanths, lungfishes, tetrapods, etc.) was neither a lobe-fin nor a ray-fin. This ancient vertebrate lineage had fins (with lepidotrichia), scales, gills, and lived in the water. Yet they also had air bladders (air-filled sacs) connected to the back of their throats that could be used for breathing air (i.e., as lungs) or for buoyancy control. The air bladders of many ray-fins no longer connect to their throats, and so they are not able to breathe air. In these ray-fins, the air bladder is used mainly for buoyancy control and is known as a swim bladder. By contrast, tetrapods have taken an alternative route: they have lost the buoyancy control function of their air bladders, and instead this organ been elaborated to form the lungs that we all use to get around on land.
<lots more at the link>
And for a more detail discussion of skeletal changes and modeling of tetrapod locomation and its evolution, here's the abstract of journal article by a Russian scientist: Evolution of tetrapod locomotion:
Abstract
Fish-like ancestors of tetrapods did not need strong limb musculature because they inhabited waters and were practically imponderable. In the primitive tetrapods, principal function of the limbs was initially restricted to passive anchoring in the course of animal movements on the substrate by means of lateral bending of the body (undulation). However, progressive development of carrying function of tetrapod limbs lead to clearing the body off the substrate which reduced friction costs and made the tetrapods less dependent on the substrate properties. Along with this, the limbs became more important as the active locomotory organs. But at the beginning, this diminished locomotory speed as the momentum caused by undulation could no longer provide additional forward sliding. Locomotory function of the tetrapod limb could be carried out due to both retraction and pronation at the shoulder joint. Relatively short humerus of the primitive tetrapods made it indifferent which of these two particular actions lead to elongation of the steps. In most of the recent tetrapods with sprawling limbs (Urodela, Lacertilia Sphenodontia, Crocodilia), step elongation was carried out mainly by retraction at the shoulder joint. Contrary to this, in Tachyglossidae (Mammalia: Monotremata) retraction is absent while pronation at the shoulder joint becomes the most important component of step elongation. This made it possible to recognize two principal types, pronatory and retractory, of locomotion on the basis of the main movement in the phase of support. A mathematical model describing changes in step length during the phase of support in both of these types is elaborated. It takes into account relative sizes of stylopodium and zeugopodium, the angles of pronation and retraction at the shoulder joint, the angle of adduction at the elbow joint, and the angle of body undulation arc. It is shown on the basis of this model, varying of which of the above parameters is advantageous and which is disadvantageous in each of the locomotory types. In the pronatory locomotory type, adduction (lateral mobility) at the elbow joint is employed. It leads to special changes in morphology of the elbow joint due to which humeral condyle becomes spherical and promotes both adduction and rotation of the entire antebrachium. In the retractory locomotory type, amplification of pronation is to be limited in order to provide step elongation, so certain morphological adaptations occur in the elbow joint which prevent adduction at this joint. For step elongation, retraction at the shoulder joint is usually more advantageous than pronation, therefore historical emergence of the pronatory type could be considered as inadaptive. However, transversal horizontal axis of rotation at the shoulder joint appeared to be a prerequisite of the subsequent appearance of the most perfect locomotion in the therian mammals with their parasagittal limbs. Transition to the parasagittal limb construction was associated with adaptation to jumping asymmetric locomotion. It caused elongation of the shoulder bone downward which lead to widening of rotation cone of the humerus and, at the same time, to reduction of the coracoid portion of the glenoid fossa, the latter became horizontal rather than lateral. As a part of this process, the longitudinal axis of the scapula was displacing caudally with destruction of the suture-like articulation of the acromion process with the clavicle. The latter became articulated with the sternum directly or via much reduced interclavicle (or via procoracoid rudiment). This increases amortisatory function of the shoulder girdle during landing at the final stage of jump.
Fish-like ancestors of tetrapods did not need strong limb musculature because they inhabited waters and were practically imponderable. In the primitive tetrapods, principal function of the limbs was initially restricted to passive anchoring in the course of animal movements on the substrate by means of lateral bending of the body (undulation). However, progressive development of carrying function of tetrapod limbs lead to clearing the body off the substrate which reduced friction costs and made the tetrapods less dependent on the substrate properties. Along with this, the limbs became more important as the active locomotory organs. But at the beginning, this diminished locomotory speed as the momentum caused by undulation could no longer provide additional forward sliding. Locomotory function of the tetrapod limb could be carried out due to both retraction and pronation at the shoulder joint. Relatively short humerus of the primitive tetrapods made it indifferent which of these two particular actions lead to elongation of the steps. In most of the recent tetrapods with sprawling limbs (Urodela, Lacertilia Sphenodontia, Crocodilia), step elongation was carried out mainly by retraction at the shoulder joint. Contrary to this, in Tachyglossidae (Mammalia: Monotremata) retraction is absent while pronation at the shoulder joint becomes the most important component of step elongation. This made it possible to recognize two principal types, pronatory and retractory, of locomotion on the basis of the main movement in the phase of support. A mathematical model describing changes in step length during the phase of support in both of these types is elaborated. It takes into account relative sizes of stylopodium and zeugopodium, the angles of pronation and retraction at the shoulder joint, the angle of adduction at the elbow joint, and the angle of body undulation arc. It is shown on the basis of this model, varying of which of the above parameters is advantageous and which is disadvantageous in each of the locomotory types. In the pronatory locomotory type, adduction (lateral mobility) at the elbow joint is employed. It leads to special changes in morphology of the elbow joint due to which humeral condyle becomes spherical and promotes both adduction and rotation of the entire antebrachium. In the retractory locomotory type, amplification of pronation is to be limited in order to provide step elongation, so certain morphological adaptations occur in the elbow joint which prevent adduction at this joint. For step elongation, retraction at the shoulder joint is usually more advantageous than pronation, therefore historical emergence of the pronatory type could be considered as inadaptive. However, transversal horizontal axis of rotation at the shoulder joint appeared to be a prerequisite of the subsequent appearance of the most perfect locomotion in the therian mammals with their parasagittal limbs. Transition to the parasagittal limb construction was associated with adaptation to jumping asymmetric locomotion. It caused elongation of the shoulder bone downward which lead to widening of rotation cone of the humerus and, at the same time, to reduction of the coracoid portion of the glenoid fossa, the latter became horizontal rather than lateral. As a part of this process, the longitudinal axis of the scapula was displacing caudally with destruction of the suture-like articulation of the acromion process with the clavicle. The latter became articulated with the sternum directly or via much reduced interclavicle (or via procoracoid rudiment). This increases amortisatory function of the shoulder girdle during landing at the final stage of jump.
for the snark, weren't you?
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