For most of earth’s history, there was no life on land.  One reason was that, until photosynthesis had contributed enough oxygen to form an ozone layer, too much ultraviolet light from the sun bombarded the land, making life impossible.  In the Siluran Period, plants and arthropods made the transition to land 100 million years before vertebrates made a similar transition.  Although moving from aquatic to terrestrial life seems like an enormous change for a fish, a variety of modern ray-finned (actinopterygian) fish, such as mudskippers and walking catfish, have adapted to terrestrial life by crawling on their fins, adapting their gills for air breathing, and/or gulping air with a swim bladder.

     Life on land presents new challenges to formerly aquatic animals:

a)     breathing oxygen from air rather than water: 

An air bladder capable of breathing atmospheric air evolved in the first bony fish (if not earlier) and lungs evolved in early sarcopterygians. Many amphibians possess gills during their larval stage (and some retain them as adults) which they use to breathe underwater.  A frog tadpole in the process of metamorphosis is depicted below.

     It is possible that the molecular changes which converted fins to limbs were not extensive.  There is one set of genes (the Hox genes) which guide the development of both fins and tetrapod limbs.  In fish, these genes are turned off after a short period of expression while in tetrapods their expression lasts longer. Mutant mice which lacked any HoxA/HoxD expression in their limbs did not produce the Sonic Hedgehog signal needed for the development of the forearms and digits. The arm consisted only of a humerus which interestingly had an “L”-like shape. Although there were cartilaginous rays which formed distal to the humerus, these were the products of an “abnormal cartilaginous plate” and not homologues of any normal forelimb bones (Kmita, 2005).

     c) more weight to bear Water obviously provides buoyancy which helps support an animal’s weight.  Not only were the limbs strengthened and the pelvis attached to the vertebral column, the vertebral column itself had to become stronger.  The adaptation of the vertebral column to bear weight on land occurred gradually as the parts of the vertebrae became larger, fused together, and replaced the notochord.  This will be discussed in greater detail when reptiles are examined.

     d) resist water loss (dessication)  Tetrapods breathe through their nose rather than mouth in order to limit water loss.  The first tetrapods would have been partially protected by the scales of their fish ancestors.  Reptiles developed keratin scales in the skin to prevent water loss.  Amphibians could not do this since their lungs, while functional, are still inefficient.  Modern amphibians depend on exchanging oxygen and carbon dioxide through their skin.  In fact, a very successful family of salamanders in North America has actually lost its lungs and respires only through the skin.

     e) change in sense organs  The lateral line system, involving a system of grooves in the skull, was used by sarcopterygians to locate animals and objects in water.  Although it was still present in the most primitive amphibians, it was not useful on land.  The eye and olfactory epithelium would have to be moistened to function in air.   Rhipidistians developed nasolacrimal duct to accomplish this.   Part of the jaw apparatus (the hyomandibular) began to conduct sound to the inner ear and became the stapes.  Although its primary function in later tetrapods would be the transmission of sound waves, in early tetrapods its primary function was to support the braincase.  It remained a solid, stout bone for quite some time and probably did not transmit sound well until early reptiles.

      In conclusion, although the transition to land involves a number of anatomical and physiological changes, they are perhaps not as great as they might seem.  Although breathing oxygen from the air is essential, the ancestors of amphibians had already accomplished this.  Although fins cannot provide the same locomotion styles as limbs, a number of modern fish have adapted their fins for some degree of terrestrial locomotion.  The sarcopterygian fish of the past would have been even better prepared for this transition, given the stout bones in their fins—bones which are homologous to the bones in tetrapod limbs.  Other adaptations to life on land (such as a strong vertebral column and the modification of sensory structures) would occur gradually and were not complete in the first amphibians.

     What promoted the evolution of sarcopterygian fish into amphibians?  First of all, any fish which can breathe air can survive in stagnant water while other fish die.  This is clearly an advantage.  The ability to move onto land, even temporarily could help a fish evade a predator or move from a pool of water which was drying out.  Finally, it should be remembered that when the first fish began their transition onto land, terrestrial environments were full of insects (many of them wingless) which had no vertebrate predators. 

     Eusthenopteron was a sarcopterygian fish (a rhipidistian, to be more precise) which had bones in its fins homologous to tetrapod limb bones (humerus, radius, ulna, femur, tibia, fibula).  The pectoral girdle (the bones which attach the arm/forelimb to the rest of the skeleton) was still attached to the skull so that there were a few bones which are lost in the first amphibians.  The organization of its skull was very similar to the first amphibians (they are much more similar to each other than either is to modern fish or modern amphibians).  Pandericthys also shared these characteristics and its skull was even more similar to that of early amphibians (in its frontal bones, shortened snout, and the position of its eyes) (Ahlberg, 1996; Ahlberg, 1998).  Other fish such as Elginerpeton and Ventastega were close to the amphibian lineage (in neither of these are fins known; future fossils might indicate that they also had fingers) (Ahlberg, 1995).

PECTORAL GIRDLES:

     In humans, the pectoral girdle consists of the scapula (shoulder blade) and clavicle (collarbone); these bones attach the arm to the axial skeleton (the skull, vertebral column, and rib cage).  Other modern tetrapods (amphibians, reptiles, birds, other mammals) not only possess a scapula but may possess a clavicle, interclavicle (lost in placental mammals), and an anterior coracoid or procoracoid (bones which were lost in placental mammals).  (Note: although the human scapula has a “coracoid”, this is actually the posterior coracoid and is not the same as the procoracoid mentioned here.)

      Early fish possessed a few additional bones, attaching the pectoral girdle to the skull (thus, they had no “neck”) called the postcleithra, anocleithrum, supracleithrum, posttemporal, and cleithrum.  In the early amphibians, some of these bones are lost (creating a neck, a separation between the skull and pectoral girdle) but the cleithrum remains the largest element (Carroll, 1988; Kardong, 2002).

Note the relative positions of the pectoral girdle and the skull in the bowfin and frog skeletons.

 FIN AND LIMB BONES

     The human arm has one bone that attaches to the pectoral girdle and composes the upper arm (the humerus), two bones in the lower arm (the radius and ulna), small wrist (carpal) bones, bones of the hand (metacarpals), and finger bones (phalanges).  The human leg has one bone that attaches to the pelvic girdle and makes up the thigh (the femur), two bones of the lower leg (the tibia and fibula), small ankle (tarsal) bones, bones of the foot (metatarsals) and toe bones (phalanges).     Eusthenopteron has the same basic organization of bones in its fins as tetrapods had in their limbs with the exception of lacking a wrist, ankle, hand and foot.  There were a number of cartilaginous fin rays in the general area that would later compose the tetrapod hands and feet.

    Sauripterus possesed a front fin which is even closer to the forelimb of the first amphibians.  (It was given the name Sauripterus because the humerus is similar to a previous find that lacked any other elements of the arm.  New fossil finds may indicate that this new fin belongs to a different species of sarcopterygian.)  In Sauripterus, the fin rays are jointed and resemble fingers that are associated with wrist elements.  There is some resemblance to fingers of Acanthostega (some researches conclude that there are 8 “fingers”  in Sauripterus, as in Acanthostega).  No hind limb is known (Daeschler, 1998).

Tiktaalik roseae was a late Devonian sarcopterygian fish from shallow-water environments which possessed a number of tetrapod characteristics. It is clearly a fish with fins, fin rays, prominent gills, scales, dermal supracleithral bones of the shoulder, and several skull features which link it to sarcopterygian fish. It is the only known fish to possess a mobile neck which was gained through the loss of a series of skull bones (the intertemporal, opercular, subopercular, and extrascapular). Its ribs were longer and wider than other sarcopterygians. Its longer snout, wide spiracular notch, and postorbital region of the skull are also features which link it to amphibians. The humerus possessed a medial process although it is not as robust as in tetrapods. Tiktaalik is the most primitive vertebrate to possess a wrist joint. There are two wrist bones (ulnare and intermedium) which articulate with radial bones (four from the ulnare and one from the intermedium although one of the radials from the ulnare articulates with intermediate radials). These wrist bones are homologous and structurally similar to the early tetrapod wrist bones. Its shoulder bones and muscles would have been capable of supporting the limb for both fin-like and arm-like movements (Daeshler, 2006; Shubin, 2006).

   Acanthostega is the oldest tetrapod known from Greenland, 360 million years ago.   While it is recognized as the most primitive amphibian, there are a number of aspects of its skeleton which link it to sarcopterygian fish and demonstrate that it was not fully adapted to locomotion on land.

     Acanthostega still retained a number of primitive features from its sarcopterygian ancestors: fully functional internal gills, gill struts, a primitive ear, no attachment of the pelvis to the vertebral column, the absence of an ankle (wrist not preserved), a shoulder bone (anocleithrum) which connected the shoulder to the skull as in fish, and a tail fin with dermal rods (as in fish; these rods were different from those of all sarcopterygians in that they were unbranched and unsegmented).  There are a number of derived features that identify it as the first amphibian: multijointed fingers and toes, the shape of its limb bones and girdles, and the loss of some opercular bones of the skull found in fish.

The braincase of Acanthostega was similar to that of fish although the occipital region had begun to fuse in the region of the notochord (Clack, 2002).

Compared to earlier osteolepiforms, Acanthostega's snout was longer, its eyes were located farther back on its head and more medially, and the otic region was shorter. Acanthostega lost its lateral rostral bone around the nostril. Much of the region around the nostril was composed of cartilage rather than bone, perhaps a juvenile trait retained from its larval stage. The ancestral skull structure the hyomandibula was modified to become the stapes (which would become one of the middle ear bones used in hearing). Bones around the external nostril were lost as fish adapted to smelling in air. Acanthostega retained the lateral line system in the skull which had served its fish ancestors as a sensory system for hunting aquatic prey. Acanthostega probably possessed at least 3 pairs of gill arches similar to modern lungfish and it probably supplemented the air absorbed from its lungs using its internal gills. The atlas was the only vertebrae that fused across the midline of the body. Although Acanthostega had a neck, the notochord's penetration of the skull made the neck less mobile. Large muscles attached to the exoccipitals to support the neck. The neural arches were reduced in the cervical region (Clack, 2002). Acanthostega's ribs were shorter and not as thick as in Ichthyostega. The interclavicle could have functioned in a way similar to the sternum of later tetrapods (Clack, 2002). Acanthostega, like other early tetrapods retained scales on its underside. The loss of scales on the dorsal surface would have made these tetrapods lighter and allowed more gas exchange through the skin. Acanthostega also had evolved a lacrimal gland, lacrimal ducts, eyelids, eyelid muscles, and a retractor bulbi muscle (Clack, 2002).

Although the shoulder girdle had separated from the skull to create a neck, Acanthostega still possessed an anocleithrum like fish (the anocleithrum is one of the bones which had attached the shoulder to the skull in fish; Clack, 2002). In Acanthostega's shoulder, the glenoid cavity was shallow. The radius was twice the length of the ulna and the ulna lacked an olecranon process(Clack, 2002). The femur was longer than the humerus. Although Acanthostega had limbs and digits, it couldn't have supported its weight and walked on these limbs since there were almost no modifications of the sacral vertebrae to interact with pelvic bones. Interestingly, it seems that the number of digits in a tetrapod limb wasn't fixed at five in the first amphibians: Acanthostega had 8 fingers, Ichthyostega had 7 toes, and Tulerpeton had six fingers (Lebedev, 1997; Clack, 2002).

Arm and Leg of Acanthostega are depicted below:

     Although Acanthostega had limbs and digits, it couldn’t have supported its weight and walked on these limbs since there were almost no modifications of the sacral vertebrae to interact with pelvic bones.   Interestingly, it seems that the number of digits in a tetrapod limb wasn’t fixed at five in the first amphibians: Acanthostega had 8 fingers, Ichthyostega had 7 toes, and  Tulerpeton had six fingers (Lebedev, 1997; ). 

ICHTHYOSTEGA

Ichthyostega is a more advanced amphibian from the Upper Devonian. Although the notochord still stretched the length of the vertebral column in Ichthyostega, it no longer penetrated the braincase, making it easier to move the head. The vertebrae had developed the processes (zygapopyses) which allowed neighboring vertebrae to articulate with one another. The ribs had increased in length. An olecranon process (the elbow) was present although the arm’s movement was limited. The femur was half the length of the humerus. After Ichtyostega, the hip socket became more or less oval in shape and humerus lots its primitive “L” shape and most of the ancestral foramina which passed through it (Clack, 2002) The anterior tectal bone around the nostril was lost. Sacral ribs specialized to attach to the pelvis, allowing the legs to support the weight of the body (Clack, 2002; Carroll)

  It still retained some primitive features such as a prominent notochord and 2 opercular bones of the skull.  The pelvis of Ichthyostega was attached to the vertebral column and was composed of the same three separate bones found in all later tetrapod pelvises: the ilium, ischium, and pubis.  (Although in adult humans there is only one “hip bone” or os coxa, fetal humans a have completely separate ilium, ischium, and pubis which fuse later in life to form each hip bone.  Many parts of the adult hip still carry these original names such as the iliac crest, ischial tuberosity, and pubic symphysis.)

--After Ichtyostega, Acanthostega, and the common ancestor of the early tetrapod Whatcheeria, the preopercular bone which had supported the opercular series of bones was lost; in the early tetrapods it was already reduced in size (Clack, 2002).

Hesperoherpeton was a later amphibian that still possessed the primitive feature of a large canal for notochord in the skull. It possessed several other primitive skull features (the otic and occipital region were still separate from the ethmosphenoid), and the shoulder girdle probably contacted the skull at the tabular bone. I ts derived characteristics included short digits, an otic notch, and larger vertebrae (the pleurocentra portion).

Tulerpeton

Tulerpeton had evolved longer limbs and its humerus was similar to the later anthracasaurs. Its hands and feet possessed 6 digits. It had lost its internal gills although it still retained the anocleithrum.