1. The origin of tetrapods and movement onto land
There are a suite of challenges associated with terrestrial living that the first tetrapods had to overcome.
These included the fact that water supports a fish’s body but air does not, so stronger skeletal structures were needed to support the body and limbs to allow movement.
In addition, gills collapse out of water so an alternative breathing system (i.e. lungs) was needed.

2. The origin of tetrapods and movement onto land
In addition, there was a constant threat of water loss from the skin or through eggs, which limited the first tetrapods to a close connection with water until hard-shelled amniotic eggs evolved.

3. The origin of tetrapods and movement onto land
Given all the apparent challenges of adapting to the land, the transition from water was for a long time viewed as a great evolutionary leap and there was a lot of speculation about the selective forces that drove the transition.
Was it to:
escape drying pools of water?
exploit new food sources on land?
escape predators in the crowded waters?

4. Modern fish out of water
It turns out that much of the speculation was based on a lack of fossils and misconceptions about the environment the first tetrapods inhabited.
Early speculation about the origins of tetrapods focused on various adaptations usefulness for animals moving and living on land, but these adaptations we now know were in fact used underwater.

5. Modern fish out of water
Despite the apparent challenges of spending time out of water a variety of modern bony fishes do so.
These include:
Eels which wriggle from one pool to another.
Walking catfish of the southeastern U.S., which can move over land using their fins for propulsion.
Gobies and sculpins: tidepool fish that spend a lot of time out of water when the tide retreats.

6. Walking Catfish

7. Climbing Perch
Climbing perch of southeast Asia and Africa walks supported by the spiny edges of its gill plates and uses its fins and tail for propulsion.

8. Climbing Perch

9. Mudskipper
The best adapted of all to the air/land boundary is the Mudskipper, which lives in the mudflats of mangroves. It can wriggle on mud and even climb tree roots.

10. Mudskipper

11. Modern fish out of water
All of these fish are jerry-rigged to survive on land and they have to live in humid environments and remain near water. They lack sturdy lobed fins, but do their best with their ray fins to propel themselves.

12. Modern fish out of water
In some marine fishes, including the fingered dragonets, frogfish and the grunt sculpin, ray fins have been modified into finger-like projections that the fish use to move along the bottom in a manner similar to a lobster.
These modified ray fins are not as sturdy or flexible as tetrapod fingers, but are a make-do solution and allow the fish to move along the seafloor.

13. Frogfish

14. The origin of tetrapods and movement onto land
The tetrapods (a monophyletic group that includes the “amphibians”, “reptiles”, birds and mammals) are all descended from an ancestral lobe-finned fish and there is an excellent collection of fossils that document the transition.

15. Tetrapod limb
The tetrapods are defined by their limbs, which have a characteristic structure.
Taking the forelimb, for example, there is first a single bone (humerus), then a pair of bones (radius and ulna), next a series of wrist bones (carpals) and finally a set of digits (phalanges).

16. Tetrapod limb
The bones found in modern tetrapod limbs are homologous bone for bone to those in lobe-finned rhipidistian fishes such as Eusthenopteron of the late Devonian period about 380 mya and to the earliest known tetrapods such as Acanthostega and Icthyostega (both approx mya) .

17. Eusthenopteron
In addition to the homologies in the limbs, the bone to bone structure of the spine in Eusthenopteron matches that in the earliest fossil tetrapods and is completely unlike that of other fishes.
Similarly a bone by bone comparison of Eusthenopteron’s skull bones matches that in the earliest tetrapods.
In tetrapods the bones covering the gills are reduced in size and the snout bones are elongated, but there is still a one to one match to the skull bones or Eusthenopteron.

18. Eusthenopteron
Lungs and gills in Eusthenopteron did not fossilize, but as its sister group the lungfishes and descendants the tetrapods possess lungs, it is reasonable to assume that Eusthenopteron possessed lungs too.

19. 17.1a Eusthenopteron

20. Panderichthys
The next fossils are a series of more tetrapod-like fish including Panderichthys from the late Devonian.
Panderichthys is a very tetrapod-like lobe-finned fish. In contrast to Eusthenopteron the body is flattened has upward facing eyes and a straight tail with a well-developed tail fin.

21. Panderichthys
Panderichthys has both gills and well-developed lungs with nostrils.
It has also lost the anal and dorsal fins of fish leaving the foot-like pectoral and pelvic fins.

22. Panderichthys

23. Panderichthys
Panderichthys’ braincase is very tetrapod-like and it possesses the characteristic teeth of later tetrapods.
These labyrinthodont teeth are named for their enfolded enamel.
Panderichthys is a “fishapod” intermediate between fish and tetrapods with a tetrapod-like skull and body, braincase, and lungs, but still retaining true fins.

24. Labyrinthodont teeth
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25. Tiktaalik
The gap between Panderichthys and the early tetrapods Acanthostega and Ichthyostega has recently been neatly bridged by the discovery of a classic intermediate form, Tiktaalik roseae.

26. Tiktaalik roseae
Discovered in 2004 on Ellesmere Island in the Canadian Arctic Tiktaalik roseae is an extremely important fossil link in the origin of tetrapods.
Another “fishapod” Tiktaalik like Panderihthys has a mixture of fish and tetrapod characteristics, but has several tetrapod characteristics that Panderichthys lacks.

27. Tiktaalik roseae
Tiktaalik has fish-like scales, as well as a fish-like palate, lower jaw and fin rays (but not toes).
Unlike a fish however it has a mobile neck and the ear is structured so that it can hear both in and out of water.
Most strikingly, in addition to the other tetrapod limb bones Tiktaalik has a wrist. Tiktaalik’s elbow could bend like ours and the wrist could bend too, which allowed the animal to make its “palm” lie flat.

28. Tiktaalik roseae
Tiktaalik’s wrist and elbow structure and the fact that Tiktaalik had very large chest muscles indicates that Tiktaalik was specialized to do push-ups.
Tiktaalik has a flat head with eyes on the top and appears to have evolved to move around in shallow water and on mudflats. Having fins that could support the body would have been a big advantage in such an environment.

29. Tiktaalik roseae 375 mya

30. Tiktaalik roseae
Tiktaalik could do push-ups, but lacks digits and so could not grasp something or throw something.
However, it reveals the early stages of the evolution of the wrist and the early stages of the forearm’s ability to rotate against the elbow (called pronation or supination depending on the direction of rotation).

32. Tiktaalik roseae
You can rotate your hand relative to your elbow because there is a ball at the end of the humerus around which the tip of the radius attaches forming a ball and socket joint.
The beginnings of this joint are present in Tiktaalik. In Tiktaalik the cup-shaped end of the radius fits onto an elongated bump on the end of the humerus, which would have allowed Tiktaalik to rotate its forearm.

33. From Wikipedia

34. The first tetrapods
The oldest tetrapods discovered are Acanthostega and Icthyostega.
Both have robust shoulder and hip bones as well as sturdy limbs and a strong spine. In both the snout is elongated and the eyes have moved further back in the skull and enlarged.
Both also possess digits, which Tiktaalik did not.
Both species, however, retain a suite of fish-like characteristics including tail-fins, lateral line systems and gill slits.

35. Acanthostega
Of the two species Acanthostega is the slightly less advanced form.
It has 8 digits and a very fish like shoulder structure. It would not have been able to move around well on land because the hand and foot were not well adapted for walking being better suited for swimming or moving along the bottom.
Acanthostega possessed gills and its ear is adapted to hearing in water.
It probably made its way along the bottom and crawled over vegetation rather than doing much crawling on land.

36. Acanthostega

37. http://www. earthhistory. org. uk/wp-content/IchthyostegaAcanthostega

38. Ichthyostega Ichthyostega was discovered in the 1930’s in Greenland.
Ichthyostega has a smaller tail fin and its legs are relatively longer than those of Acanthostega. Although the legs probably wouldn’t have supported it well on land, but would have enabled it to move around on the bottom in shallow water.
It has 7 digits rather than the five of modern tetrapods.

39. 17.1 B and C

40. What is clear from Acanthostega and Icthyostega is that the limbs of the earliest tetrapods first evolved not for walking on land, but for walking under water.
Understanding that, the retention of fish-like features such as the lateral line system and tail fins by the earliest tetrapods makes sense. The first tetrapods apparently behaved much like modern salamanders spending most of their time in water and only occasionally emerging onto land.
It also renders old arguments about tetrapods having to have robust limbs to chase prey on land or to crawl from pool to pool moot.

41. From the primitive tetrapod forms a great radiation of more terrestrial forms occurred in the Carboniferous.
From these two lineages of tetrapods arose one leading to the modern amphibians the other to the modern amniotes.

42. Tetrapod characteristcis
Unique derived features of the tetrapods
Paired limbs: forelimbs with digits, carpals, radius+ulna, humerus; hindlimbs digits, tarsals, tibia+fibula, femur.
Mobile neck: pectoral girdle separated from the skull. In ancestors the pectoral girdle attached directly to the skull.
Hyomandibular bone previously used to support the jaw now used in hearing. Called the stapes it conducts sound in the ear.
First cervical vertebra (the atlas) specialized to allow the skull to nod.