The origin of tetrapods and The origin of tetrapods and movement onto landmovement onto land

There are a suite of challenges associated with There are a suite of challenges associated with terrestrial living that the first tetrapods had to terrestrial living that the first tetrapods had to overcome.overcome.

These included the fact that water supports a fish’s These included the fact that water supports a fish’s body but air does not, so stronger skeletal structures body but air does not, so stronger skeletal structures were needed to support the body and limbs to allow were needed to support the body and limbs to allow movement. movement.

In addition, gills collapse out of water so an In addition, gills collapse out of water so an alternative breathing system (i.e. lungs) was needed.alternative breathing system (i.e. lungs) was needed.

The origin of tetrapods and The origin of tetrapods and movement onto landmovement onto land

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

The origin of tetrapods and The origin of tetrapods and movement onto landmovement onto land

Given all the apparent challenges of adapting to Given all the apparent challenges of adapting to the land, the transition from water was for a long the land, the transition from water was for a long time viewed as a great evolutionary leap and time viewed as a great evolutionary leap and there was a lot of speculation about the selective there was a lot of speculation about the selective forces that drove the transition.forces that drove the transition.

Was it to:Was it to: escape drying pools of water?escape drying pools of water? exploit new food sources on land?exploit new food sources on land? escape predators in the crowded waters? escape predators in the crowded waters?

Modern fish out of waterModern fish out of water

It turns out that much of the speculation was It turns out that much of the speculation was based on a lack of fossils and misconceptions based on a lack of fossils and misconceptions about the environment the first tetrapods about the environment the first tetrapods inhabited.inhabited.

Early speculation about the origins of tetrapods Early speculation about the origins of tetrapods focused on various adaptations usefulness for focused on various adaptations usefulness for animals moving and living on land, but these animals moving and living on land, but these adaptations we now know were in fact used adaptations we now know were in fact used underwater. underwater.

Modern fish out of waterModern fish out of water

Despite the apparent challenges of spending Despite the apparent challenges of spending time out of water a variety of modern bony fishes time out of water a variety of modern bony fishes do so. do so.

These include:These include: Eels which wriggle from one pool to another.Eels which wriggle from one pool to another. Walking catfish of the southeastern U.S., which can Walking catfish of the southeastern U.S., which can

move over land using their fins for propulsion.move over land using their fins for propulsion. Gobies and sculpins: tidepool fish that spend a lot of Gobies and sculpins: tidepool fish that spend a lot of

time out of water when the tide retreats.time out of water when the tide retreats.

Walking Catfishhttp://www.cfnews13.com/uploadedImages/Stories/Local/walking%20catfish.jpg

Climbing PerchClimbing Perch

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

Climbing Perchhttp://static.howstuffworks.com/gif/willow/climbing-perch-info0.gif

MudskipperMudskipper

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

Mudskipperhttp://www.naturephoto-cz.com/photos/mraz/mudskipper-05a22018.jpg

Modern fish out of waterModern fish out of water

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

Modern fish out of waterModern fish out of water

In some marine fishes, including the fingered In some marine fishes, including the fingered dragonets, frogfish and the grunt sculpin, ray dragonets, frogfish and the grunt sculpin, ray fins have been modified into finger-like fins have been modified into finger-like projections that the fish use to move along the projections that the fish use to move along the bottom in a manner similar to a lobster.bottom in a manner similar to a lobster.

These modified ray fins are not as sturdy or These modified ray fins are not as sturdy or flexible as tetrapod fingers, but are a make-do flexible as tetrapod fingers, but are a make-do solution and allow the fish to move along the solution and allow the fish to move along the seafloor.seafloor.

Frogfishhttp://www.eco-divers.com/galleries/d/256-3/12.jpg

The origin of tetrapods and The origin of tetrapods and movement onto landmovement onto land

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

Tetrapod limbTetrapod limb

The tetrapods are defined by their limbs, The tetrapods are defined by their limbs, which have a characteristic structure. which have a characteristic structure.

Taking the forelimb, for example, there is Taking the forelimb, for example, there is first a single bone (humerus), then a pair first a single bone (humerus), then a pair of bones (radius and ulna), next a series of of bones (radius and ulna), next a series of wrist bones (carpals) and finally a set of wrist bones (carpals) and finally a set of digits (phalanges).digits (phalanges).

Tetrapod limbTetrapod limb

The bones found in modern tetrapod limbs The bones found in modern tetrapod limbs are homologous bone for bone to those in are homologous bone for bone to those in lobe-finned rhipidistian fishes such as lobe-finned rhipidistian fishes such as EusthenopteronEusthenopteron of the late Devonian of the late Devonian period about 380 mya and to the earliest period about 380 mya and to the earliest known tetrapods such as known tetrapods such as AcanthostegaAcanthostega and and IcthyostegaIcthyostega (both approx. 360-365 (both approx. 360-365 mya) .mya) .

EusthenopteronEusthenopteron In addition to the homologies in the limbs, the bone to In addition to the homologies in the limbs, the bone to

bone structure of the spine in bone structure of the spine in EusthenopteronEusthenopteron matches matches that in the earliest fossil tetrapods and is completely that in the earliest fossil tetrapods and is completely unlike that of other fishes.unlike that of other fishes.

Similarly a bone by bone comparison of Similarly a bone by bone comparison of Eusthenopteron’s Eusthenopteron’s skull bones matches that in the skull bones matches that in the earliest tetrapods.earliest tetrapods.

In tetrapods the bones covering the gills are reduced in In tetrapods the bones covering the gills are reduced in

size and the snout bones are elongated, but there is still size and the snout bones are elongated, but there is still a one to one match to the skull bones or a one to one match to the skull bones or EusthenopteronEusthenopteron..

EusthenopteronEusthenopteron

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

17.1a Eusthenopteron

Figure 25.01a

PanderichthysPanderichthys

The next fossils are a series of more The next fossils are a series of more tetrapod-like fish including tetrapod-like fish including PanderichthysPanderichthys from the late Devonian.from the late Devonian.

Panderichthys is a very tetrapod-like lobe-Panderichthys is a very tetrapod-like lobe-finned fish. In contrast to finned fish. In contrast to EusthenopteronEusthenopteron the body is flattened has upward facing the body is flattened has upward facing eyes and a straight tail with a well-eyes and a straight tail with a well-developed tail fin.developed tail fin.

PanderichthysPanderichthys

PanderichthysPanderichthys has both gills and well- has both gills and well-developed lungs with nostrils.developed lungs with nostrils.

It has also lost the anal and dorsal fins of It has also lost the anal and dorsal fins of fish leaving the foot-like pectoral and fish leaving the foot-like pectoral and pelvic fins.pelvic fins.

Panderichthys http://archeowiesci.files.wordpress.com/2008/09/panderichthys_bw.jpg

PanderichthysPanderichthys Panderichthys’ braincase is very tetrapod-like Panderichthys’ braincase is very tetrapod-like

and it possesses the characteristic teeth of later and it possesses the characteristic teeth of later tetrapods. tetrapods.

These labyrinthodont teeth are named for their These labyrinthodont teeth are named for their enfolded enamel.enfolded enamel.

Panderichthys is a “fishapod” intermediate Panderichthys is a “fishapod” intermediate between fish and tetrapods with a tetrapod-like between fish and tetrapods with a tetrapod-like skull and body, braincase, and lungs, but still skull and body, braincase, and lungs, but still retaining true fins.retaining true fins.

Labyrinthodont teeth http://www.google.com/imgres?imgurl=http://www.palaeos.com/Vertebrates/Lists/Glossary/Images/labyrinthodont.gif&imgrefurl=http://www.palaeos.com/Vertebrates/Lists/Glossary/GlossaryJL.html&h=199&w=300&sz=20&tbnid=Qek9daL31XEKGM::&tbnh=77&tbnw=116&prev=/images%3Fq%3Dlabyrinthodont%2Bteeth%2Bimages&hl=en&usg=__qB1LuNWSy3UmLGc90oYDkAQzItY=&ei=hOPLSf7YC9TulQeR5NTQCQ&sa=X&oi=image_result&resnum=1&ct=image&cd=1

TiktaalikTiktaalik

The gap between Panderichthys and the The gap between Panderichthys and the early tetrapods early tetrapods AcanthostegaAcanthostega and and IchthyostegaIchthyostega has recently been neatly has recently been neatly bridged by the discovery of a classic bridged by the discovery of a classic intermediate form, intermediate form, Tiktaalik roseae.Tiktaalik roseae.

Tiktaalik roseaeTiktaalik roseae

Discovered in 2004 on Ellesmere Island in the Discovered in 2004 on Ellesmere Island in the Canadian Arctic Tiktaalik roseae is an extremely Canadian Arctic Tiktaalik roseae is an extremely important fossil link in the origin of tetrapods.important fossil link in the origin of tetrapods.

Another “fishapod” Tiktaalik like Panderihthys Another “fishapod” Tiktaalik like Panderihthys has a mixture of fish and tetrapod has a mixture of fish and tetrapod characteristics, but has several tetrapod characteristics, but has several tetrapod characteristics that characteristics that PanderichthysPanderichthys lacks. lacks.

Tiktaalik roseaeTiktaalik roseae TiktaalikTiktaalik has fish-like scales, as well as a fish-like has fish-like scales, as well as a fish-like

palate, lower jaw and fin rays (but not toes).palate, lower jaw and fin rays (but not toes).

Unlike a fish however it has a mobile neck and Unlike a fish however it has a mobile neck and the ear is structured so that it can hear both in the ear is structured so that it can hear both in and out of water.and out of water.

Most strikingly, in addition to the other tetrapod Most strikingly, in addition to the other tetrapod limb bones limb bones TiktaalikTiktaalik has a wrist. has a wrist. Tiktaalik’sTiktaalik’s elbow could bend like ours and the wrist could elbow could bend like ours and the wrist could bend too, which allowed the animal to make its bend too, which allowed the animal to make its “palm” lie flat. “palm” lie flat.

Tiktaalik roseaeTiktaalik roseae

Tiktaalik’sTiktaalik’s wrist and elbow structure and the fact wrist and elbow structure and the fact that that TiktaalikTiktaalik had very large chest muscles had very large chest muscles indicates that indicates that Tiktaalik Tiktaalik was specialized to do was specialized to do push-ups.push-ups.

TiktaalikTiktaalik has a flat head with eyes on the top and has a flat head with eyes on the top and appears to have evolved to move around in appears to have evolved to move around in shallow water and on mudflats. Having fins that shallow water and on mudflats. Having fins that could support the body would have been a big could support the body would have been a big advantage in such an environment.advantage in such an environment.

Tiktaalik roseae 375 mya

Tiktaalik roseaeTiktaalik roseae

TiktaalikTiktaalik could do push-ups, but lacks could do push-ups, but lacks digits and so could not grasp something or digits and so could not grasp something or throw something.throw something.

However, it reveals the early stages of the However, it reveals the early stages of the evolution of the wrist and the early stages evolution of the wrist and the early stages of the forearm’s ability to rotate against the of the forearm’s ability to rotate against the elbow (called pronation or supination elbow (called pronation or supination depending on the direction of rotation).depending on the direction of rotation).

http://ovrt.nist.gov/projects/vrml/h-anim/joint1A.gif

Tiktaalik roseaeTiktaalik roseae

You can rotate your hand relative to your elbow You can rotate your hand relative to your elbow because there is a ball at the end of the because there is a ball at the end of the humerus around which the tip of the radius humerus around which the tip of the radius attaches forming a ball and socket joint.attaches forming a ball and socket joint.

The beginnings of this joint are present in The beginnings of this joint are present in TiktaalikTiktaalik. In Tiktaalik the cup-shaped end of the . In Tiktaalik the cup-shaped end of the radius fits onto an elongated bump on the end of radius fits onto an elongated bump on the end of the humerus, which would have allowed the humerus, which would have allowed TiktaalikTiktaalik to rotate its forearm.to rotate its forearm.

From Wikipedia

The first tetrapodsThe first tetrapods

The oldest tetrapods discovered are The oldest tetrapods discovered are AcanthostegaAcanthostega and and IcthyostegaIcthyostega. .

Both have robust shoulder and hip bones as well as Both have robust shoulder and hip bones as well as sturdy limbs and a strong spine. In both the snout is sturdy limbs and a strong spine. In both the snout is elongated and the eyes have moved further back in the elongated and the eyes have moved further back in the skull and enlarged.skull and enlarged.

Both also possess digits, which Both also possess digits, which TiktaalikTiktaalik did not. did not.

Both species, however, retain a suite of fish-like Both species, however, retain a suite of fish-like characteristics including tail-fins, lateral line systems and characteristics including tail-fins, lateral line systems and gill slits.gill slits.

AcanthostegaAcanthostega Of the two speciesOf the two species Acanthostega Acanthostega is the slightly less is the slightly less

advanced form. advanced form.

It has 8 digits and a very fish like shoulder structure. It It has 8 digits and a very fish like shoulder structure. It would not have been able to move around well on land would not have been able to move around well on land because the hand and foot were not well adapted for because the hand and foot were not well adapted for walking being better suited for swimming or moving along walking being better suited for swimming or moving along the bottom. the bottom.

Acanthostega possessed gills and its ear is adapted to Acanthostega possessed gills and its ear is adapted to hearing in water.hearing in water.

ItIt probably made its way along the bottom and crawled probably made its way along the bottom and crawled over vegetation rather than doing much crawling on land.over vegetation rather than doing much crawling on land.

Acanthostegahttp://universe-review.ca/I10-72-Acanthostega.jpg

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

IchthyostegaIchthyostega Ichthyostega was discovered in the 1930’s in Ichthyostega was discovered in the 1930’s in

Greenland.Greenland.

Ichthyostega Ichthyostega has a smaller tail fin and its legs are has a smaller tail fin and its legs are relatively longer than those of relatively longer than those of AcanthostegaAcanthostega. . Although the legs probably wouldn’t have Although the legs probably wouldn’t have supported it well on land, but would have enabled supported it well on land, but would have enabled it to move around on the bottom in shallow water. it to move around on the bottom in shallow water.

It has 7 digits rather than the five of modern It has 7 digits rather than the five of modern tetrapods.tetrapods.

17.1 B and C

Figure 25.01b

What is clear from What is clear from AcanthostegaAcanthostega and and IcthyostegaIcthyostega is that is that the limbs of the earliest tetrapods first evolved not for the limbs of the earliest tetrapods first evolved not for walking on land, but for walking under water.walking on land, but for walking under water.

Understanding that, the retention of fish-like features Understanding that, the retention of fish-like features such as the lateral line system and tail fins by the earliest such as the lateral line system and tail fins by the earliest tetrapods makes sense. The first tetrapods apparently tetrapods makes sense. The first tetrapods apparently behaved much like modern salamanders spending most behaved much like modern salamanders spending most of their time in water and only occasionally emerging of their time in water and only occasionally emerging onto land.onto land.

It also renders old arguments about tetrapods having to It also renders old arguments about tetrapods having to have robust limbs to chase prey on land or to crawl from have robust limbs to chase prey on land or to crawl from pool to pool moot.pool to pool moot.

From the primitive tetrapod forms a great From the primitive tetrapod forms a great radiation of more terrestrial forms occurred radiation of more terrestrial forms occurred in the Carboniferous.in the Carboniferous.

From these two lineages of tetrapods From these two lineages of tetrapods arose one leading to the modern arose one leading to the modern amphibians the other to the modern amphibians the other to the modern amniotes.amniotes.

Tetrapod characteristcisTetrapod characteristcis

Unique derived features of the tetrapodsUnique derived features of the tetrapods Paired limbs: forelimbs with digits, carpals, Paired limbs: forelimbs with digits, carpals,

radius+ulna, humerus; hindlimbs digits, tarsals, radius+ulna, humerus; hindlimbs digits, tarsals, tibia+fibula, femur.tibia+fibula, femur.

Mobile neck: pectoral girdle separated from the skull. Mobile neck: pectoral girdle separated from the skull. In ancestors the pectoral girdle attached directly to In ancestors the pectoral girdle attached directly to the skull.the skull.

Hyomandibular bone previously used to support the Hyomandibular bone previously used to support the jaw now used in hearing. Called the stapes it jaw now used in hearing. Called the stapes it conducts sound in the ear.conducts sound in the ear.

First cervical vertebra (the atlas) specialized to allow First cervical vertebra (the atlas) specialized to allow the skull to nod.the skull to nod.