Transcript The origin of tetrapods and movement onto land

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.
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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.
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In addition, gills collapse out of water so an
alternative breathing system (i.e. lungs) was
needed.
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.
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.
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Was it to:
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escape drying pools of water?
exploit new food sources on land?
escape predators in the crowded waters?
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.
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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.
Modern fish out of water
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Despite the apparent challenges of spending
time out of water a variety of modern bony fishes
do so.
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These include:
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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.
Walking Catfish
http://www.cfnews13.com/uploadedImages/Stories/Local/walking%20catfish.jpg
Climbing Perch
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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.
Climbing Perch
http://static.howstuffworks.com/gif/willow/climbing-perch-info0.gif
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.
Mudskipper
http://www.naturephoto-cz.com/photos/mraz/mudskipper-05a22018.jpg
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.
Modern fish out of water
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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.
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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.
Frogfish
http://www.eco-divers.com/galleries/d/256-3/12.jpg
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.
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).
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. 360-365
mya) .
Eusthenopteron
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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.
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Similarly a bone by bone comparison of
Eusthenopteron’s skull bones matches that in the
earliest tetrapods.
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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.
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.
Figure 25.01a
17.1a Eusthenopteron
Panderichthys
 The
next fossils are a series of more
tetrapod-like fish including Panderichthys
from the late Devonian.
 Panderichthys
is a very tetrapod-like lobefinned fish. In contrast to Eusthenopteron
the body is flattened has upward facing
eyes and a straight tail with a welldeveloped tail fin.
Panderichthys
 Panderichthys
has both gills and welldeveloped lungs with nostrils.
 It
has also lost the anal and dorsal fins of
fish leaving the foot-like pectoral and
pelvic fins.
Panderichthys
http://archeowiesci.files.wordpress.com/2008/09/panderichthys_bw.jpg
Panderichthys
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Panderichthys’ braincase is very tetrapod-like
and it possesses the characteristic teeth of later
tetrapods.
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These labyrinthodont teeth are named for their
enfolded enamel.
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Panderichthys is a “fishapod” intermediate
between fish and tetrapods with a tetrapod-like
skull and body, braincase, and lungs, but still
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
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.
Tiktaalik roseae
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Discovered in 2004 on Ellesmere Island in the
Canadian Arctic Tiktaalik roseae is an extremely
important fossil link in the origin of tetrapods.
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Another “fishapod” Tiktaalik like Panderihthys
has a mixture of fish and tetrapod
characteristics, but has several tetrapod
characteristics that Panderichthys lacks.
Tiktaalik roseae
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Tiktaalik has fish-like scales, as well as a fish-like
palate, lower jaw and fin rays (but not toes).
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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.
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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.
Tiktaalik roseae
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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.
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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.
Tiktaalik roseae 375 mya
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).
http://ovrt.nist.gov/projects/vrml/h-anim/joint1A.gif
Tiktaalik roseae
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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.
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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.
From Wikipedia
The first tetrapods
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The oldest tetrapods discovered are Acanthostega and
Icthyostega.
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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.
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Both also possess digits, which Tiktaalik did not.
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Both species, however, retain a suite of fish-like
characteristics including tail-fins, lateral line systems and
gill slits.
Acanthostega
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Of the two species Acanthostega is the slightly less
advanced form.
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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.
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Acanthostega possessed gills and its ear is adapted to
hearing in water.
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It probably made its way along the bottom and crawled
over vegetation rather than doing much crawling on land.
Acanthostega
http://universe-review.ca/I10-72-Acanthostega.jpg
http://www.earthhistory.org.uk/wp-content/IchthyostegaAcanthostega.jpg
Ichthyostega
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Ichthyostega was discovered in the 1930’s in
Greenland.
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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.
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It has 7 digits rather than the five of modern
tetrapods.
Figure 25.01b
17.1 B and C
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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.
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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.
 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.
Tetrapod characteristcis
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Unique derived features of the tetrapods
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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.