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Lecture 7

lecture 7

12 Pages
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Department
Biology
Course Code
BIO356H5
Professor
Michaelde Braga

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Lecture 7- Origin of Tetrapod
Early tetrapods and how they arrived; Paleozoic amphibian lineages and some of their plausible
evolutionary scenarios (as far as living amphibians apply)
Article reading (2007)- will touch on briefly this lecture: how the tetrapod limb evolved –
Genetic arguement supports a lot of the anatomical and morphological arguements that
have been around for a very long time – how limbs appear similar
Very detailed and technical genetic language but try get a sense of the relationships between the different disciplines:
get a sense of whats it like
1.)Sigillaria tree stump (not a tree, it’s a fern)- most dominant plant life of the Carboniferous period; extremely
abundant in Nova Scotia-have extreme tide between low and high tides so a lot of erosion takes place of the
Nova Scotia shoreline and expose areas that contain trees that have preserved remains of the earliest
tetrapod
2.)Tetrapod arrives somewhere in the Devonian. Fish was still the most predominant life
form at that time (whether actinopterygian or sarcopterygian fish-> ones that led to the
tetrapods); tetrapods not that abundant in terms of distribution but were starting to show
up (found only in areas associated w/ the Nova Scotia style environment)
3.)Earliest tetrapods are represented by: Icthyostega and Acanthostega (best known)
show very similar anatomy to Eusthenopteron – a sarcopterygian (lobe-finned fish);
similarity expressed throughout the skeleton but there are some differences too
Icthyostega and Acanthostega have proper tetrapod limbs but Eusthenopteron does not
Earliest amphibians:
NO NECK
have a fish –like head
fairly long tail which is dorso-ventrally expanded and compressed so would swim by
undulating their tail
www.notesolution.com
probably were not good at moving around on land - limbs arent very big and most likely
use of limbs was to move from one pond to another- basically lived in the water and were
tied to the water for reproduction
Turpleton another one from Russia so as we keep looking, keep finding more of them
4.)Phylogeny :
Acanthostega – most basal member of the tetrapods; mostly known from its skull
Icthyostega- best known and most complete
Turpleton- only fragmentary remains of the skull
Crown group tetrapods –includes all of the tetrapods that have given rise to all of the
living groups that are around today
5.)What must happen when switching from fish to tetrapod:
In water- helps you maintain architecture so no need to have a well-developed
appendicular skeleton and theres no need to have ribs because your body is supported by
the water youre living in. When leave the water and move to land , no more buoyancy
and gravity pulls you down so if you dont have ribs to protect your internal organs,
they’ll be squished; Didnt have a diaphragm so cant breathe if lungs being squeezed.
Development of expanded ribs that supported the thoracic or dorsal region of the body were
important
No water for locomotion so cant rely on moving around using a tail or fins – need limbs!
Food sources- may have been the reason why these organisms came onto land; source of
food in the water may have been inconsistent or theres direct competition that prevented
these organisms from taking advantage of their environment to their fullest. Ability to
gulp air or exchange of gases permitted them to come out of the water and walk around
and may have come across food sources.
Dentition implied they were not herbivorous- were carnivorous so earliest tetrapods depended on
other animals for food (insects, arthropods already on land). But they didnt do much feeding in
water anyways. They likely laid in ambush on water and any slow swimming or unsuspecting fish
that swam by were attacked by them- sprung out of a hiding place
www.notesolution.com
Very large jaws and quick closing (similar to crocodile); peglike dentition – no chewing;
designed to grip and hold prey and then bolted down their food
Sensory system: in water, lateral line system in fish is the most effective way of
identifying whats around them. Effecient because molecules of different materials can
dissolve readily in water and these organisms swim through and pick up these molecules
to recognize whats around them. On land, not easily accomplished so the lateral line
system need to be supplemented. Hearing needed to have evolved. Hearing in early
tetrapods not like in present tetrapods : no external ear or tympanum. They were low to
ground so their head was in contact to the ground and felt the vibrations on the ground
from approaching prey/predator and responded to the vibrations rather than actual sound
Not very fast, no neck so couldnt turn very rapidly
Eyes on top of the head so they werent designed to see anything coming around. Eyesight
wasnt very good- would be useful if they buried under a pond and could see things above
them but thats it. But on land, can only see very big things so eyes served very little
purpose
Key thing is limb changes: typical fish- like limb to the typical tetrapod-like limb. The
earliest tetrapods may have had multiple digits. The seratotrichia – extension of the fin in
fish- are the precursors and served same function as digits in the early tetrapods. Since
Ichthyostega had only 5 digts on their hand, it was thought this was the primitive number
of all the early tetrapods. 5 is primitive (meaning in a cladogram, it was given state 0). All
tetrapods started w/ 5 and anything beyond that or less than that would have been derived
b/c all the other organisms would be considered more highly evolved than Ichthyostega
was. Acanthyostega was then found and had many more digits on its hand so what is
primitive? Find out how many digits are more useful to walk around in land.
Bones have Pizioelectric property: bones respond to stress by becoming more heavily calcified.
Bones that are not under stress becomes smaller and eventually disappear (as far as their function
is concerned). If you have stress imparted on certain elements, those elements will retain their
rigidity and their strength. So its possible w/ the many number of digits in tetrapod limbs to start
off , that the digits most exposed to the stress were about 4 or 5 digits and all of the other digits
became unnecessary (may have even created some disadvantage to locomotion).
Natural selection works against deleterious effects but doesnt get rid of items that are useless as
long as theyre not harmful. If theres a characteristic / trait that is harmful and reduces the ability
www.notesolution.com

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Description
Lecture 7- Origin of Tetrapod Early tetrapods and how they arrived; Paleozoic amphibian lineages and some of their plausible evolutionary scenarios (as far as living amphibians apply) Article reading (2007)- will touch on briefly this lecture: how the tetrapod limb evolved Genetic arguement supports a lot of the anatomical and morphological arguements that have been around for a very long time how limbs appear similar Very detailed and technical genetic language but try get a sense of the relationships between the different disciplines: get a sense of whats it like 1.)Sigillaria tree stump (not a tree, its a fern)- most dominant plant life of the Carboniferous period; extremely abundant in Nova Scotia-have extreme tide between low and high tides so a lot of erosion takes place of the Nova Scotia shoreline and expose areas that contain trees that have preserved remains of the earliest tetrapod 2.)Tetrapod arrives somewhere in the Devonian. Fish was still the most predominant life form at that time (whether actinopterygian or sarcopterygian fish-> ones that led to the tetrapods); tetrapods not that abundant in terms of distribution but were starting to show up (found only in areas associated w the Nova Scotia style environment) 3.) Earliest tetrapods are represented by: Icthyostega and Acanthostega (best known) show very similar anatomy to Eusthenopteron a sarcopterygian (lobe-finned fish); similarity expressed throughout the skeleton but there are some differences too Icthyostega and Acanthostega have proper tetrapod limbs but Eusthenopteron does not Earliest amphibians: NO NECK have a fish like head fairly long tail which is dorso-ventrally expanded and compressed so would swim by undulating their tail www.notesolution.com probably were not good at moving around on land - limbs arent very big and most likely use of limbs was to move from one pond to another- basically lived in the water and were tied to the water for reproduction Turpleton another one from Russia so as we keep looking, keep finding more of them 4.) Phylogeny : Acanthostega most basal member of the tetrapods; mostly known from its skull Icthyostega- best known and most complete Turpleton- only fragmentary remains of the skull Crown group tetrapods includes all of the tetrapods that have given rise to all of the living groups that are around today 5.) What must happen when switching from fish to tetrapod: In water- helps you maintain architecture so no need to have a well-developed appendicular skeleton and theres no need to have ribs because your body is supported by the water youre living in. When leave the water and move to land , no more buoyancy and gravity pulls you down so if you dont have ribs to protect your internal organs, theyll be squished; Didnt have a diaphragm so cant breathe if lungs being squeezed. Development of expanded ribs that supported the thoracic or dorsal region of the body were important No water for locomotion so cant rely on moving around using a tail or fins need limbs! Food sources- may have been the reason why these organisms came onto land; source of food in the water may have been inconsistent or theres direct competition that prevented these organisms from taking advantage of their environment to their fullest. Ability to gulp air or exchange of gases permitted them to come out of the water and walk around and may have come across food sources. Dentition implied they were not herbivorous- were carnivorous so earliest tetrapods depended on other animals for food (insects, arthropods already on land). But they didnt do much feeding in water anyways. They likely laid in ambush on water and any slow swimming or unsuspecting fish that swam by were attacked by them- sprung out of a hiding place www.notesolution.com
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