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BIO356H5 (3)
Lecture 7

lecture 7

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Michaelde Braga

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 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
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