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09 28 07 - Appendicular Skeleton I

09 28 07 - Appendicular Skeleton I - Chapter 9 The...

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Chapter 9: The Postcranial Skeleton, Appendicular Skeleton The pelvic and pectoral appendages and their girdles Paired appendages- fins(fish) or limbs in tetrapods Function - Fins used to stabilize, steer, or propel - Limbs in terrestrial forms use to walk and grasp Evolutionary origin: First animals to have paired appendages: osteostracans - pectoral fins First animals to have 2 paired appendages: placoderms Hypothesis 1: Pectoral fins evolved first, followed by pelvic. This is supported by existing fossil record *not expected to remember Hypothesis 2: Pectoral and pelvic fins evolved simultaneously; Supported by anatomical similarity of pectoral and pelvic fins in most organisms Fin fold hypothesis ” is still considered (it is a pretty old idea). - A jawless vertebrate had folds of skin along the dorsal midline, and ventrally on the tail. Between the head and the anus, the folds were paired in a ventrolateral position. According to this hypothesis, the fin folds broke up, leading to pairs of fins. Hypothesis 3: the spines that form on the outer ridge of the skin folds became the fin rays. Discredited because these spines form in epidermis which is not how they are formed in the embryo.
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