L02jawless-jaw11COLOR

L02jawless-jaw11COLOR - PRIMITIVE FISHES Origin of fishes...

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Unformatted text preview: PRIMITIVE FISHES Origin of fishes A. From What? B. When? C. How? D. Where? Phylum Chordata 1. Notochord - flexible dorsal rod for support. Present at some stage in all chordates (usually in embryonic development). Present in adults of many fishes: sharks, rays, sturgeons 2. Dorsal, hollow nerve chord 3. Pharyngeal gill slits - present in embryos of all vertebrates Phylum Chordata Subphylum Urochordata: tunicates Subphylum Cephalochordata: lancelets Subphylum Craniata Infraphylum Vertebrata A. From What? 1) Fishes probably arose from invertebrate chordate ancestors Tunicates Lancelets Subph. Cephalochordata Subph. Urochordata A. From What? 1) Fishes probably arose from invertebrate chordate ancestors Tunicates Lancelets Subph. Cephalochordata Subph. Urochordata B. When? 1) Chordates date from early to mid Cambrian (600 mya) 2) First good vertebrate (really craniate) fossils - late Cambrian/Ordovician (500 mya) 3) BUT these first fossil bones were of external armor, characteristic of early jawless fishes (no internal bones) 4) So, vertebrates probably originated during the early Cambrian explosion (600 mya) Pikaia gracilens - Mid Cambrian Burgess shale Earliest accepted chordate Discovered in 1911 in British Columbia First classified as a polychaete worm Re-classified as a chordate based on presence of primitive proto- notochord Similar to a lancelet, may have swam like an eel C. How? Neoteny = retention of larval features into the adult stage Stages: 1) Ancestors (tunicates?) had: a) sessile adult stage b) free swimming larval stage for dispersal 2) Larval stage became more active; more vertebrate-like 3) Eventually, larvae capable of reproduction evolved ( = neoteny) adult with tunicate larval features proto-vertebrate, development of a cranium first fish lancelet adult tunicate How first fishes may have evolved from primitive chordate ancestors D. Where? 1) Generally believed to be in ocean because: a) habitat of other chordates b) marine deposits contain most early vertebrate fossils Suphylum Craniata Infraphylum Vertebrata Characteristics: 1. Most have a bony skeleton (except Agnatha and Chondrichthyes) 2. Vertebral column 3. Distinct cranium; skull with brain....
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L02jawless-jaw11COLOR - PRIMITIVE FISHES Origin of fishes...

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