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Chordates

Chordates. Chapter 23. I. Ancestry and Evolution. Anatomical, developmental, and molecular evidence indicate chordates arose about 570 mya from lineage related to echinoderms. One lineage of fossil echinoderms has pharyngeal slits and other chordate characteristics.

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Chordates

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  1. Chordates Chapter 23

  2. I. Ancestry and Evolution • Anatomical, developmental, and molecular evidence indicate chordates arose about 570 mya from lineage related to echinoderms. • One lineage of fossil echinoderms has pharyngeal slits and other chordate characteristics

  3. CharacteristicsA. Notochord • Rodlike, semirigid tissue enclosed in sheath extending length of body between gut and nervous system that stiffens body or acts as skeletal axis • Found at some embryonic stage (usually 1st part of endoskeleton to appear) of all chordates • Persists throughout life of protochordates and jawless fishes but in vertebrates, it becomes the invertebral discs between vertebrae

  4. B. Dorsal Tubular Nerve Cord • Nerve cord is in dorsal position as opposed to ventral position in invertebrates • Anterior end enlarges to form brain

  5. C. Pharyngeal Pouches and Slits • Slits form in pharynx in aquatic chordates which form filter feeding apparatus in protochordates • Fishes added a capillary network which eventually evolved into gills • In amniotes, grooves do not penetrate into pharynx

  6. D. Post-Anal Tail • Post-anal tail along with muscles, provide motility for larvae • Tail increased in fishes but reduced or vestigial in later lineages

  7. Subphylum UrochordataA. Diversity • 3000 species found in all seas and all depths • Larvae bear the chordate characteristics but lose all but pharyngeal slits as adults, which are sessile • 1. Ascidians--sea squirts live on rocks, pilings in intertidal areas and are filter-feeders • 2. Thalacians—salps live in open-ocean and filter-feed • 3. Larvacea—resemble larval forms of tunicates and filter-feed

  8. Subphylum CephalochordataA. Characteristics • Lancelets (amphioxus) are slender, laterally flattened, translucent animals about 5-7 cm. Long • Live in sandy bottoms of coastal areas around the world

  9. B. Form and Function • Filters water through pharyngeal slits • Closed circulatory system without heart • Nerve cord above notochord • Considered living descendant of ancestors that produced cephalochordates and vertebrates

  10. Subphylum VertebrataA. Characteristics • 1. Endoskeleton • Grows with individual, jointed to allow scaffolding for muscles • Skull and rib cage enclose and protect organs • Tough integument also protect individual • Cartilage probably 1st endoskeleton material because it promotes fast growth; remains in sharks • Bone can store minerals and has added strength needed for terrestrial life

  11. 2. Pharynx & Efficient Respiration • As protovertebrates shifted from filter feeders to predators, pharynx modified into muscular feeding apparatus that could pump water • Circulation in internal gills improved with addition of capillary beds and aortic arches, which increased metabolic rate

  12. 3. Advanced Nervous System • Switch to predation created selective pressure for paired eyes with lenses and inverted retinas, pressure receptors, paired ears, electroreceptors, and chemical receptors • 2 new vertebrate innovations in cells (extremely rare event in evolution) which resulted in cranium, cranial nerves, branchial skeleton, and aortic arches; also give rise to nose, eyes, ears, taste receptors, and lateral line mechanoreceptors

  13. 4. Paired Limbs • Pectoral and pelvic appendages originated as swimming stabilizers • Jointed limbs that developed are suited for life on land and permit fine movement

  14. Evolutionary History1. Fossils • Only 1 invertebrate chordate fossil is known • It has both a notochord, muscle bans, and is similar to amphioxus

  15. 2. Garstang’s Hypothesis • Came up with the theory of paedomorphosis, the evolutionary retention of larval traits in an adult body • This occurs in some amphibians • Suggested that evolution occurs at some larval stages which is supported by embryological evidence

  16. 3. Amphioxus • Thought to be sister of vertebrates but more evidence indicates that it is a direct ancestor to earliest vertebrates

  17. 4. Lamprey Larvae • Amnocoete (larval stage of lampreys) resembles amphioxus • It has the most primitive set of organs seen in vertebrates:2-chambered heart, 3-part brain, median nostril, auditory vesicles, thyroid, pituitary gland, liver, gall bladder, and pancreatic tissue

  18. 5. Jawless Ostracoderms • Earliest articulated vertebral skeletons • Small,heavily armored, jawless fishlike animal • May have fed on invertebrates along ocean bottom

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