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

II.Characteristics A. 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 1 st 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

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

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

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

III.Subphylum Urochordata A. 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

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

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

V.Subphylum Vertebrata A. 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 1 st endoskeleton material because it promotes fast growth; remains in sharks  Bone can store minerals and has added strength needed for terrestrial life

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

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

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

B.Evolutionary History 1. Fossils  Only 1 invertebrate chordate fossil is known  It has both a notochord, muscle bans, and is similar to amphioxus

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

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

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

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