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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. 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
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 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
IV. 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
B. Form and Function
Filters water through pharyngeal slitsClosed circulatory system without heartNerve cord above notochordConsidered living descendant of ancestors that
produced cephalochordates and vertebrates
V. 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
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 History1. Fossils
Only 1 invertebrate chordate fossil is knownIt 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