Chordates - Santa Paula High School
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Transcript Chordates - Santa Paula High School
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 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 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 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 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