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