Download Subphylum Vertebrate

CHAPTER 23
Chordates
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The Chordates: Characteristics
Structural Plan
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Name chordata comes from the notochord
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5 Hallmark chordate characteristics
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Rodlike, semirigid tissue enclosed in a sheath
In most cases, extends the length of body lying
between the gut and the nervous system
Mainly serves to stiffen the body, providing
skeletal scaffolding for the attachment of
swimming muscles
Dorsal, tubular nerve cord
Notochord
Pharyngeal slits
Endostyle
Postanal tail
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The Chordates: Characteristics
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Chordates share features with some
invertebrates:
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Bilateral symmetry
Anterioposterior axis
Coelom
Tube-within-a-tube body plan
Metamerism
Cephalization
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Traditional and Cladistic Classification of the Chordates
Traditional and Cladistic Systems Diverge
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Traditional classification
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Cladistic systems
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Convenient way to indicate the taxa included in
each major group
Some traditional taxa (Agnatha and Reptilia) no
longer used
Reptiles are paraphyletic because they do not
contain all of the descendants of recent common
ancestor
Reptiles, birds and mammals compose a
monophyletic clade called Amniota
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Traditional and Cladistic Classification of the Chordates
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Reptiles can only be grouped as amniotes that are
not birds or mammals
No derived characters that group only reptiles to
the exclusion of birds and mammals
Likewise, agnathans (hagfishes and lampreys) are
paraphyletic
 Most common recent ancestor is also an
ancestor of all remaining vertebrates
In contrast, the branches of a phylogenetic tree
represent real lineages with geological
information
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Traditional and Cladistic Classification of the Chordates
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Traditional classification
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Protochordata (Acraniata) are separated from
Vertebrata (Craniata) that have a skull
Vertebrates may be divided into Agnatha (jawless)
and Gnathostomata (having jaws)
Vertebrates are also divided into Amniota, having
an amnion, and Anamniota lacking an amnion
Gnathostomata is subdivided into Pisces with fins
and Tetrapoda, usually with two pair of limbs
Many of these groupings are paraphyletic
Alternative monophyletic taxa are suggested
Some cladistic classifications exclude Myxini
(hagfishes) from the group Vertebrata because
they lack vertebrae, although retaining them in
Craniata since they do have a cranium.
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Five Chordate Hallmarks
Notochord
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Always found at some embryonic stage
First part of the endoskeleton to appear in
the embryo
Serves as an axis for muscle attachment
Can bend without shortening and permits
undulation
In protochordates and jawless vertebrates,
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Persists throughout life
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Five Chordate Hallmarks
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In vertebrates
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Series of cartilaginous or bony vertebrae
form from mesenchymal cells derived from
blocks of mesodermal cells lateral to
notochord
In most vertebrates
Notochord displaced by vertebrae
 Remnants may persist between or within
vertebrae
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Five Chordate Hallmarks
Dorsal Tubular Nerve Cord
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In most invertebrate phyla
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In chordates
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Nerve cord is solid and ventral to alimentary canal
Single, tubular cord is dorsal to alimentary canal
Anterior end enlarges to form the brain
Cord is produced in embryo by infolding of
ectodermal cells on the dorsal side of body
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Five Chordate Hallmarks
Pharyngeal Pouches and Slits
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Pharyngeal slits lead from pharyngeal cavity
to the outside
Form by the inpocketing of the ectoderm and
the evagination of endoderm of pharynx
In aquatic chordates
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2 pockets break through to form pharyngeal slit
In amniotes
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Pockets may not break through and only grooves
are formed
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Five Chordate Hallmarks
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In tetrapods
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Pharyngeal pouches give rise to a variety of
structures, including the Eustachian tube, middle
ear cavity, tonsils and parathyroid glands
Perforated pharynx functions as filterfeeding apparatus in protochordates
Fishes added a capillary network with thin
gas-permeable walls
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Led to evolution of gills
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Five Chordate Hallmarks
Endostyle or Thyroid Gland
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Recently, the endostyle was recognized as a
shared chordate character
Endostyle or its derivative, the thyroid gland,
found in all chordates
Some cells in endostyle secrete iodinated
proteins homologous with the iodinatedhormone-secreting thyroid gland of adult
lampreys and the remainder of vertebrates
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Five Chordate Hallmarks
Postanal Tail
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Postanal tail, plus musculature, provided
motility for larval tunicates and Amphioxus
to swim.
Efficiency increased in fishes but became
smaller or vestigial in later lineages
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Ancestry and Evolution
History
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Earliest protochordates were soft-bodied and
would not have left many fossils
Most work has been conducted on
developmental stages where early features
are conserved
A theory that chordates evolved within the
protostome lineage was discarded due to
embryonic evidence
Deuterostomes are a natural grouping that
have a common origin in Precambrian seas
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Ancestry and Evolution
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Anatomical, developmental, and molecular
evidence indicate that chordates arose about
570 million years ago from a lineage related
to echinoderms and hemichordates
Molecular data suggest that a clade
containing both echinoderms and
hemichordates is the sister group of
chordates
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Subphylum Urochordata: Tunicata
Diversity
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Approximately 3000 species of tunicates
identified
Occur in all seas and at all depth
Most are sessile as adults although a few are
free-living
Tunic is a tough, nonliving test that
surrounds them and contains cellulose
In most species, only the larval form bears all
the chordate hallmarks
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Adults lose many of these characters
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Subphylum Urochordata: Tunicata
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During adult metamorphosis
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Notochord and tail disappear
Dorsal nerve cord is reduced
Urochordata is divided into 3 classes
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Ascidiacea
Appendicularia
Thaliacea
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Subphylum Urochordata: Tunicata
Form and Function of Ascidians
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Called sea squirts because they discharge a
jet of water when disturbed
Most attached to rocks or pilings as adults
Among most abundant intertidal animals
Colonial and solitary ascidians have their
own test
Compound forms share a common test
In some compound ascidians
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Each has own incurrent siphon but share the
excurrent siphon
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Subphylum Urochordata: Tunicata
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Mantle lines the tunic
Incurrent or oral siphon marks anterior side
Excurrent or atrial siphon marks dorsal side
Water entering the incurrent siphon passes
through a ciliated perforated pharynx with an
elaborate basketwork
Water flows though slits into atrial cavity and
out through excurrent siphon
Feeding depends on the formation of a
mucous net that is secreted by the endostyle
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Subphylum Urochordata: Tunicata
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Cilia on gill bars of pharynx pull mucus into a
sheet
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Particles trapped in sheet are worked into a rope
and carried back to the esophagus and stomach
Heart drives blood first in one direction, then
in reverse
Organisms also concentrate very rare
elements, such as vanadium, in dramatically
high concentrations
Nervous system has one nerve ganglion and
a plexus of nerves on dorsal side of pharynx
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Subphylum Urochordata: Tunicata
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Subneural gland samples incoming water
and may have an endocrine function
Hermaphroditic with a single ovary and a
single testes
Fertilization is external
Adult sea squirts
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Tadpole larvae
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Retain 1 of the 5 chordate features: pharyngeal
slits
Have all 5 chordate characteristics
Larva does not feed, but swims awhile before
attaching and developing into a sessile adult
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Subphylum Urochordata: Tunicata
Form and Function of Thalacians
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Salps are pelagic with a lemon-shaped,
transparent body
Pump water through body by muscular
contraction rather than ciliary action
Alternate sexual and asexual generations
Increase in number rapidly with abundant
food supply
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Subphylum Urochordata: Tunicata
Form and Function of Appendicularian
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Resemble the larval stages of other tunicates
Each builds a delicate hollow sphere of mucus
interlaced with passages for water entry
Phytoplankton and bacteria trapped on a
feeding filter inside sphere are drawn into
mouth through a tube
When filters become clogged with wastes,
they are left behind and a new sphere is built
Paedomorphic
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Sexually mature individuals that retain the larval
body form of ancestors
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Subphylum Cephalochordata
Diversity
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Lancelets
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Live in sandy bottoms of coastal waters
around the world
Originally bore the generic name Amphioxus,
but by priority are now in the genus
Branchiostoma
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Slender, laterally flattened, translucent animals
about 5–7 cm long
Still referred to by general name, amphioxous
About 25 species of amphioxus are described
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5 occur in North American coastal waters
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Subphylum Cephalochordata
Form and Function
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Amphioxus has the five distinctive
characteristics of chordates in simple form
Water enters the mouth driven by cilia in the
buccal cavity and pharynx
Water passes through pharyngeal slits where
food is trapped in mucus secreted by the
endostyle
Food is moved through gut via cilia
concentrated in areas called the ileocolic ring
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Subphylum Cephalochordata
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Food particles separated from mucus are
passed into hepatic cecum where they are
phagocytized
Filtered water leaves body by an atriopore
Closed circulatory system is complex but
lacks a heart
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Blood is pumped by peristaltic contractions in
ventral aorta, passes upward through branchial
arteries in pharyngeal bars to paired dorsal aortas
Blood moves by microcirculation through tissues
and returns to ventral aorta
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Subphylum Cephalochordata
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Blood lacks erythrocytes and hemoglobin
and mainly transports nutrients
Hollow nerve cord lies above the notochord
Pairs of spinal nerve roots emerge at each
trunk segment
Sense organs are simple, including an
unpaired ocellus that functions as a
photoreceptor
Anterior nerve cord is not enlarged, yet is
homologous to vertebrate brain
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Subphylum Cephalochordata
Reproduction
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Sexes are separate
Gametes are set free in the atrium and pass
out through atriopore
Fertilization is external
Cleavage is holoblastic and a gastrula forms
by invagination
Larvae soon hatch and gradually assume the
shape of adults
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Subphylum Cephalochordata
Basic Plan
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Amphioxus possesses features that suggest
the vertebrate plan
Cecum is a diverticulum resembling the
vertebrate pancreas in secreting digestive
enzymes
Trunk muscles resemble vertebrate patterns
Possess basic circulatory plan of more
advanced chordates
Many zoologists consider amphioxus a living
descendant of ancestors that gave rise to
both cephalochordates and vertebrates
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Would make them the living sister group of the
vertebrates
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Subphylum Vertebrate (Craniata)
Adaptations That Guided Vertebrate Evolution
 Earliest vertebrates
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Were substantially larger than protochordates
Considerably more active than protochordates
Characterized by increased speed and mobility
resulting from modifications of skeletal structures
and muscles
Higher activity level and size of vertebrates
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Requires structures specialized in the location,
capture, and digestion of food and adaptations
designed to support a high metabolic rate
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Subphylum Vertebrate (Craniata)
Musculoskeletal Modifications
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Most vertebrates possess both an
exoskeleton and endoskeleton of cartilage or
bone
Endoskeleton permits almost unlimited body
size with much greater economy of building
materials
Endoskeleton forms excellent jointed
scaffolding for attachment of segmented
muscles
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Subphylum Vertebrate (Craniata)
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Segmented body muscles (myomeres)
changed from V-shaped muscles of
cephalochordates to W-shaped muscles of
vertebrates
Also unique to vertebrates are presence of
fin rays of dermal origin in fins, aiding in
swimming
Endoskeleton probably composed initially of
cartilage and later gave way to bone
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Subphylum Vertebrate (Craniata)
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Endoskeleton of living hagfishes, lampreys,
sharks and their kin, and even some “bony”
fishes, such as sturgeons, mostly composed
of cartilage
Structural strength of bone is superior to
cartilage
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Makes it ideal for muscle attachment in areas of
high mechanical stress
Perhaps bone evolved, in part, as a means of
minereal regulation
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Phosphorus and calcium are used for many
physiological processes
In particularly high demand in organisms with
high metabolic rates
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Subphylum Vertebrate (Craniata)
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Some of the most primitive fishes, including
Ostracoderms and placoderms were partly
covered in a bony, dermal armor
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Modified in later fishes as scales
Many of the bones encasing brain of
advanced vertebrates develop from cells that
originate from the dermis
Most vertebrates are protected with
keratinized structures derived from the
epidermis
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Reptilian scales, hair, feathers, claws, and horns
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Subphylum Vertebrate (Craniata)
Physiology
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Modifications of digestive, respiratory,
circulatory, and excretory systems to meet
increased metabolic demand
Perforated pharynx evolved as a filterfeeding device in early chordates
Water with suspended food particles was
drawn through the pharynx by ciliary action
and trapped by mucus secreted by the
endostyle
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Subphylum Vertebrate (Craniata)
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In larger, predatory vertebrates, pharynx was
modified into a muscular apparatus that
pumped water through pharynx
With the origin of highly vascularized gills,
function of pharynx shifted to primarily gas
exchange
To manage increased ingestion of food
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Gut shifted from movement of food by ciliary
action to muscular action
Accessory digestive glands, the liver and
pancreas, produced secretions that aided
digestion
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Subphylum Vertebrate (Craniata)
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Transport of nutrients gases, and other
substances was enhanced by
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Ventral 3-chambered heart
 Sinus venosus
 Atrium
 Ventricle
Erythrocytes containing hemoglobin
Vertebrates possess paired, glomerular
kidneys to remove metabolic waste products
and regulated body fluid composition
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Subphylum Vertebrate (Craniata)
New Head, Brain, and Sensory Systems
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Shift from filter feeding to active predation
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Anterior end of nerve cord enlarged as a
tripartite brain
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Required new sensory, motor, and integrative
controls for location and capture of larger prey
Forebrain, midbrain, and hindbrain
Brain was protected by cartilaginous or bony
cranium
Paired special sense organs for vision,
equilibrium, and sound evolved
Other receptors that evolved
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Mechanoreceptors, chemoreceptors,
electroreceptors, and olfactory receptors
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Subphylum Vertebrate (Craniata)
Neural Crest, Ectodermal Placodes, and Hox Genes
 Development of vertebrate head and special
sense organs
 Largely result of two embryonic innovations
present only in vertebrates
 Neural crest and ectodermal placodes
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Subphylum Vertebrate (Craniata)
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Neural crest
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Derived from a population of ectodermal cells
lying along length of the embryonic neural tube
Contributes to formation of many different
structures
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Most of the cranium, pharyngeal skeleton, tooth dentine,
some cranial nerves, ganglia, Schwann cells, and some
endocrine glands
May also regulate development of adjacent tissue,
such as tooth enamel and pharyngeal muscles
(branchiomeres)
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Subphylum Vertebrate (Craniata)
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Ectodermal placodes
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Plate-like ectodermal thickenings appearing on
either side of neural tube
Give rise to olfactory epithelium, lens of eye, inner
ear epithelium, some ganglia, some cranial
nerves, lateral-line mechanoreceptors, and
electroreceptors
Placodes also induce formation of taste buds
Vertebrate head with sensory structures located
adjacent to mouth (later equipped with preycapturing jaws) stemmed from the creation of new
cell types
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Subphylum Vertebrate (Craniata)
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Hox Genes
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Recent studies of the distribution of homeoboxcontaining genes that control the body plan of
chordate embryos suggest that the Hox genes
were duplicated at about the time of the origin of
vertebrates
One copy of Hox genes is found in Amphioxus
and other invertebrates
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Living gnathostomes have 4 copies
Perhaps additional copies of genes that control
body plan provided genetic material free to evolve
a more complex kind of animal
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Subphylum Vertebrate (Craniata)
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The Search for the Vertebrate Ancestral Stock
Jawless ostracoderms from the early
Paleozoic vertebrate fossil record
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Share organ system development with living
vertebrates
Indicates organ systems must have originated in
early vertebrate and invertebrate lineage
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Subphylum Vertebrate (Craniata)
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Haikouella lanceolata
Small fishlike creature known from over
300 fossil specimens provides a wealth of
information on evolution of vertebrates
 Possessed a notochord, pharynx, and
dorsal nerve cord
 Had pharyngeal muscles, paired eyes, and
an enlarged brain
 Not a vertebrate because it lacks
distinctive vertebrate characteristics such
as a cranium, an ear, and telencephalon
(anterior lobe of the brain)
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Subphylum Vertebrate (Craniata)
It is transitional in morphology between
cephalochordates and vertebrates
 Some hypothesize Haikouella is the sister
taxon of vertebrates
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Evolutionary History
Garstang’s Hypothesis of Chordate
Larval Evolution
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Chordates pursued 2 paths in early evolution
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1st path led to the sedentary urochordates
The 2nd to active, mobile cephalochordates and
vertebrates
In 1928, Walter Garstang of England,
suggested the chordate ancestral lineage
retained into adulthood the larval form of
sessile tunicate-like animals
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Paedomorphosis: Evolutionary retention of larval
traits in an adult body
 Occurs in some amphibians
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Evolutionary History
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Garstang’s hypothesis has been challenged
recently
Molecular data along with information from
fossils suggests
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Ancestor of deuterostomes was free-swimming
The sessile ascidians represent a derived body
form
Free-swimming appendicularians are most similar
in body form to ancestral chordates
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Evolutionary History
Position of Amphioxus
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Long been considered the closest living
relative to earliest vertebrates
Now not considered a direct ancestor
although it may closely resemble ancestor
Lacks a brain and specialized sensory
equipment of vertebrates
No gills in the pharynx and no mouth for
pumping water
Recent studies of homeobox containing genes
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Suggest that ancestor of both amphioxus and
vertebrates was cephalized
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Evolutionary History
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Many zoologists still consider
cephalochordates closest living relative of
vertebrates
However, Amphioxus is unlike the most
recent common ancestor of vertebrates
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Lacks the tripartite brain, chambered heart,
special sensory organs, muscular gut and
pharynx, and neural crest tissue inferred to have
been present in ancestor
Larger fins of some extinct cephalochordates
suggest they were more free-swimming than
modern Amphioxus
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Evolutionary History
Ammocoete Larvae of Lampreys as a
Model of the Primitive Vertebrate Body Plan
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Lampreys have a larval stage, ammocoete, that
closely resembles the amphioxus
Ammocoete larvae were originally considered
petromyzontidan adults
Ammocoete larval mouth resembles the
amphioxus but draws water in by muscular
pumping
Endostyle, mucus, body muscles, notochord
and circulatory system closely resemble
amphioxus
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Evolutionary History
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Ammocoetes equipped with
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2-chambered heart
3-part brain
Median nostril
Auditory vesicles
Thyroid and pituitary gland
Extensive pharyngeal filaments (serve in
respiration)
True liver, gallbladder, and pancreatic tissue
Ammocoete larva
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Has the most primitive condition of this set of
vertebrate structures
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Evolutionary History
The Earliest Vertebrates
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Ostracoderms
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Until recently, earliest known vertebrate fossils
were armored jawless fishes called ostracoderms
Found in the late Cambrian deposits in United
States, Bolivia and Australia
Small, heavily armored, jawless, and lacked
paired fins
During the last 10 years, several 530-million-yearold fossils were discovered in the Chengjiang
deposits belonging to one or two fishlike
vertebrates
 Myllokunmingia and Haikouichthys
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Evolutionary History
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These fossils push back the origin of vertebrates
to at least the early Cambrian
Fossils showed many vertebrate characters
including a heart, paired eyes, otic capsules, and
rudimentary vertebrae
Earliest Ostracoderms
 Equipped with bone in dermis
 Lacked paired fins that later fishes used for
stability
 Not considered to be a natural evolutionary
assemblage, but a convenience for describing
several groups of heavily armored extinct
jawless fishes, such as the heterostracans
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Evolutionary History
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Heterostracans
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Early group of ostracoderms
Represent an awkward design that probably
filtered particles from the ocean bottom
Sucked in water by muscular pumping
Some believe they may have been able to feed on
soft-bodied animals
Devonian saw a major radiation of heterostracans
producing numerous peculiar-looking forms,
never evolved jaws or paired fins
Extinct near end of Devonian
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Evolutionary History
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Osteostracans
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Coexisted with heterostracans
Developed paired pectoral fins that stabilized
movement
Jawless, toothless mouth
Sensory lateral line, paired eyes, and inner ears
with semicircular canals
Head was well armored, but lacked axial skeleton
or vertebrae
A typical osteostracan was Cephalaspis
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Small marine animal covered with a heavy, dermal armor
of cellular bone, including a single-piece head shield
Likely had a sophisticated nervous system and
sense organs, similar to those of modern
lampreys
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Evolutionary History
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Anapsids
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More streamlined than other Ostracoderms
Impressive radiation in Silurian and Devonian
periods
Streamlined and more closely resembled modern
lamprey
All Ostracoderms became extinct by end of
Devonian period
For decades strange microscopic tooth-like fossils
called conodonts have been used to date paleozoic
marine sediments without having any idea what
kind of creature originally possessed these
elements
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Evolutionary History
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Complete conodont animals have been
discovered (early 1980s)
 Phosphatized toothlike elements, W-shaped
myomeres, cranium, notochord, and paired eye
and otic capsules, conodonts clearly indicate
they belong to vertebrate clade
 Exact position is unclear
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Evolutionary History
Early Jawed Vertebrates
 Gnathostomes
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Living agnathans (jawless vertebrates), the
lampreys and hagfishes, often called
cyclostomes
Gnathostomes constitute a monophyletic
group
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All living and extinct jawed vertebrates
Presence of jaws is a derived character state
shared by all jawed fishes and tetrapods
Agnathans, defined by the absence of jaws, is
paraphyletic
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Evolutionary History
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Evidence indicates jaws arose by modification
of 1st two cartilaginous gill arches
Both gill arches and jaws form from upper and
lower bars that bend forward and are hinged
Both are derived from neural crest cells rather
than from mesodermal tissue as are most
bones
Jaw musculature is homologous to the
musculature that originally supported gills
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Evolutionary History
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Mandibular arch may have first become
enlarged to assist gill ventilation
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Perhaps to meet increasing metabolic demands of
early vertebrates
Placoderms appeared in the early Devonian
and were heavily armored
Acanthodians are included in a clade that
underwent a great radiation into bony fishes
that dominate waters today
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