bis+2c+lecture+34+dec+3+2012+pm
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Fall 2012
Lecture 34. Chordata
Description of deuterostomes
Brief description of echinoderms
Introduction to Chordates
Students should be able to:Recognize an echinoderm
explain how members of the 3
chordate subphyla differ
describe the position and functionof the notochord
explain how the vertebral columnforms in development and how it
relates to the notochord
explain how jaws evolved and whatevidence supports this idea
Urochordata,Cephalochordata,
Vertebrata
Teach yourself about development: http://bit.ly/dev-bio
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Know the deuterostome features
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Echinodermata
bottom-dwelling marine organisms; predators, grazers, particle
feeders; 7,000 extant species; 13,000 fossil speciescalcareousendoskeleton, composed of plates or ossiclesderived from mesoderm, and covered in epidermis. The skeletal plates
(ossicles) are connected by a form of collagenwhich can bestiff or
flexible!neuronal control of body tonewithout muscle action.
,
Sea lily
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Mouth on oral
(under) side
regions with tube
feet = ambulacra
two stomachs in
asteroids
Hard calciumossicles in the
skin
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Sea stars (Asteroidea) common, largely predatory
Fight: combined tube foot forces versus bivalve adductor muscles
and the role of the fluffy (cardiac) stomach
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Class Echinoidea: sea urchins, heart urchins, sand dollars
( =spine form
) 5 ambulacra
5 inter-ambulacra
aboral view
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Class Holothuroidea: sea cucumbers
( =
sea cucumber form
)
5 ambulacral andinterambulacral
regions externallyvisible in some
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pharyngeal slits
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What are chordates?
Sea squirts, lancelets, jawless fishes, jawed fishes, and
tetrapods (amphibians, reptiles, mammals)
Deuterostomes with
Notochord
Dorsal tubular nerve cord
Post-anal tail
Pharyngeal slits
endostyle/thyroid
at least in embryo
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Chordate features
Four synapomorphies
notochord
pharyngeal slits
Pharyngeal slits
openings between the pharynx and the exterior
associated with filter feeding or respiration
endostyle/thyroid
dorsal hollow nerve post-anal tail
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Why are pharyngeal slits not considered a
synapomorphy of chordates?
a) Because they do not occur in all chordates.b) Because they evolved convergently in
hemichordates and cephalochordates.
c) Because they have been further modifiedwithin the chordates.
d) Because they occur in other deuterostomes.
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Urochordates or tunicates
sea squirts (ascidians) and relatives sea squirts are bag-like, with enlargedperforated pharynx for filter-feeding
Why a chordate?
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Because urochordate larvae show all the chordate characters
1. notochord2. dorsal hollow nerve3. postanal tail4. endostyle/thyroid
Is this an ancestral
urochordate body plan?
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Free-swimming colony above andlarvacean (solitary) below)
sessile
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Vertebrates have colonized a wide variety of environments
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Hagfishes:
scavengers on dead
animal carcasses
no bone, no jaws tongue with rasping teeth can tie themselves in knot
for greater leverage when
tearing food from prey
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produce copious amounts ofslime
have a notochord
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Lamprey: bloodsucking parasite of fish!
no bone, no jaws sucker-like mouth and
rasping teeth
larvae are mud-dwellingfilter feeders
have a notochordsurrounded by
cartilaginous arches.
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explain the position of the notochord explain where the nerve cord lies show a vertebra
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Chordate phylogeny including two extinct groups of fish
21: Cranium2: Vertebrae 3: Bone
4: Jaws 5: Bone lost
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4
5
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How do vertebrae form
developmentally? segmented muscle blocks non-segmented
notochord in
embryo
end ofmuscle
block
secretescalciuminto space
between
blocks
remnants ofnotochord persist
as inter-vertebraldiscs
two muscleblocks contribute
to a single
vertebra
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Ostracoderms: heavily armored, extinct jawless fish
with bony plates in the skin, including around the gills
probably filter feeders and scavengers
radiated in Silurian and Devonian (~400 mya)
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JAWS EVOLVED from bony plates in
the skin that held the gill slits open.
Gill arches:
cartilaginous
gill supports
Anterior gill arches
became modified as
bony jaws
Additional gill
arches added, and
teeth acquired
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Placoderms: armor-plated predators
with jaws and teeth-like structures
first jawed vertebrates
originated in Silurian, extinct by end of Devonian (360 mya)
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Dunkeleosteous: 9 meters long, major predator
Some placoderms were large!
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A note on the evolution of bone
Regarding the positions of vertebrae and bone on thevertebrate cladogram, organisms can have vertebrae madeout of cartilage. This is what occurs in lampreys. Theyare not vertebrae in the sense of the human skeleton, butthey are bits of cartilage that sit above the notochord asneural arches or spines. Lampreys do not have bone.
Professor Wainwright says: Cartilage is not necessarily anontogenetic or evolutionary precursor of bone. It's a versatile,high performance skeletal material in its own right. Ofcourse there are many adult cartilaginous structures, butalso, lots of bones do not develop by replacing cartilage (alldermal bones for example). A number of non-vertebratemetazoans have cartilage and no bone (molluscs are a goodexample).
Professor Wainwright also says that bone first occurred in thedermal skull of ostracoderms, a paraphyletic group, but eventhere it was not in the vertebrae (these were cartilaginous).
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