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

13

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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