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Lecture 2

Origins of VertebratesChapter 2, KK or MH

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Chordate Characteristics Vertebrates are phylum Chordata, subphylum Vertebrata.

Phylum Chordata can be defined as those animals with anotochord.

Subphylum Vertebrata are those chordates with a brain

(cephalization) surrounded by a cranium.

Fig. 2.4 KK, Other

chordate

characteristics:

- hollow dorsalnerve cord

- gill slits in pharynx

- complete gut

- post-anal tail

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Vertebrate Characteristics

bilaterally symmetry, cephalization

cranium surrounding brain hollow dorsal nerve cord above notochord, with anterior brain

adjacent to 3 special sense organs

anterior mouth, complete gut, post-anal tailgill slits in the

anterior part of gut or pharynx coelom (around gut, heart, and lungs if present)

segmented (metamerism)

large size

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Vertebrate Origins The origin of vertebrates, and their relationship to other animal

taxa, is poorly known.

This is because vertebrate ancestors were small, softorganisms that do not fossilize well and lived long ago (>500mybp). Good fossils are rare. On the other hand, we know

vertebrate evolution comparatively well.

Calendar analogy. If the earth is 4.6 billion years old, and weexpress the timing of events on a calendar year:

the first vertebrates arose in late November (500 million years ago)

land animals in early December

mammals in mid-December

our genus (Homo) about 8 pm on New Years Eve.

Homo sapiens appears during the last hour before midnight

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The Geologic Time Scale

KK Fig. 1.31

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Life on Earth 4.6 billion years

Vertebrates appear in the fossil record about 500 MYBPearlyPaleozoic Era (Cambrian Period)

Era Period MYBP first

appearance

prominent

Group

Cenozoic Neogene

Paleogene

23

66

Homo

mammals

Mesozoic Cretaceous

Jurassic

Triassic

146

200

251

mammals,birds

reptiles

Paleozoic Permian

Carboniferous

Devonian

SilurianOrdovician

Cambrian

291

359

416

444488

542

reptiles

amphibians

fish with jaws

agnathans

amphibians

fish with jaws

Pre-Cambrian chordates

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The 3 subphyla of the phylum Chordatawere all present in the Cambrian period

Urochordata

Cephalochordata

Vertebrata

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Subphylum

CephalochordataAmphioxus

Nature 453: 957 (June 19, 2008)

TheAmphioxus genome.

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A fossil chordate from

the Cambrian period,

western Canada.

Vertebrate Characteristicsshared by Amphioxus

1.notochord under hollowdorsal nerve chord

2.complete gut and post-analtail

3.midgut cecum (homologueof liver?)

4.perforated pharynx5.segmentation

6.coelomate

KK Figure 2.16, MH Fig. 2.2

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Features of Amphioxus that are not

vertebrate-like

no cranium (or other skeleton)

no special sense organs and associated brain

no kidneys

no heart

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Subphylum - Urochordata

(tunicates or sea squirts)KK Fig. 2.24

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Why are tunicates classified as Chordata?

Because their planktonic larval stage has a notochord.

(KK Fig. 2.23, H&G Fig. 2.1)

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More distant relatives: DeuterostomesAnimals can be categorized as deuterostomes or protostomes based on their

early embryonic development. Chordates are among the deuterostomes.

KK. Fig. 2.3

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Three characteristics of Deuterostomes:- radial (as opposed to spiral) cleavage

- indeterminate cleavage (as opposed to determinate)

- the blastopore is at the posterior, and becomes the anus

rather than the mouth

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Phylum Hemichordata (acorn worms)

KK Fig. 2.8

Hemichordates are detuterostomes and

used to be considered Chordata, but the

stomochord is no longer considered to be

homologous with the notochord of

chordates. They do have a perforated

pharynx.

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Phylum Echinodermata

Sea Urchin

Starfish

Crinoid (sea lily)

The largest phylum of deuterostomes other than

chordates.

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KK Fig. 2.1 Chordata is a smallphylum compared toArthropoda, and the other

deuterostome phyla are

even smaller

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Possible Origins of Vertebrates

One school of thought has held that

vertebrate ancestors were motile filter-

feeders from which several moresedentary filter feeding groups evolved,

like echinoderms, hemichordates,

cephalochordates and tunicates

evolved.

KK 2.32

KK 2.31

Another view emphasizes the evolution of larval

forms, and suggests that a cephalochordate-like

ancestor of vertebrates arose from tunicate-like

ancestor by paedomorphosis.

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The Origin of Vertebrates(According to Kardong)

New molecular studies

point to Urochordates as

being the closest living

relatives of vertebrates,

not cephalochordates.

KK 2.33