2 - Origins
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Transcript of 2 - Origins
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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