Lecture22

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Lecture 22. Evolution of Multicellularity,
Colonization of Land.
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reading: Chapter 5
Vendian Period 650-543 Ma
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before 600 Ma
fossils are rare, simple
mostly trace fossils
worm-like creatures moving across sediment surfaces
radially-symmetric impressions of animals
575-544 Ma
wider diversity of fossils
radially symmetric fossils and
bilaterally symmetric fossil (bilaterians)
Biological interpretation controversial.
Several distinct body plans, many don’t exist today.
Vendian=Ediacaran
Diverse algae, simple animals.
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Vendian Fauna
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Cyclomedusa
- most common and widespread Vendian fossil
- few mm to m in diameter
- bottom-dwelling, like a sea anemone
Cnidaria
stinging cells
sea anemones, jellies, sea pens
radial symmetry
simple body cavity
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Charnia
- one of the largest Vendian fossils
- up to a m in length
- disk shaped holdfast attaching to the bottom
- similar to “sea pens”
Vendian Fauna, cont.
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Eoporpita
- thick tentacles surrounding central body
- 6 cm across
- cnidarian? (jellyfish)
Nemiana
- simplest of all Vendian fossils
- sac-like body
- never found alone, always in colonies
- alga or sea anemone?
Vendian Fauna, cont.
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Pteridinium
- not known what it once was
- usually found squished flat
- bottom-dweller
Echinoderms
sea urchins, sea stars
exoskeleton with plates
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Arkarua
- small disk-like fossil
- echinoderm?
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Vendian Fauna, cont.
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Spriggina
- soft-bodied, only found in the Precambrian
- 3 cm long
- annelid worm or arthropod
Arthropodsegmented body
appendages on segments
molting exoskeleton
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Dickinsonia
- annelid worm or a cnidarian?
Annelid worm
earthworms, leaches
complex body cavity
Vendian Fauna, cont.
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Tribrachidium
- bizarre, disk-shaped fossil
- three-part symmetry
- may be cnidarian or echinoderm
Molluscshells
body cavity, muscular foot
mantle and gills
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Kimberella
- tough shell that stuck up out of the
sediment covering the organism
- box jellyfish or mollusc?
Duoshantuo Formation 570 Ma
Sponge
sessile, tube shaped
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no true tissues
Exceptionally preserved fossils in phosphate minerals.
Preserves soft-tissue fossils, cellular structures.
Nuclei and cytoplasmic contents (organelles) preserved!
Well-preserved algae/seaweed.
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Sponges most primitive metazoans multicellular animals with differentiated tissues
few occurrences in Precambrian
Preserved animal embryos bilaterian animals, although
no adults are seen!
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Terminal Proterozoic
Small shelly fossils
unknown what they were
Microdictryon
Cloudina
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Cambrian 543-490 Ma
First 10-12 Ma:
- few animal taxa
- some Vendian organisms survived
- diverse sponges, rare cnidarians
- small skeletal fossils
- complex tracks, trails, & burrows (more complex behaviors)
After that:
- abundant crown group animals
- diversification of crown group animals
- wide expansion of mineralized skeletons
- arthropods the most abundant fossils
- > 80 skeletal taxa
- some with 3-fold symmetry
- mollusc spiral shells, arthropods, bivalve shells of brachiopods
Burgess Shale
Yoho National Park, Canada
40 Ma after start of Cambrian.
Exceptional preservation of soft-bodied organisms.
Wide diversity of fossil invertebrates.
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Anomalocaris
large >60 cm
arthropod-like predator
Burgess Shale, cont.
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Marrella
small arthropod similar to a trilobite
one of the most common fossils
Trilobites
several species
some soft appendages preserved
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Burgess Shale, cont.
Vauxia
sponge with branches
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Tuzoia
bivalve crustacean
similar to brine shrimp
Wiwaxia
not sure what it is
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Burgess Shale, cont.
Ottoia
common worm
carnivorous
burrowed
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Hallucigenia
one side has tentacles one has spines
tentacles have claws at the end, so they
were probably the “feet”
don’t know which end is the head, which the tail
Snowball Earth Episodes
Early evolution of animals coincides with global glaciations.
Possibly 3 glaciation events.
Duoshantuo animal embryos and sponges at 570 Ma.
What Factors Contributed to the Cambrian Explosion?
1. Increasing oxygen concentrations in the Neoproterozoic
- limits size due to energy yields
- larger organisms need more oxygen
- early animals limited by diffusion
- animals need at least 50% PAL O2
2. Decreased temperatures from cold T’s
- global glaciations 800-550 Ma
- disruption in biosphere
- carbon isotopes disrupted
3. Origination of body armor
- arms race of predatory relationships
4. Origination of developmental pathways
- metazoans have modular body plans
5. Abundant food sources
- microbial mats and stromatolites
Land Plants Are Related to Green Plants
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hornworts
liverworts
mosses
vascular plants
Chara
Coleochaetae
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Colonization of Land
Need to obtain traits
- to deal with lack of water
- to deal with gravity
- how to obtain nutrients
- new reproductive strategies
Traits Needed For Animals to Colonize Land
First land vertebrates retained a tail fin, suggesting they spent
most of their time in water.
First land animals are amphibians.
A strong support system
- bones, vertebrae
Locomotion
- limbs (adapt fins to legs)
Able to breathe air
- get rid of gills, lungs evolved from the swim bladder
Prevent drying out
- need tear glands to wet eyes
- thick skin, scales, keratin coating to prevent drying out
Mechanism for hearing
- fish have hair cells that sense vibration
- evolution of the ear (fluid filled channel with hair cells)
Lay eggs on dry land ~310 Ma
- amniotic egg
Traits Needed For Plant to Colonize Land
Waxy cuticles - keeps plant from drying out
Vascular tissue - transports water to tissues
Woody tissue - supports the structure against gravity
Stomata - allow for gas exchange, minimizing drying out
Pollen, seeds, flowers, fruit - new reproductive strategies,
wind pollination (then insect, animal pollination)
Early land plants (liverworts, hornworts, mosses)
-water is a critical part of the life cycle
-no cuticle, vascular tissue, woody tissue
-reproductive cells swim
-can’t grow tall, restricted to wet environments
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Lecture 23. Mass Extinctions, K-T boundary,
Buckyballs.
reading: Chapter 5
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