Tuesday November 27, 2012 (The Phanerozoic Eon: The Paleozoic Era - Life Explodes)
Outline 14: Paleozoic Life - West Virginia Universitypages.geo.wvu.edu/~kammer/g3/Outline14.pdf ·...
Transcript of Outline 14: Paleozoic Life - West Virginia Universitypages.geo.wvu.edu/~kammer/g3/Outline14.pdf ·...
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Outline 14: Paleozoic Life
Radiation of the Animal Phyla
Cambrian Life
• The first animals evolved about 100 my before the start of the Cambrian. These are the Ediacaran fossils of the latest Proterozoic.
• None of these animals had hard parts.
• Base of the Cambrian defined by first animals with hard parts.
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Life at the end of the Proterozoic
Cambrian Life
• Early Cambrian fossils consist mostly of trilobites, brachiopods, archaeocyathids, and small little shells.
Cambrian trilobites cruising on Saturday night
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Typical Cambrian trilobites
Modern horseshoe crabs look similar to trilobites, but they are not closely related. Example of a “living fossil.” Trilobites are extinct.
A living Inarticulate Brachiopod. Very common fossils in the Cambrian.
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Modern Inarticulate Brachiopods in their burrows
Modern Inarticulate Brachiopods for dinner in southeastern Asia.
Cambrian Archaeocyathids:
Reef-Forming Animals
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Examples of small shelly fossils from the Early Cambrian. Scale bars are 0.1 mm.
Cambrian Life
• The Middle Cambrian Burgess Shale records the “Explosion of Life.” All known phyla had appeared by then.
• A phylum is a major body plan. Examples: Mollusca, Annelida, Arthropoda, Chordata, etc.
Kevin Peterson, Dartmouth
Animals got their start in the Ediacaran, followed by the Cambrian “Explosion of Life.”
Sponges
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The Cambrian Explosion made the cover of TIME.
Burgess Shale Fossils
• Most are soft-bodied fossils, a very rare kind of fossilization.
• Of today’s 32 living phyla, 15 are found in the Burgess Shale. The other 17 are microscopic or too delicate to be preserved.
• Another 10 extinct phyla are also found in the Burgess Shale.
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Burgess Shale Fossils
• Assume that all 32 living phyla were alive in the Middle Cambrian.
• Add the 10 extinct phyla for a total of 42 phyla. That’s more phyla than today!
• Thus, Cambrian phyla were more diverse than today.
A Paradox
• There were more body plans (phyla) near the start of animal life than today.
• However, there were many fewer species.
• This doesn’t match the expectation of slow evolutionary diversification of life.
The Pattern of Animal Evolution
• Initial radiation of phyla.
• Reduction by natural selection.
• No new phyla since the Cambrian.
• Diversification within remaining phyla.
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A Hypothesis
• The genome of early animals was less rigid, not as “hardwired” as later animals. Adaptive mutations were more possible.
• A wide variety of body plans were produced by mutations.
• Natural selection eliminated some of these body plans.
A Hypothesis
• Body plans that survived became the modern phyla.
• 500 m.y. of evolution has made genomes more rigid and more species rich.
• Mutations required to make a new body plan would be lethal. Phyla were locked in.
The Burgess Shale of British Columbia, Canada: record of the Cambrian Explosion
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Mt. Stephen in Yoho National Park, Canada
Geologists at the Burgess Shale quarry
Trilobites!
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Paleontologist collecting a slab of fossils
Trilobites with preserved legs and antennae
The strange animals of the Middle Cambrian Burgess Shale
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Opabinia and Amwiskia, representatives of two extinct phyla
Opabinia
The first sea scorpion on the attack!
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Marella, extinct class of arthropods
Marella, extinct class of arthropods
Marella as Cambrian road kill
(or a squished bug?)
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Yohoia, an extinct class of arthropods
Specimens of lobopods
Living and fossil lobopods
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Burgess Shale worm Ottoia
A spiny “worm,” Wiwaxia
Hallucigenia, a spiny lobopod
Which way is up?
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Hallucigenia
Original Interpretation.
Correct Interpretation
Anomalocaris, the largest predator of the Cambrian and an extinct phylum.
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Trilobite with a bite mark, possibly from Anomalocaris
Anomalocaris in hot pursuit of Marella
Pikaia
Pikaia, an early chordate
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Pikaia, a chordate from the Burgess Shale
Yunnanozoan, a chordate from the early Cambrian of China
Primitive chordates: Tunicates or Sea Squirts. Adults have a pharynx with gill slits. Larval forms are free-swimming and have a notochord. Fish are thought to have evolved from the larval form by precocious sexual maturation.
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Chordate evolution
Branchiostoma, the lancelet; a primitive living chordate
Invertebrates after the Cambrian
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Phylum Cnidaria: colonial corals
Phylum Cnidaria: horn coral
Skeleton of a modern coral
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A living sea anemone, relative of corals
Living coral reefs
Living coral reefs
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Phylum Bryozoa - fossils
Phylum Bryozoa – living animals
Phylum Brachiopoda
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Phylum Mollusca
BIVALVIA
Mollusca: Class Bivalvia
Fossil marine bivalve, Kansas
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Phylum Mollusca: Class Gastropoda
Phylum Mollusca: Class Cephalopoda
Nautilus
Nautilus
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A Paleozoic Cephaplopod
Phylum Arthropoda
An Ordovician Trilobite
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A Silurian Trilobite
The Devonian Trilobite Phacops rana
The compound eye of Phacops rana
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A death assemblage of Phacops rana
Eurypterid or “Sea Scorpian”, Silurian of New
York
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A Cenozoic crab
Phylum Echinodermata
Crinoid Blastoid
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A living crinoid at a depth of 692 m, Bahamas
Slab of Mississippian crinoids – note the long stems for feeding high above the substrate
AsteroidOphiuroid
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Starfish feeding on bivalves
Devonian starfish
Echinoids:
sand dollar (left)
sea biscuit (below)
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Holothurian: sea cucumber