Burgess Shale

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Burgess Shale
Paleozoic Life Invertebrates
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Cambrian Explosion (Revolution?)

• Beginning of the Paleozoic Era
• animals with skeletons appeared abruptly in the
  fossil record
• Sudden and rapid appearance of new animals in the
  fossil record
• rapid, however, only in the context of geologic
  time
• millions of years during the Early Cambrian
  Period

Archaeooides, an enigmatic spherical Cambrian
fossil from the Mackenzie Mountains, Northwest
Territories, Canada specimen is several
millimeters in size
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Sharp Contrast

• The sudden appearance of shelled animals
• contrasts sharply with the biota living during
  the preceding Proterozoic Eon
• Up until the evolution of the Ediacaran fauna
• Earth was populated primarily by single-celled
  organisms
• The Ediacaran fauna
• consists primarily of
• multicelled soft-
• bodied organisms

• Restoration of the Ediacaran Environment

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Major Event in Earth's History

• Whatever the ultimate cause of the Cambrian
  explosion,
• the appearance of a skeletonized fauna and the
  rapid diversification of that fauna during the
  Early Cambrian was a major event in Earth's
  history

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Why Skeletons

• (1) protection against ultraviolet radiation,
  allowing animals to move into shallower waters
• (2) prevent drying out in an intertidal
  environment
• (3) protection against predators
• recent evidence of actual fossils of predators
  and specimens of damaged prey
• indicates that the impact of predation during the
  Cambrian was great

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Cambrian Predator

• Reconstruction of Anamalocaris
• a predator from the Early and Middle Cambrian
• about 45 cm long and probably fed on trilobites
• gripping appendages presumably carried food to
  its mouth

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Wounded Trilobite

• Wounds to the body of the trilobite Olenellus
  robsonensis
• wounds have healed, demonstrating that they
  occurred when the animal was alive and were not
  inflicted on an empty shell

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Cambrian Marine Community

• New body plans evolved
• Higher percentage of such experiments than any
  other period of geologic history
• Almost all the major invertebrate phyla evolved
  during the Cambrian Period
• many were represented by only a few species
• Majority of Cambrian skeletonized life
• trilobites
• brachiopods
• archaeocyathids

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Cambrian Marine Community

• Floating jellyfish, swimming arthropods,
  benthonic sponges, and scavenging trilobites

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Trilobites

• Most conspicuous element of the Cambrian marine
  invertebrate community
• about half of the total fauna
• benthonic
• mobile
• sediment-deposit feeders
• Appeared in the Early Cambrian and rapidly
  diversified
• reached their maximum diversity in the Late
  Cambrian
• mass extinctions near the end of the Cambrian,
  never fully recovered

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Trilobite Extinctions

• No consensus on what caused the trilobite
  extinctions
• reduction of shelf space How?
• increased competition
• rise in predators
• A cooling of the seas may have played a role
• particularly for the extinctions that took place
  at the end of the Ordovician Period

http//www.nmsu.edu/geology/zuhl/trilobite.jpg
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Cambrian Brachiopods

• Cambrian brachiopods
• not abundant until the Ordovician Period

http//www.palaeontology.geo.uu.se/Mainpages/Brach
iopoda/brach Ordovician Brachiopod
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Archaeocyathids

• Archaeocyathids, an extinct group of sponges
• benthonic sessile suspension feeders
• constructed reeflike structures
• The rest of the Cambrian fauna consisted of
  representatives of the other major phyla,
• including many organisms that were short-lived
  evolutionary experiments

http//www.carleton.ca/tpatters/teaching/intro/ca
mbrian/cambrianex16.html
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Cambrian Reeflike Structure

• Restoration of a Cambrian reeflike structure
  built by archeocyathids

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Burgess Shale Soft-Bodied Fossils

• In 1909, Charles D. Walcott
• geologist and head of the Smithsonian Institution
  
• discovered the first soft-bodied fossils from the
  Burgess Shale
• a discovery of immense importance in deciphering
  the early history of life
• Walcott and his collecting party split open
  numerous blocks of shale
• yielding the impressions of a number of
  soft-bodied organisms
• beautifully preserved on bedding planes

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Burgess Shale

• Presented a much more complete picture of a
  Middle Cambrian community
• What conditions led to the remarkable
  preservation of the Burgess Shale fauna?

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Reason for the Preservation

• Animals preserved in the Burgess Shale
• lived in and on mud banks
• that formed along the top of a steep submarine
  escarpment
• Periodically, this unstable area would slump
• slide down the escarpment as a turbidity current
• mud and animals carried with it were deposited in
  a deep-water anaerobic environment

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Rare Preservation Burgess Shale

• Ottoia, a carnivorous worm

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Rare Preservation Burgess Shale

• Wiwaxia, a scaly armored sluglike creature whose
  affinities remain controversial

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Rare Preservation Burgess Shale

• Hallucigenia, a velvet worm

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Rare Preservation Burgess Shale

• Waptia, an anthropod

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Strangeness of the Burgess Biota

• The reason members of the Burgess Shale biota
  look so strange to us
• no living organisms possess their basic body plan
  
• therefore many of them have been placed into new
  phyla

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Ordovician Marine Community

• A major transgression that began during the
  Middle Ordovician (Tippecanoe sequence)
• resulted in the most widespread inundation of the
  N. A. craton
• This vast epeiric sea, which experienced a
  uniformly warm climate during this time
• opened numerous new marine habitats
• soon filled by a variety of organisms

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Middle Ordovician Seafloor Fauna

• Cephalopods, crinoids, colonial corals,
  trilobites, and brachiopods

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Brachiopods

• Brachiopods
• present since the Cambrian
• began a period of major diversification in the
  shallow-water marine environment during the
  Ordovician

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Graptolites

• Excellent guide fossils
• especially abundant
• most graptolites were planktonic
• most individual species existed for less than a
  million years
• Due to the fragile nature of their organic
  skeleton
• most commonly found in black shales

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Conodonts

• well-known small toothlike fossils
• composed of the mineral apatite
• (calcium phosphate)
• the same mineral that composes bone

Although conodonts have been known for more than
130 years, their affinity has been the subject
of debate until the discovery of the conodont
animal in 1983!
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Mass Extinctions

• End of the Ordovician a time of mass extinctions
  in the marine realm
• gt 100 families of invertebrates became extinct
• What caused such an event?
• many geologists think these extinctions were the
  result of the extensive glaciation that occurred
  in Gondwana at the end of the Ordovician Period

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Silurian and Devonian Marine Communities

• The mass extinction at the end of the Ordovician
• was followed by rediversification
• recovery of many of the decimated groups
• brachiopods, bryozoans, gastropods, bivalves,
  corals, crinoids, and graptolites

Lots of massive reef builders
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Middle Devonian Reef

• corals, cephalopods, trilobites, crinoids, and
  brachiopods

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Ammonoids

• Excellent guide fossils !
• Devonian through Cretaceous
• distinctive suture patterns
• short stratigraphic ranges
• widespread distribution

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Another Mass Extinction

• Another mass extinction occurred near the end of
  the Devonian
• worldwide near-total collapse of the massive reef
  communities
• most extensive in the marine realm

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Another Mass Extinction

• The tropical groups were most severely affected
• in contrast, the polar communities were seemingly
  little affected
• Apparently, an episode of global cooling
• was largely responsible for the extinctions near
  the end of the Devonian
• WHY?

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Actors in Extinctions

• During such a cooling, the disappearance of
  tropical conditions
• would have had a severe effect on reef and other
  warm-water organisms
• Cool-water species, on the other hand, could have
  simply migrated toward the equator
• The closing of the Iapetus Ocean and the orogenic
  events of the Late Devonian
• undoubtedly also played a role in these
  extinctions
• by reducing the area of shallow shelf
  environments where many marine invertebrates lived

Remember the connection between the tectonic and
faunal changes!
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Carboniferous and Permian Marine Communities

• The Carboniferous invertebrate marine community
  (Mississipian and Pennsylvanian)
• responded to the Late Devonian extinctions by
  renewed adaptive radiation and rediversification

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Mississippian Marine Life

• blastoids
• lacy bryozoans

• crinoids

• brachiopods

• small corals

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Permian Period

• Paleogeography of North America during the
  Permian Period

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Permian Patch-Reef Community

• From Glass Mountains of West Texas
• algae, productid brachiopods, cephalopods,
  sponges, and corals

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Early Permian

• Shallow sea still covered western United States
  and SW Texas
• Organic reef complex flourished at edge of 300 m
  deep basin
• El Capitan massive reef core

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Permian Reefs and Basins

• Location of the west Texas Permian basins and
  surrounding reefs

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Guadalupe Mountains
www.rozylowicz.com/retirement/guadalupe/guadalupe.
html
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Reef Cross-section
Cross-section from shallow to deep water deposits
www.nps.gov/gumo/gumo/geology.html
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Reef Cross-section
www.science.ubc.ca/eoswr/slidesets/guad/slidefile
s/guadc4.html
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El Capitan
Massive reef at edge of basin
www.rozylowicz.com/retirement/guadalupe/guadalupe.
html
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El Capitan
Massive reef at edge of basin
www.desertusa.com/guad/
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Reef Complex
www.science.ubc.ca/eoswr/slidesets/guad/slidefile
s/guadc6.html
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Permian Basin

• Prolific oil producer
• Ancient basins near reef source of organic
  material
• Deformation during Late Paleozoic provided
  trapping structures

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The Permian Marine Invertebrate Extinction Event

• Greatest recorded mass-extinction event
• occurred at the end of the Permian Period
• Before the Permian ended
• roughly 50 of all marine invertebrate families
  and about 90 of all marine invertebrate species
  became extinct

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Phanerozoic Diversity

• Diversity of marine invertebrate and vertebrate
  families

• 3 episodes of Paleozoic mass extinction are
  visible
• with the greatest occurring at the end of the
  Permian Period

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Permian Mass Extinction

• What caused such a crisis for both marine and
  land-dwelling organisms?
• Various hypotheses have been proposed,
• but no completely satisfactory answer has yet
  been found
• Some scenarios put forth to explain the
  extinctions include
• (1) a meteorite impact such as occurred at the
  end of the Cretaceous Period
• (2) a widespread marine regression resulting from
  glacial conditions

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Permian Mass Extinction

• (3) a reduction in shelf space due to the
  formation of Pangaea
• (4) climatic changes
• (5) oceanographic changes such as anoxia,
  salinity changes, and turnover of deep-ocean
  waters
• It appears that the Permian mass extinction
• took place over an 8-million-year interval at the
  end of the Permian Period
• which would seemingly rule out a meteorite
  impact see reading for essay three for more
  discussion of this

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Biota Dramatically Changed

• Regardless of the ultimate cause of the Permian
  mass extinctions,
• the fact is that Earth's biota was dramatically
  changed
• Triassic marine faunas were of low diversity
• but the surviving species tended to be abundant
• and widely distributed around the world