Paleozoic Life and Earth History

1
Paleozoic Life and Earth History
Invertebrates
2
Study of Paleozoic Life

• The history of Paleozoic life
• a system of interconnected biologic and geologic
  events
• Evolution and plate tectonics are the forces that
  drove this system
• The opening and closing of ocean basins,
• transgressions and regressions of epeiric seas,
• the formation of mountain ranges,
• and the changing positions of the continents
• these had a profound effect on the evolution of
  the marine and terrestrial communities

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The Cambrian Explosion

• At the beginning of the Paleozoic Era,
• animals with skeletons appeared rather abruptly
  in the fossil record
• In fact, their appearance is described
• as an explosive development of new types of
  animals
• and is referred to as the "Cambrian explosion"

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Sharp Contrast

• The sudden appearance of shelled animals during
  the Early Cambrian
• contrasts sharply with the biota living during
  the Proterozoic Eon
• Up until the evolution of the Ediacaran fauna,
• Earth was populated primarily by single-celled
  organisms
• The Ediacaran fauna,
• which is found on all continents except
  Antarctica,
• consists primarily of multicelled soft-bodied
  organisms

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Triggering Mechanism

• The mechanism that triggered this event is still
  unknown
• it was likely a combination of factors
• both biological and geological
• For example, geologic evidence
• indicates Earth was glaciated one or more times
  during the Proterozoic,
• followed by global warming during the Cambrian
• These global environmental changes may have
  stimulated evolution
• and contributed to the Cambrian explosion

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Why Skeletons

• Along with the question of
• why did animals appear so suddenly in the fossil
  record
• is the equally intriguing one of
• why they initially acquired skeletons
• and what selective advantage this provided
• A variety of explanations
• about why marine organisms evolved skeletons have
  been proposed,
• but none is completely satisfactory or
  universally accepted

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Advantages of an Exoskeleton

• The formation of an exoskeleton
• confers many advantages on an organism
• It provides protection against ultraviolet
  radiation, allowing animals to move into
  shallower waters
• (2) it helps prevent drying out in an intertidal
  environment
• (3) it provides protection against predators
• Recent evidence of actual fossils of predators
  and specimens of damaged prey,
• as well as antipredatory adaptations in some
  animals,
• indicates that the impact of predation during the
  Cambrian was great

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Cambrian Predation

• Anomalocaris

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Evidence of Predation
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Advantages of an Exoskeleton

• With predators playing an important role in the
  Cambrian marine ecosystem,
• any mechanism or feature that protected an animal
  would certainly be advantageous
• and confer an adaptive advantage to the organism
• (4) A fourth advantage is that a supporting
  skeleton
• allows animals to increase their size
• and provides attachment sites for muscles

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Mineralized Skeletons Were Successful

• Whatever the reason,
• the acquisition of a mineralized skeleton was a
  major evolutionary innovation
• allowing invertebrates to successfully occupy a
  wide variety of marine habitats

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Marine Invertebrate Communities

• In order to understand the evolution of the
  marine invertebrate communities through time,
• concentrating on the major features and changes
  that took place
• We need to briefly examine the nature and
  structure of living marine communities
• so that we can make a reasonable interpretation
  of the fossil record

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The Present Marine Ecosystem

• In analyzing the present-day marine ecosystem,
• we must look at where organisms live,
• how they get around,
• as well as how they feed
• Organisms that live in the water column above the
  seafloor are called pelagic
• They can be divided into two main groups
• the floaters, or plankton,
• and the swimmers, or nekton

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Plankton

• Plankton are mostly passive and go where currents
  carry them
• Plant plankton
• such as diatoms, dinoflagellates, and various
  algae,
• are called phytoplankton and are mostly
  microscopic
• Animal plankton are called zooplankton and are
  also mostly microscopic
• Examples of zooplankton include foraminifera,
  radiolarians, and jellyfish

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Nekton

• The nekton are swimmers
• and are mainly vertebrates
• such as fish
• the invertebrate nekton
• include cephalopods

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Benthos

• Organisms that live on or in the seafloor make up
  the benthos
• They can be characterized
• as epifauna (animals) or epiflora (plants),
• for those that live on the seafloor,
• or as infauna,
• which are animals living in and moving through
  the sediments

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Sessile and Mobile

• The benthos can be further divided
• into those organisms that stay in one place,
  called sessile,
• and those that move around on or in the seafloor,
  called mobile

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Marine Ecosystem

• Where and how animals and plants live in the
  marine ecosystem

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Feeding Strategies

• The feeding strategies of organisms
• are also important in terms of their
  relationships
• with other organisms in the marine ecosystem
• There are basically four feeding groups
• suspension-feeding animals remove or consume
  microscopic plants and animals as well as
  dissolved nutrients from the water
• herbivores are plant eaters
• carnivore-scavengers are meat eaters
• and sediment-deposit feeders ingest sediment and
  extract the nutrients from it

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Marine Ecosystem
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Organism's Place

• We can define an organism's place
• in the marine ecosystem
• by where it lives
• and how it eats
• For example, an articulate brachiopod
• is a benthic,
• epifaunal suspension feeder,
• whereas a cephalopod
• is a nektonic carnivore

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Trophic Levels

• An ecosystem includes several trophic levels,
• which are tiers of food production and
  consumption
• within a feeding hierarchy
• The feeding hierarchy
• and hence energy flow in an ecosystem comprise
• a food web of complex interrelationships among
• the producers,
• consumers,
• and decomposers

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Primary Producers

• The primary producers, or autotrophs,
• are those organisms that manufacture their own
  food
• Virtually all marine primary producers are
  phytoplankton
• Feeding on the primary producers
• are the primary consumers, which are mostly
  suspension feeders

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Other Consumers

• Secondary consumers feed on
• the primary consumers,
• and thus are predators, while tertiary consumers,
  which are also predators, feed on the secondary
  consumers
• Besides the producers and consumers,
• there are also transformers and decomposers
• These are bacteria that break down the dead
  organisms
• that have not been consumed
• into organic compounds that are then recycled

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Marine Food Web

• Showing the relationships
• among the
• producers,
• consumers,
• and decomposers

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When the System Changes

• When we look at the marine realm today,
• we see a complex organization of organisms
• interrelated by trophic interactions
• and affected by changes in the physical
  environment
• When one part of the system changes, the whole
  structure changes,
• sometimes almost insignificantly,
• other times catastrophically

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Changing Marine Ecosystem

• As we examine the evolution of the Paleozoic
  marine ecosystem,
• keep in mind how geologic and evolutionary
  changes can have a significant impact on its
  composition and structure

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Changing Marine Ecosystem

• For example, the major transgressions onto the
  craton
• opened up vast areas of shallow seas
• that could be inhabited
• The movement of continents
• affected oceanic circulation patterns
• as well as causing environmental changes

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Cambrian Skeletonized Life

• Although almost all the major invertebrate phyla
  evolved during the Cambrian Period
• many were represented by only a few species

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Cambrian Skeletonized Life

• the organisms that comprised the majority of
  Cambrian skeletonized life were
• trilobites,
• inarticulate brachiopods,
• and archaeocyathids

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Trilobites
Cambrian Permian
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Trilobites

• Trilobites
• were by far the most conspicuous element of the
  Cambrian marine invertebrate community
• and made up about half of the total fauna

• Trilobites were
• benthic
• mobile
• sediment-deposit feeders
• that crawled or swam along the seafloor

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Trilobites

• They first appeared in the Early Cambrian,
• rapidly diversified,
• reached their maximum diversity in the Late
  Cambrian,
• and then suffered mass extinctions near the end
  of the Cambrian
• from which they never fully recovered
• As yet no consensus exists on what caused the
  trilobite extinctions,

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Trilobite Extinctions

• but a combination of factors were likely
  involved,
• including possibly a reduction of shelf space,
• increased competition,
• and a rise in predators
• It has also been suggested
• that a cooling of the seas may have played a
  role,
• particularly for the extinctions that took place
  at the end of the Ordovician Period

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Cambrian Brachiopods

• Cambrian brachiopods
• were mostly primitive types called inarticulates
• They secreted a chitinophosphate shell,
• Inarticulate brachiopods
• also lacked a tooth-and-socket-arrangement along
  the hinge line of their shells
• They were
• benthic
• Epifaunal
• Suspension-feeders

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Archaeocyathids

• The third major group of Cambrian organisms
• were the archaeocyathids
• These organisms
• were benthic sessile suspension feeders
• that constructed reeflike structures

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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
  craton
• This vast epeiric sea,
• which experienced a uniformly warm climate during
  this time,
• opened numerous new marine habitats
• that were soon filled by a variety of organisms

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The Tippecanoe Sequence
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Striking Changes in Ordovician

• the Ordovician was characterized by the adaptive
  radiation of many other animal phyla,
• such as articulate brachiopods,
• bryozoans,
• and corals
• with a consequent dramatic increase in the
  diversity of the total shelly fauna
• and the start of large-scale reef building

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Middle Ordovician Seafloor Fauna

• Recreation of a Middle Ordovician seafloor fauna
  with cephalopods, crinoids, colonial corals,
  trilobites, and brachiopods

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Acritarchs

• The Ordovician was also a time
• of increased diversity and abundance of the
  acritarchs
• organic-walled phytoplankton of unknown affinity
• which were the major phytoplankton group of the
  Paleozoic Era
• and the primary food source of the suspension
  feeders

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Ordovician Reef Builders

• During the Cambrian, archaeocyathids
• were the main builders of reeflike structures,
• but beginning in the Middle Ordovician
• bryozoans, stromatoporoids,
• and tabulate and rugose corals
• assumed that role

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Biostratigraphic Correlation

• Three Ordovician fossil groups have proved to be
  particularly useful for biostratigraphic
  correlation
• the articulate brachiopods,
• graptolites,
• and conodonts
• The articulate 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

• Most graptolites were
• planktonic animals carried about by ocean
  currents
• Because most graptolites were planktonic
• and most individual species existed for less than
  a million years,
• graptolites are excellent guide fossils
• They were especially abundant
• during the Ordovician and Silurian periods

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Graptolites
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Graptolite
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Conodonts

• Conodonts are a group of 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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Conodonts

• The conodont animal
• preserved as a carbonized impression 40 x 2 mm
• in the Lower Carboniferous Granton Shrimp Bed in
  Edinburgh, Scotland

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Conodont
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Mass Extinctions

• The end of the Ordovician was a time of mass
  extinctions in the marine realm
• More than 100 families of marine invertebrates
  became extinct,
• and in North America alone,
• approximately one-half of the brachiopods and
  bryozoans died out
• 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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Mass Extinctions

• Mass extinctions,
• geologically rapid events in which an unusually
  high percentage of the fauna and/or flora becomes
  extinct
• have occurred throughout geologic time
• for instance, at or near the end of the
• Ordovician,
• Devonian,
• Permian,
• and Cretaceous periods
• and are the focus of much research and debate

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Silurian and Devonian Marine Communities

• The mass extinction at the end of the Ordovician
  was followed by rediversification
• and recovery of many of the decimated groups
• Brachiopods, bryozoans, gastropods, bivalves,
  corals, crinoids, and graptolites
• were just some of the groups that rediversified
• beginning during the Silurian

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Silurian and Devonian Reefs

• The Silurian and Devonian reefs
• were dominated by
• tabulate and colonial rugose corals and
  stromatoporoids
• While the fauna of these Silurian and Devonian
  reefs
• was somewhat different from that of earlier reefs
  and reeflike structures,
• the general composition and structure are the
  same as in present-day reefs

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Eurypterids

• The Silurian and Devonian periods
• were also the time when eurypterids
• arthropods with scorpion-like bodies and
  impressive pincers
• were abundant, especially in brackish and
  freshwater habitats

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Ammonoids

• Ammonoids are excellent guide fossils for the
  Devonian through Cretaceous

• short stratigraphic ranges,
• and widespread distribution
• a subclass of the cephalopods,
• evolved from nautiloids during the Early Devonian
  
• and rapidly diversified

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Another Mass Extinction

• Another mass extinction occurred near the end of
  the Devonian
• and resulted in a worldwide near-total collapse
  of the massive reef communities
• On land, however, the seedless vascular plants
• were seemingly unaffected,
• although the diversity of freshwater fish
• was greatly reduced
• Thus, extinctions at this time
• were most extensive in the marine realm,
• particularly in the reef and pelagic communities

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Global Cooling

• The demise of the Middle Paleozoic reef
  communities
• serves to highlight the geographic aspects
• of the Late Devonian 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

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Rapid Recovery

• The brachiopods and ammonoids
• quickly recovered
• and again assumed important ecological roles,
• while other groups, such as the lacy bryozoans
  and crinoids,
• reached their greatest diversity during the
  Carboniferous
• With the decline of stromatoporoids and tabulate
  and rugose corals,
• large organic reefs virtually disappeared
• and were replaced by small patch reefs

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Mississippian Patch Reefs

• These patch reefs were dominated by
• crinoids, blastoids, lacy bryozoans, brachiopods,
  and calcareous algae
• and flourished during the Late Paleozoic
• In addition, bryozoans and crinoids contributed
  large amounts of skeletal debris
• to the formation of the vast bedded limestones
• that constitute the majority of Mississippian
  sedimentary rocks

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The Permian Marine Invertebrate Extinction Event

• The 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 for 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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• Source Gibbs, On the Termination of Species,
  Scientific American Nov. 2001

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Casualties

• Fusulinids, rugose and tabulate corals, several
  bryozoan and brachiopod orders,
• as well as trilobites and blastoids
• did not survive the end of the Permian
• All of these groups had been very successful
  during the Paleozoic Era
• In addition, more than 65 of all amphibians and
  reptiles,
• as well as nearly 33 of insects on land also
  became extinct

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Mass Extinction

• 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,
• (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

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Permian Mass Extinction

• It appears that the Permian mass extinction took
  place over an 8-million-year interval
• which would seemingly rule out a meteorite impact
• although, meteorite impact may have contributed
  to the Permian mass extinction
• The second and third hypotheses can probably be
  eliminated
• because most of the collisions of the continents
  had already taken place by the end of the Permian
  
• and the large-scale formation of glaciers took
  place during the Pennsylvanian Period

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Climatic Changes

• Currently, many scientists think that a
  large-scale marine regression
• coupled with climatic changes in the form of
  global warming
• due to an increase in carbon dioxide levels
• may have been responsible for the Permian mass
  extinctions
• In this scenario, a widespread lowering of sea
  level occurred near the end of the Permian
• thus greatly reducing the amount of shallow shelf
  space for marine organisms
• and exposing the shelf to erosion

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Impact from the Deep

• A new model suggest intense global warming could
  trigger deaths in the sea and on land
• Trouble begins with widespread volcanic activity
  that releases enormous volumes of carbon dioxide
  and methane
• The gases cause rapid global warming
• A warmer ocean absorbs less oxygen from the
  atmosphere
• low oxygen (anoxia) destabilizes the chemocline,
  where oxygenated water meets water permeated with
  hydrogen sulfide (H2S) generated by
  bottom-dwelling anaerobic bacteria
• as H2S concentrations build and oxygen falls, the
  chemocline rises abruptly to the ocean surface

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Impact from the Deep

• green and purple photosynthesizing sulfur
  bacteria, which consume H2S and normally live at
  chemocline depth, now inhabit the H2S-rich
  surface waters while oxygen-breathing ocean life
  suffocates
• H2S also diffuses into the air, killing animals
  and plants on land
• and rising to the troposphere to attack the
  planets ozone layer
• without the ozone shield, the suns ultraviolet
  (UV) radiation kills remaining life

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Burgess Shale Soft-Bodied Fossils

• On August 30 and 31, 1909,
• Charles D. Walcott,
• geologist and head of the Smithsonian
  Institution,
• discovered the first soft-bodied fossils
• from the Burgess Shale (British Columbia, Canada)
  
• a discovery of immense importance in deciphering
  the early history of life
• yielded the impressions of a number of
  soft-bodied organisms
• beautifully preserved on bedding planes
• soft-bodied animals that lived some530 million
  years ago
• present a much more complete picture of a Middle
  Cambrian community
• than deposits containing only fossils of the hard
  parts of organisms

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Sixty Percent Soft-Bodied

• 60 of the total fossil assemblage of more than
  100 genera is composed of soft-bodied animals,
• a percentage comparable to present-day marine
  communities
• The site of deposition of the Burgess Shale
• was located at the base of a steep submarine
  escarpment
• Periodically, this unstable area
• would slump and slide down the escarpment
• as a turbidity current