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Chapter 12
Paleozoic Life History:
Invertebrates
Burgess
Shale
• Diorama of
the
environment
and biota
– of the
Phyllopod
bed of the
Burgess
Shale,
• British
Columbia,
Canada
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,
– a discovery of immense importance in deciphering
the early history of life
• Walcott and his collecting party split open
numerous blocks of shale,
– many of which yielded the impressions
– of a number of soft-bodied organisms
– beautifully preserved on bedding planes
Thousands of Fossil Specimens
• Walcott returned to the site the following
summer
– and located the shale stratum
– that was the source of his fossil-bearing rocks
– in the steep slope above the trail
• He quarried the site
– and shipped back
– thousands of fossil specimens
– to the United States National Museum of Natural
History,
– where he later catalogued and studied them
More Complete Picture of a
Middle Cambrian Community
• The importance of Walcott's discovery
– is that it allowed geologists a rare glimpse into a
world previously almost unknown
– that of the soft-bodied animals that lived some
530 million years ago
• The beautifully preserved fossils
–
–
–
–
from the Burgess Shale
present a much more complete picture
of a Middle Cambrian community
than deposits containing only fossils of the hard
parts of organisms
Sixty Percent Soft-Bodied
• In fact, 60% of the total fossil assemblage
– of more than 100 genera is composed of softbodied animals,
– a percentage comparable to present-day marine
communities
• What conditions led to the remarkable
preservation of the Burgess Shale fauna?
• The site of deposition of the Burgess Shale
– was located at the base of a steep submarine
escarpment
Reason for the Preservation
• The animals
–
–
–
–
whose exquisitely preserved fossil remains
are found in the Burgess Shale
lived in and on mud banks
that formed along the top of this escarpment
• Periodically, this unstable area
– would slump and slide down the escarpment
– as a turbidity current
• At the base, the mud and animals carried with it
– were deposited in a deep-water anaerobic
environment devoid of life
Carbonaceous Impressions
• In such an environment,
– bacterial degradation did not destroy the buried
animals
– and they were compressed by the weight of the
overlying sediments
– and eventually preserved as carbonaceous
impressions
Study of Paleozoic Life
• We will examine the history of Paleozoic life
– as 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
had a profound effect on the evolution
of the marine and terrestrial communities
Tremendous Biologic Change
• A time of tremendous biologic change
– began with the appearance of skeletonized animals
– near the Precambrian-Cambrian boundary
• Following this event, marine invertebrates
– began a period of adaptive radiation and evolution
– during which the Paleozoic marine invertebrate
community greatly diversified
• Indeed, the history of the Paleozoic marine
invertebrate community
– was one of diversification and extinction,
– culminating at the end of the Paleozoic Era
– in the greatest mass extinction in Earth history
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" by most scientists
The Cambrian Explosion
• This sudden and rapid appearance
– of new animals in the fossil record
– is rapid, however, only in the context of geologic
time,
– having taken place over millions of years
– during the Early Cambrian Period
Not a Recent Discovery
• Early geologists observed
– that the remains of skeletonized animals
– appeared rather abruptly in the fossil record
• Charles Darwin addressed this problem
–
–
–
–
–
in On the Origin of Species
and observed that,
without a convincing explanation,
such an event was difficult to reconcile
with his newly expounded evolutionary theory
Sharp Contrast
• The sudden appearance of shelled animals
– during the Early Cambrian
– 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,
• which is found on all continents except Antarctica,
– consists primarily of multicelled soft-bodied
organisms
Soft-Bodied Organisms
• Microscopic calcareous tubes,
• presumably housing worm-like suspension feeding
organisms,
– have also been found at some localities
• In addition, trails and burrows,
• which represent the activities of worms
• and other sluglike animals
– are also found associated
– with Ediacaran faunas throughout the world
• The trails and burrows
– are similar to those made by present-day softbodied organisms
Time Between Fauna
• Until recently, it appeared that
– a fairly long time period existed
– between the extinction of the Ediacaran fauna
– and the evolution of the first Cambrian fossils
• That gap has been considerably narrowed
– in recent years with the discovery
– of new Proterozoic fossiliferous localities
– that continue right to the base of the Cambrian
Hotly Debated Topic
• Nonetheless, the cause of the sudden
appearance
– of so many different animal phyla
– during the Early Cambrian
– is still a hotly debated topic
• Newly developed molecular techniques
–
–
–
–
that allow evolutionary biologists
to compare the similarity of molecular sequences
of the same gene from different species
is being applied to the phylogeny of many
organisms
Early Invertebrate History
• In addition, new fossil sites
– and detailed stratigraphic studies
– are shedding light
– on the early history and ancestry
– of the various invertebrate phyla
Triggering Mechanism
• The Cambrian explosion
– probably had its roots firmly planted in the
Proterozoic
• However, the mechanism
– that triggered this event is still unknown and
– 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
Hypotheses for Shell Development
• These global environmental changes
– may have stimulated evolution
– and contributed to the Cambrian explosion
• A change in ocean chemistry may have favored
– the evolution of a mineralized skeleton
• Skeletonized forms may have been a response
– to the evolution of predators
• Recent work on Hox genes, which are
• sequences of genes that control the development of individual regions
of the body,
–
–
–
–
shows that the basic body plans for all animals
was apparently established
by the end of the Cambrian explosion,
and was only slightly modified since then
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 life history
The Emergence of a Shelly Fauna
• The earliest organisms with hard parts
– are Proterozoic calcareous tubes
– found associated with Ediacaran faunas
– from several locations throughout the world
• These are followed by other microscopic
skeletonized fossils
– from the Early Cambrian
– and the appearance of large skeletonized animals
– during the Cambrian explosion
Lower Cambrian Shelly Fossil
• A conical sclerite* of Lapworthella from
Australia
* a piece of
armor
covering
– This
specimen
is several
millimeters
in size
Lower Cambrian Shelly Fossil
• Archaeooides, an enigmatic spherical fossil
from the Mackenzie Mountains, Northwest
Territories, Canada
– This
specimen
is several
millimeters
in size
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
Advantages of an Exoskeleton
•
The formation of an exoskeleton confers
many advantages on an organism:
(1) It provides protection against ultraviolet
radiation, allowing animals to move into
shallower waters;
(2) it helps prevent drying out in an intertidal
environment
(3) A supporting skeleton allows an increase in size
and provides a place for attachment of muscles;
(4) it provides protection against predators
Advantages of an Exoskeleton
•
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
Cambrian Predator
• Reconstruction of Anamalocaris
– a predator from the Early and Middle Cambrian
– is shown feeding on Opabinia
• It was 45 cm long and probably
– used its gripping appendages to carry food to its
circular mouth
Wounded Trilobite
• Wounds to the body of the trilobite Olenellus
robsonensis
– The wounds have healed, demonstrating that they
occurred when the animal was alive and were not
inflicted on an empty shell
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
It Is Unknown Why Organisms
Evolved Mineralized Skeletons
• There currently is no clear answer about
– why marine organisms evolved mineralized
skeletons
– during the Cambrian explosion and shortly
thereafter
• They undoubtedly evolved
– because of a variety of biologic and environmental
factors
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
Paleozoic Invertebrate
Marine Life
• Having considered the origin, differentiation,
and evolution
–
–
–
–
of the Precambrian-Cambrian marine biota,
we now examine the changes
that occurred in the marine invertebrate community
during the Paleozoic Era
Marine Invertebrate Communities
• Rather than focusing on
– the history of each invertebrate phylum,
– we will survey the evolution
– of the marine invertebrate communities through
time,
– concentrating on the major features and changes
that took place
• To do that, 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
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
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
Nekton
• The nekton are swimmers
– and are mainly vertebrates
• such as fish;
– the invertebrate nekton
• include cephalopods
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
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
Marine Ecosystem
• Where and how animals and plants live in the
marine ecosystem
Plankton:
Sessile epiflora:
Nekton: fish
cephalopod
seaweed
Jelly fish
Sessile epifauna:
Benthos: d-k
bivalve
coral
crinoid
Marine Ecosystem
Infauna:
worm,
bivalve
Mobile epifauna: gastropod, starfish
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
Marine Ecosystem
coral
crinoid
bivalve
Suspension feeders:
Marine Ecosystem
worm
sedimentdeposit feeder
Herbivores: gastropod
Carnivores-scavengers: starfish
An 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 benthonic,
– epifaunal suspension feeder,
• whereas a cephalopod
– is a nektonic carnivore
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
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
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
Marine Food Web
• Showing the
relationships
–
–
–
–
among the
producers,
consumers,
and
decomposers
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
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
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
– and caused environmental changes
Cambrian Marine Community
• The Cambrian Period was a time
– during which many new body plans evolved
– and animals moved into new niches
• As might be expected, the Cambrian
– witnessed a higher percentage of such experiments
– than any other period of geologic history
Cambrian Skeletonized Life
• Although almost all the major invertebrate
phyla
– evolved during the Cambrian Period
– many were represented by only a few species
• While trace fossils are common
– and echinoderms diverse,
• the organisms that comprised the majority of
Cambrian skeletonized life were
– trilobites,
– inarticulate brachiopods,
– and archaeocyathids
Cambrian Marine Community
• Floating jellyfish, swimming arthropods,
benthonic sponges, and scavenging trilobites
Reconstruction
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
–
–
–
–
benthonic
mobile
sediment-deposit feeders
that crawled or swam along the seafloor
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
Trilobite Extinctions
• A combination of factors were likely involved
in the extinctions,
– including a possible 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
Cambrian Brachiopods
• Cambrian brachiopods
– were mostly primitive types called inarticulates
• They secreted a chitinophosphate shell,
– composed of the organic compound chitin
– combined with calcium phosphate
• Inarticulate brachiopods
– also lacked a tooth-and-socket-arrangement
– along the hinge line of their shells
Articulate Brachiopods
• The articulate brachiopods,
–
–
–
–
which have a tooth-and-socket arrangement,
were also present
but did not become abundant
until the Ordovician Period
Archaeocyathids
• The third major group of Cambrian organisms
– were the archaeocyathids
• These organisms
–
–
–
–
–
were benthonic sessile suspension feeders
that constructed reeflike structure
at the beginning of the Cambrian
Archaeocyathids went extinct
at the end of the Cambrian
Cambrian Reeflike Structure
• Restoration of a Cambrian reeflike structure
built by archeocyathids
Other Cambrian Fauna
• The rest of the Cambrian fauna
–
–
–
–
consisted of representatives
of the other major phyla,
including many organisms
that were short-lived evolutionary experiments
Primitive Echinoderm
• Helicoplacus was a
primitive echinoderm
– that became extinct 20
million years after its first
appearance about 510
million years ago
– and was a representative of
one of several short-lived
echinoderm classes
– Such organisms illustrate
the “experimental” nature
of the Cambrian
invertebrate fauna
The Burgess Shale Biota
• No discussion of Cambrian life
– would be complete without mentioning
– one of the best examples
– of a preserved soft bodied fauna and flora,
• the Burgess Shale biota
• As the Sauk Sea transgressed
– from the Cordilleran shelf
– onto the western edge of the craton
• Early Cambrian sands were covered
– by Middle Cambrian black muds
– that allowed a diverse soft-bodied benthic
community to be preserved
Soft-Bodied Animals and Plants
• These fossils were discovered in 1909 by
Charles Walcott
– near Field, British Columbia
• They represent one of the most significant
fossil finds of the 20th century
– because they consist of impressions of soft-bodied
animals and plants
– which are rarely preserved in the fossil record
Rare Preservation: Burgess Shale
• Ottoia, a carnivorous worm
Rare Preservation: Burgess Shale
• Wiwaxia, a scaly
armored sluglike
creature whose
affinities remain
controversial
Rare Preservation: Burgess Shale
• Hallucigenia, a velvet worm
Rare Preservation: Burgess Shale
• Waptia, an anthropod
Rarely Preserved Organisms
• This discovery therefore
–
–
–
–
provides us with a valuable glimpse
of rarely preserved organisms
as well as the soft-part anatomy
of many extinct groups
Reinterpretation
• In recent years, the reconstruction,
classification, and interpretation
–
–
–
–
of many of the Burgess Shale fossils
have undergone a major change
that has led to new theories and explanations
of the Cambrian explosion of life
• Recall that during the Neoproterozoic
multicellular organisms evolved,
– and shortly thereafter animals with hard parts
– made their first appearance
Basic Body Plans
• These were followed by
– an explosion of invertebrate groups
– during the Cambrian,
– many of which are now extinct
• These Cambrian organisms
– represent the rootstock
– and basic body plans
– from which all present-day invertebrates evolved
How Many Phyla?
• The question that paleontologists are still
debating is
– How many phyla arose during the Cambrian?
– At the center of that debate are the Burgess Shale
fossils
• For years, most paleontologists
–
–
–
–
placed the bulk of the Burgess Shale organisms
into existing phyla,
with only a few assigned to phyla
that are now extinct
Cambrian Phyla
• Thus, the phyla of the Cambrian world
– were viewed as being essentially the same in
number
– as the phyla of the present-day world,
– but with fewer species in each phylum
• According to this view, the history of life
– has been simply a gradual increase in the diversity
of species
– within each phylum through time
• The number of basic body plans
– has therefore remained more or less constant
– since the initial radiation of multicelled organisms
Explosion of Varied Lifeforms
• This view, however, has been challenged
– by other paleontologists
– who think that the initial explosion of varied lifeforms in the Cambrian
– was promptly followed by a short period of
experimentation
– and then extinction of many phyla
• The richness and diversity of modern life-forms
– are the result of repeated variations of the basic body
plans
– that survived the Cambrian extinctions
Strangeness of the
Burgess Shale Biota
• In other words, life was much more diverse
– in terms of phyla
– during the Cambrian
– than it is today
• The reason members of the Burgess Shale biota
–
–
–
–
look so strange to us
is that no living organisms
possess their basic body plan,
and therefore many of them have been reassigned
into new phyla
Reassignment to Extant Phyla
• Discoveries of Cambrian fossils
– at localities such as Sirius Passet, Greenland, and
Yunnan, China,
– have resulted in reassignment
– of some Burgess Shale specimens back into extant
phyla
• If these reassignments to known phyla prove to
be correct,
– then no massive extinction event followed the
Cambrian explosion,
– and life has gradually increased in diversity
through time
No Clear Answer
to This Debate
• Currently, there is no clear answer to this
debate,
– and the outcome will probably be decided
– as more fossil discoveries are made
Ordovician Marine Community
• A major transgression that began
– during the Middle Ordovician (Tippecanoe
sequence)
– resulted in 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
Striking Changes in Ordovician
• Both sedimentation patterns and fauna
– underwent striking changes
– from the Cambrian to the Ordovician,
• Whereas the Cambrian invertebrate community
– was dominated by trilobites, inarticulate
brachiopods, and archaeocyathids,
• 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
Middle Ordovician Seafloor Fauna
• Recreation of a Middle Ordovician seafloor
fauna with cephalopods, crinoids, colonial
corals, trilobites, and brachiopods
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
Upper Ordovician Acritarch
• Acritarch from the
Upper Ordovician
Sylvan Shale,
Oklahoma
• Acritarchs are
organic-walled
phytoplankton
– and were the
primary food source
for suspension
feeders during the
Paleozoic Era
Upper Ordovician Acritarch
• Acritarch from the
Upper Ordovician
Sylvan Shale,
Oklahoma
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
• Many of these reefs
–
–
–
–
were small patch reefs similar in size
to those of the Cambrian
but of a different composition,
whereas others were quite large
Suspension Feeders
Dominated Reefs
• As with present-day reefs,
– Ordovician reefs exhibited a high diversity of
organisms
– and were dominated by suspension feeders
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
Brachiopods
Brachiopods
• Brachiopods became a conspicuous element
– of the invertebrate fauna
– during the Ordovician
– and in succeeding Paleozoic periods
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
• Graptolites are most commonly found in black
shales
– preserved as carbonaceous impressions
Graptolites
Conodonts
• Conodonts are a group
– of well-known, small tooth-like fossils
– composed of the mineral apatite
• (calcium phosphate)
– the same mineral that composes bone
• Although conodonts have been known for more
than 150 years,
– their affinity has been the subject of debate
– until the discovery of the conodont animal in 1983
Conodonts
• Conodonts are microscopic tooth-like fossils
• Cahabagnathus sweeti, Copenhagen Formation
– Middle Ordovician, Monitor Range, Nevada
Conodonts
• Conodonts are microscopic toothlike fossils
• Scolopodus, sp., Shingle Limestone,
– Shingle Pass, Nevada
Conodonts
• The conodont animal
– preserved as a carbonized impression 40mm x 2 mm
– in the Lower Carboniferous Granton Shrimp Bed in
Edinburgh, Scotland
Conodont Animal
• Several specimens of carbonized impressions
–
–
–
–
–
of the conodont animal
from Lower Carboniferous rocks of Scotland
reveal that it is a member of a group
of primitive jawless animals
assigned to the phylum Chordata
• Study of the specimens
– indicates that the conodont animal
– was probably an elongate swimming organism
Excellent Guide Fossils
• The wide distribution
– and short stratigraphic range of individual
conodont species
– make them excellent fossils
– for biostratigraphic zonation and correlation
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
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
– during the Silurian
Massive Reef Builders
• Recall that the Silurian and Devonian
– were times of major reef building
• While most of the Silurian radiations of
invertebrates
– represented repopulation of niches,
• organic reef builders diversified in new ways,
– building massive reefs
– larger than any produced
– during the Cambrian or Ordovician
Repopulation
• This repopulation
–
–
–
–
–
–
was probably caused in part to renewed
transgressions over the craton,
and although a major drop in sea level
occurred at the end of the Silurian,
the Middle Paleozoic sea level
was generally high
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 reef-like structures,
the general composition and structure
are the same as in present-day reefs
Middle Devonian Reef
• Reconstruction of a Middle Devonian reef
from the Great Lakes area
– with corals, cephalopods, trilobites, crinoids,
and brachiopods
Eurypterids and Ammonoids
• 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
• Ammonoids,
– a subclass of the cephalopods,
– evolved from nautiloids
– during the Early Devonian and rapidly diversified
Silurian Brackish Water Community
• Restoration of a Silurian brackish
water scene
– near Buffalo New York
– with algae, eurypterids,
gastropods, worms,
and shrimp
Ammonoids
• Ammonoids are excellent guide fossils
–
–
–
–
for the Devonian through Cretaceous periods
with their distinctive suture patterns,
short stratigraphic ranges,
and widespread distribution
Ammonoid Cephalopod
• A late Devonian ammonoid cephalopod
– from Erfoud,
Morocco
– The distinctive suture
pattern, short
stratigraphic range,
and wide geographic
distribution make
ammonoids excellent
guide fossils
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,
• Thus, extinctions at this time
– were most extensive among marine life,
– particularly in the reef and pelagic communities
Global Cooling
• The demise of the Middle Paleozoic reef
communities
– highlights the geographic aspects
– of the Late Devonian mass extinction
• The tropical groups were most severely
affected;
– in contrast, the higher latitude communities were
seemingly little affected
• Apparently, an episode of global cooling
– was largely responsible for the extinctions
– near the end of the Devonian
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
Carboniferous and Permian
Marine Communities
• The Carboniferous invertebrate marine
community
–
–
–
–
–
responded to the Late Devonian extinctions
in much the same way as
the Silurian invertebrate marine community
responded to the Late Ordovician extinctions
that is, by renewed adaptive radiation and
rediversification
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
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
Mississippian Marine Life
• Based on a fossil site in the Upper
Mississippian at Crawfordville, Indiana
• Invertebrate animals shown include
– blastoids
– crinoids
– lacy bryozoans
– brachiopods
– small corals
Restricted Permian
Marine Faunas
• The Permian invertebrate marine faunas
–
–
–
–
–
resembled Carboniferous faunas,
but were not as widely distributed
because of the restricted size of the shallow seas
on the cratons and the reduced shelf space
along the continental margins
Permian Period
• Paleogeography
of North
America during
the Permian
Period
Productids
• The spiny and odd-shaped productids
–
–
–
–
dominated the brachiopod assemblage
and constituted an important part
of the reef complexes
that formed in the Texas region during the Permian
Permian Patch-Reef Community
• From Glass Mountains of
West Texas
– including algae, productid
brachiopods, cephalopods,
sponges, and corals
Fusulinids
• The fusulinids
• spindle-shaped foraminifera
–
–
–
–
which first evolved during the Late Mississippian
and greatly diversified during the Pennsylvanian,
experienced a further diversification
during the Permian
Fusulinid
• Fusulinids are spindleshaped, benthonic
foraminifera that are
excellent guide fossils
for the Pennsylvanian
and Permian periods
Fusulinids Are Important
Guide Fossils
• Because of their
– abundance, diversity, and worldwide occurrence,
– fusulinids are important guide fossils
– for Pennsylvanian and Permian strata
• Bryozoans, sponges, and some types of
calcareous algae
– also were common elements of the Permian
invertebrate fauna
Mass Extinctions
• Throughout geologic history
– various plant and animal species have become
extinct
• Extinction is a common feature of the fossil
record,
– and the rate of extinction through time has
fluctuated only slightly.
– Just as new species evolve,
– others become extinct.
Mass Extinctions
• There have been however,
– brief intervals in the geologic past
• during which mass extinctions have eliminated
– large numbers of species.
• Extinctions of this magnitude
– could only occur due to radical changes
– in the environment
– on a regional or global scale
Common Themes
• When we examine different mass extinctions
– of the geologic past
• several common themes stand out
• Mass extinctions typically have affected
– life in both sea and on land
• Tropical organisms
– particularly in the marine realm
– are apparently more affected
• Some animal groups repeatedly experience
– mass extinctions.
Mass Extinctions
• We when examine mass extinctions
– for the past 650 million years,
• we see the first extinction event
– involved only acritarchs.
• Several extinction events
– occurred during the Cambrian
– and only affected marine invertebrates
• particularly trilobites.
Paleozoic Mass Extinctions
• Three other marine mass extinctions took place
during the Paleozoic Era:
– End of the Ordovician
• involving many invertebrates
– Near the end of the Devonian
• affecting major barrier reef-building organisms
• and primitive armored fish
– End of the Permian
• the most severe
• about 90% of all marine invertebrate species and
• more than 65% of all land animals became extinct
Mesozoic Mass Extinctions
• The Mesozoic Era experienced several mass
extinctions
– The most devastating occurring
– at the end of the Cretaceous,
– when almost all large animals
• dinosaurs, flying reptiles, seagoing animals
– became extinct.
• Many scientists think the terminal Cretaceous mass
extinction
– was caused by a meteorite impact
Cenozoic Mass Extinctions
• Several mass extinctions occurred during the
Cenozoic Era:
– The most severe was at the end of the Eocene
Epoch
– and is correlated with global cooling
– and climate change.
• The most recent extinction event
– occurred near the end of the Pleistocene Epoch.
Gradual Events
• Although many scientists think of marine mass extinctions
– as sudden events from a geologic perspective
• they were gradual from a human perspective,
– occurring over hundreds of thousands
– and even millions of years
• Furthermore, many geologists think
– that climate changes, rather than a single catastrophic event
– were primarily responsible
– particularly at the marine realm
• Evidence of glacial episodes
– and other signs of climate change
– such as global warming
• have been correlated with extinction events
– in the fossil record
Permian Mass Extinction
• The greatest recorded mass extinction
– to affect Earth
– occurred at the end of the Permian Period.
• Roughly 50% of all marine vertebrates families
– and about 90% of all marine invertebrate species
– became extinct
Phanerozoic Diversity
• Diversity for marine invertebrate and vertebrate
families
– 3 episodes of
Paleozoic
mass
extinctions
are visible
– with the
greatest
occurring at
the end of
the Permian
Period
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
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
• Because the extinction event extended over
– millions of years,
– a meteorite impact can be reasonably discounted.
Permian Mass Extinction
• A reduction in shelf space due to the formation of
Pangaea,
– and a widespread marine regression
– can also be rejected
• By the end of the Permian, most continental collisions
– had already taken place
– before the mass extinctions began in earnest.
• The widespread glaciation that took place in the
Carboniferous
– was already waning in the Permian
Anoxic Waters
• Currently, many scientists think
–
–
–
–
that an episode of deep-sea anoxia
and increased CO2 levels
resulted in a highly stratified ocean
during the Late Permian.
• There was little circulation
– of oxygen-rich surface waters
– into the deep ocean.
– Stagnant waters also covered the shelf region.
• There is also evidence
– of increased global warming
– during the Late Permian,
– which would contribute to a stratified global ocean
Carbon Dioxide
• During this time, widespread volcanic eruptions
–
–
–
–
–
and continental fissure eruptions
also took place,
further releasing additional carbon dioxide
into the atmosphere
and contributing to increased climatic instability and
ecological collapse
• By the end of the Permian,
– a near collapse of both the marine and terrestrial ecosystem
had occurred.
• Probably an combination of interconnected and related
geologic and biologic events
– was responsible
Summary
• Multicelled organisms presumably
– had a long Precambrian history during which
they lacked hard parts
• Invertebrates with hard parts
– suddenly appeared during the Early Cambrian
– in what is called the Cambrian explosion
• Skeletons provided such advantages
–
–
–
–
as protection against predators
and support for muscles,
enabling organisms to grow large
and increase locomotor efficiency
Summary
• Hard parts probably evolved
– as a result of various geologic factors
– rather than a single cause
• Marine organisms are classified
– as plankton
• if they are floaters,
– nekton
• if they swim,
– and benthos
• if they live on or in the seafloor
Summary
• Marine organisms can be divided
– into four basic feeding groups:
– suspension feeders,
• which consume microscopic plants and animals as
well as dissolved nutrients from water;
– herbivores, which are plant eaters;
– Carnivore-scavengers, which are meat eaters;
– and sediment-deposit feeders,
• which ingest sediment and extract nutrients from it
Summary
• The marine ecosystem consists of various
trophic levels
– of food production and consumption
• At the base are primary producers,
– upon which all other organisms are dependent
• The primary consumers
– feed on the primary producers
• higher level consumers
– can feed upon primary producers
• The decomposers are bacteria
– that break down the complex organic
compounds
– of dead organisms
– and recycle them within the ecosystem
Summary
• The Cambrian invertebrate community
– was dominated by three major groups,
• the trilobites,
• inarticulate brachiopods,
• and archaeocyathids
• Little specialization existed among the
invertebrates,
– and the most phyla were represented
– by only a few species
Summary
• The Middle Cambrian Burgess Shale
– contains one of the finest examples
– of a well-preserved soft-bodied biota in the world
• The Ordovician marine invertebrate
community
– marked the beginning of the dominance
– by the shelly fauna
– and the start of large-scale reef building
• The end of the Ordovician Period
– was a time of major extinctions
– for many invertebrate phyla
Summary
• The Silurian and Devonian periods
– were times of diverse faunas
– dominated by reef-building animals,
• while the Carboniferous and Permian periods
– saw a great decline in invertebrate diversity
• Mass extinctions are times
– when anomalously high numbers of organisms
– go extinct in a short period of time.
• Such events have occurred several times
– during the past 650 million years
Summary
• A major extinction occurred at the end of
the Paleozoic Era,
– affecting the invertebrates as well as the
vertebrates
• Its cause is still being debated