R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
CHAPTER 12
PALEOZOIC LIFE HISTORY:
INVERTEBRATES
OUTLINE
INTRODUCTION
WHAT WAS THE CAMBRIAN EXPLOSION?
THE EMERGENCE OF A SHELLY FAUNA
PALEOZOIC INVERTEBRATE MARINE LIFE
The Present Marine Ecosystem
Cambrian Marine Community
PERSPECTIVE: Trilobites—Paleozoic Arthropods
The Burgess Shale Biota
Ordovician Marine Community
Silurian and Devonian Marine Communities
Carboniferous and Permian Marine Communities
MASS EXTINCTIONS
The Permian Mass Extinction
SUMMARY
CHAPTER OBJECTIVES
The following content objectives are presented in Chapter 12:
 Animals with skeletons appeared abruptly at the beginning of the Paleozoic Era and
experienced a short period of rapid evolutionary diversification.
 The present marine ecosystem is a complex organization of organisms that interrelate
and interact not only with each other, but also with the physical environment.
 The Cambrian Period was a time of many evolutionary innovations during which
almost all the major invertebrate phyla evolved.
 The Ordovician Period witnessed striking changes in the marine community, resulting
in a dramatic increase in diversity of the shelly fauna, followed by a mass extinction
at the end of the Ordovician.
 The Silurian and Devonian periods were a time of rediversification and recovery for
many of the invertebrate phyla as well as a time of major reef building.
 Following the Late Devonian extinctions, the marine community again experienced
renewed adaptive radiation and diversification during the Carboniferous and Permian
periods.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
 Mass extinctions occur when anomalously high numbers of species go extinct in a
short period of time. The greatest recorded mass extinction in Earth’s history
occurred at the end of the Permian Period.
LEARNING OBJECTIVES
To exhibit mastery of this chapter, students should be able to demonstrate comprehension
of the following:
 the sudden appearance of animals in the fossil record, known as the Cambrian
explosion
 the importance of the Burgess Shale fauna
 the acquisition and significance of hard parts and the role of predators in the Early
Paleozoic marine community
 the ways in which organisms live, eat, and move, and their relationships with each
other within the marine ecosystem
 the relationships between producers, consumers, transformers, and decomposers in
the marine food web
 changes within the Paleozoic invertebrate marine community during different
geologic periods
 possible causes of mass extinctions
 the affected and relatively unaffected groups, possible causes, and significance of the
Permian marine invertebrate extinction event
CHAPTER SUMMARY
1. 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. Hard parts probably
evolved as a result of various geologic and biologic factors rather than a single
cause.
Figure 12.1 Lower Cambrian Shelly Fossils
Figure 12.2 Cambrian Predation
Table 12.1
The Major Invertebrate Groups and Their Stratigraphic Ranges
Enrichment Topic 1. The Cambrian Explosion
German researchers stated that the tremendous increase in biodiversity during the
Cambrian explosion was actually caused by life itself. Climate modeler Werner von Bloh
believed that weathering of rock by early land plants removed carbon dioxide from the
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
atmosphere, cooling the Earth, and setting the stage for the radiation of life. He stated
that a drop to 30 degrees Celsius resulted in the appearance of higher life forms. The
hypothesis is controversial, because many geologists believe the rock record supports a
warm Cambrian Period. Von Bloh’s model is consistent with the Gaia hypothesis,
however. “Cool Cambrian Triggers Life,” Geotimes, Dec. 2003, v.48 n.12 p.11
2. Marine organisms are classified as plankton if they are floaters, nekton if they
swim, and benthos if they live on or in the seafloor.
Figure 12.3 Marine Ecosystem
3. 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; carnivores, which are meat
eaters; and sediment-deposit feeders which ingest sediment and extract nutrients
from it.
4. The marine ecosystem consists of various trophic levels of food production and
consumption. At the base are primary producers, on which all other organisms are
dependent. Feeding on the primary producers are the primary consumers, which in
turn are fed on by higher levels of consumers. The decomposers are bacteria that
break down the complex organic compounds of dead organisms and recycle them
within the ecosystem.
Figure 12.4 Marine Food Web
5. The Cambrian invertebrate community was dominated by three major groups, the
trilobites, inarticulate brachiopods, and archeocyathids. Little specialization existed
among the invertebrates, and most phyla were represented by only a few species.
The Middle Cambrian Burgess Shale contains one of the finest examples of a wellpreserved soft-bodied biota in the world.
Figure 12.5 Cambrian Marine Community
Figure 12.6 Archaeocyathids
Figure 12.7 The Primitive Echinoderm Helicoplacus
Figure 12.8 Fossils from the Burgess Shale
Enrichment Topic 2. Trilobites
Richard Fortey, a retired trilobite paleontologist of the Natural History Museum in
London, discussed in detail the various types and sizes of trilobites, as well as the
environments in which they lived. Also included in the text are descriptions of Fortey’s
own research and paleontological career, as well as his comments on rivals Simon
Conway Morris and the late Stephen Jay Gould. Trilobite: Eyewitness to Evolution
(2001).
The University of California Museum of Paleontology Berkeley hosts a website with
photographs, descriptions, and links, for a more in-depth look at trilobites.
http://www.ucmp.berkeley.edu/arthropoda/trilobita/trilobita.html
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
6. The Ordovician marine invertebrate community marked the beginning of
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.
Figure 12.9 Middle Ordovician Marine Community
Figure 12.10 Late Ordovician Acritarchs
Figure 12.11 Representative Brachiopods and Graptolites
Figure 12.12 Conodonts and the Conodont Animal
7. The Silurian and Devonian Periods were times of diverse faunas dominated by
reef-building animals. Following the Late Devonian extinctions, the marine
community again experienced an adaptive radiation and diversification during the
Carboniferous and Permian periods.
Figure 12.13 Middle Devonian Marine Reef Community
Figure 12.14 Silurian Brackish Water Community
Figure 12.15 Ammonoid Cephalopod
Figure 12.16 Late Mississippian Marine Community
Figure 12.17 Permian Patch-Reef Marine Community
Figure 12.18 Fusulinids
8. A major extinction occurred at the end of the Paleozoic Era, affecting the
invertebrates as well as the vertebrates. Its cause is still the subject of debate.
Figure 12.19 Phanerozoic Marine Diversity
Enrichment Topic 3. Mass Extinctions
The Permian extinction was the greatest recorded mass extinction of all time. In his
essay, Neil deGrasse Tyson explored the possible causes for extinction events, including
the possibility of periodicity in extinctions and extraterrestrial sources. In addition to the
meteor impact hypotheses, Tyson also explores supernovae explosions, galactic
collisions, and black hole encounters. This essay offers a look at the unusual—and
fortunately improbable—potential causes of extinction events. “Knock ‘Em Dead,”
Natural History, May 2005, v.114 n.4 p.25-28, 70.
Enrichment Topic 4. What Caused the Permian Extinction?
Benton’s When Life Nearly Died: The Greatest Mass Extinction of All Time (2005) offers
an overview of the Permian mass extinction in greater detail.
Although currently not in favor with scientists, the bollide impact has been proposed
many times for the cause of Permian extinction. Sediments from the Permian boundary
have been found to contain microscopic fragments of metals with an extraterrestrial
signature. (“Pieces of a Pulvarizer,” Science News, Nov. 22, 2003, v. 164 p.323) An
impact site in Australia, the Bedout dome, has been proposed, although some scientists
are skeptical. (“Australian Crater Implicated in Global Rubout,” Discover, Jan. 2005,
v.26 n.1 p.40; Wright, “The Day Everything Died,” Discover, April 2005 v.26 n.4 p.6471)
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
In recent years, other data and hypotheses have emerged. Some researchers noted that
the venting of hydrogen sulfide gas may have built up and poisoned land animals. (“Last
Gasp,” Science News, May 28, 2005 v.167 n.22 p.339)
LECTURE SUGGESTIONS
The Burgess Shale
1. Incorporate Stephen J. Gould's Wonderful Life: The Burgess Shale and the Nature
of History for excellent commentaries on science, evolution and extinction. Gould
discussed contingencies in the geologic record, or the "what if” factor. The fauna of
the Burgess Shale contained many species that were evolutionary “dead ends.” If
some of these species had survived and given rise to new species through natural
selection, how might life be different today?
2. Using Gould's book, lead the students in a discussion about the role that serendipity
plays in science. Walcott, the man credited with the great Burgess Shale
discoveries, never really had the opportunity to advance his research because of
other commitments. Would Walcott have interpreted this fauna differently if he had
sufficient time to conduct thorough research? Gould suggested that Walcott was
limited by scientific vision at that time; however in later essays (“This View of
Life”) in Natural History magazine, Gould acknowledged that it is difficult to judge
historical decisions when viewed through a modern lens.
3. How have the Burgess Shale fauna been interpreted since their discovery? The
Burgess Shale organisms have been reinterpreted and revised many times, even
since the publication of Gould’s book. Use the Burgess Shale to illustrate how
science progresses, and continues to modify as new data become available.
Reef Communities
Have students discuss the development of reef communities throughout the Paleozoic
Era. Students can investigate the factors that make reef communities successful, diverse
ecosystems.
1. Why are reef communities important? Where is the greatest diversity on Earth?
In which environments do reef communities exist? Is there a connection between
the two?
2. Students can reconstruct ancient reef communities to show the progression of life
forms within the reefs. Why didn’t archaeocyathids survive into the Silurian?
Why did reef communities proliferate in the Devonian?
3. Have students predict what will happen to reef communities in the Mesozoic Era.
Will reefs survive into the Triassic? How do students know this?
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
The Permian Extinction
To convey the magnitude of the Permian extinction, have students collect and bring to
class bottle caps, shells, paper clips, or marbles. Other small, inexpensive items may also
be substituted.
1. Use one item (shells, etc.) to represent marine invertebrate species, and another item
(bottle caps, etc.) to represent land animals. On a display table, arrange 100 shells to
represent 100 marine invertebrates of the Permian. Similarly arrange 100 bottle
caps to represent 100 Permian land animals.
2. After discussion of the Permian extinction, ask student volunteers to REMOVE 90
of the shells and 65 bottle caps from the display table. The very few shells and
bottle caps remaining represent the organisms that survived the greatest mass
extinction in the geologic record.
3. Ask students to predict what will happen in the next geologic era, the Mesozoic.
How will life respond to relatively few species (when compared to the former
diversity) inhabiting the planet?
4. Students can discuss possible commonalities among the surviving organisms of the
Permian extinction.
CONSIDER THIS
1. This chapter introduces the important concept of extinction, especially as a
clustered event. Given the number of species that have gone extinct throughout
geologic time, how important is it for humans to try to save species that are
currently endangered? Should we be focused on saving only large animals from
extinction, or do the smaller organisms play an important role in an ecosystem?
2. Why have most students heard about the extinction at the end of the Mesozoic,
but fewer have previously learned about the extinction event at the end of the
Paleozoic? Is this related to the organisms that became extinct at the end of each
era?
3. Why are there several hypotheses for the Permian extinction? Is the extinction the
result of one major event, or do you believe that several factors contributed to
“The Great Dying?”
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
IMPORTANT TERMS
benthos
carnivore-scavenger
herbivore
nekton
plankton
primary producer
sediment-deposit feeder
suspension feeder
SUGGESTED MEDIA
Videos
1. Lost Worlds, Vanished Lives: The Rare Glimpses, BBC Video
2. Miracle Planet: Extinction and Rebirth, The Science Channel
3. Miracle Planet: New Frontiers, The Science Channel
4. Triumph of Life: The Four Billion Year War, PBS Home Video
Slides and Demonstration Aids
1. Evolution of Life on Earth, slide set, Educational Images, Ltd.
2. Fossil Collection, Earth Science Educator’s Supply
3. Fossils of the Precambrian and Lower Paleozoic, JLM Visuals
CHAPTER 12 – ANSWERS TO QUESTIONS IN TEXT
Multiple Choice Review Questions
1.
2.
3.
4.
5.
a
a
d
e
b
6.
7.
8.
9.
10.
c
e
e
c
c
11. a
12. d
13. a
14. c
Short Answer Essay Review Questions
15. Major transgressions onto the craton open up vast areas of shallow seas that can be
inhabited by marine organisms, and marine invertebrate life flourishes. Conversely,
the movement of continents over polar regions can facilitate glaciation, and
possibly cause extinctions, particularly among tropical marine communities.
16. Many organisms survived the mass extinctions at the end of the Permian and
Cretaceous and their niches were not left vacant. The Cambrian Earth was
practically a blank slate waiting to be filled with different types of organisms. New
body plans evolved, and animals moved into new niches.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
Additionally, many of the Cambrian organisms had shells, which allowed for better
preservation when compared to the soft-bodied Proterozoic organisms. Therefore,
the difference in Cambrian and Proterozoic organisms is more pronounced in the
fossil record.
17. Shells may have appeared because the change in the chemistry of the oceans
favored the evolution of a mineralized skeleton. In the form of an exoskeleton
(shell) the organisms would be protected from ultraviolet radiation, drying out, and
predators. Organisms with shells may have escaped predation, and therefore
survived to reproduce. Shells also allowed the organisms to grow bigger and attach
muscles. Shells not only provide protection for the organisms, they are also
advantageous to paleontologists because hard parts fossilize much more easily than
soft parts.
18. It is difficult to assess what is not present—or has yet to be discovered—in the
fossil record. The lack of hard parts before the Cambrian explosion means that
there are fewer soft-bodied specimens preserved in the fossil record, not that the
life forms didn’t exist. It is possible that the Cambrian is a recording of the
diversity of life that was already present—but that didn’t fossilize because of lack
of hard parts.
19. The Cambrian marine community was dominated by three groups of organisms:
trilobites, archaeocyathids, and inarticulate brachiopods. The end-of-Cambrian
extinction caused a decrease in the number of trilobites, which never really
recovered in diversity, and the extinction of archaeocyathids. During the
Ordovician, inarticulate brachiopods were replaced (via natural selection) by
articulate forms. The Ordovician fauna was much more diverse, with adaptive
radiation of corals and bryozoans plus other groups.
20. An episode of deep-sea anoxia and increased oceanic CO2 levels resulted in a
highly stratified ocean during the Late Permian. There is also evidence of
increased global warming. This would contribute to a stratified ocean because
warming of high latitudes would reduce or eliminate down-welling of cold, dense,
oxygenated waters from the polar areas. Widespread volcanic and continental
fissure eruptions may have added CO2 into the atmosphere, contributing to climate
instability and ecologic collapse.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
Apply Your Knowledge
1. Student responses will vary, but should be consistent with Figure 12.4, page 246.
2. The Earth’s past can inform the present situation. When biodiversity is severely
impacted, a mass extinction occurs. Several mass extinctions occurred during the
Paleozoic, and greatly reduced biodiversity. These mass extinctions have
common themes: extinctions affected life on both land as well as in the sea, and
tropical organisms were particularly susceptible to extinctions.
Although some extinctions are thought to have been caused by extraterrestrial
impacts (such as the Cretaceous extinction), climate changes are associated with
others. Global warming or cooling events have been associated with extinction
events in the Earth’s geologic past. Following a mass extinction, though, the
geologic record reveals that the surviving organisms rediversify to occupy
available niches.
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