Historical Geology
Chapter 12 Paleozoic Life History: Invertebrates
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.
 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.
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 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
Historical Geology
Chapter 12 Paleozoic Life History: Invertebrates
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 well-preserved softbodied 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
What are Trilobites? Click here for a more in-depth look: http://www.trilobites.info/trilobite.htm
Historical Geology
Chapter 12 Paleozoic Life History: Invertebrates
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 reefbuilding 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.64-71)
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)