Historical Geology

 

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.

¨     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 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 well-preserved 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

 

   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.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)

 

 

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?

 

 

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?”

 

IMPORTANT TERMS

benthos	nekton	sediment-deposit feeder
carnivore-scavenger	plankton	suspension feeder
herbivore	primary producer

 

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.    a	6.    c	11.    a
2.    a	7.    e	12.    d
3.    d	8.    e	13.    a
4.    e	9.    c	14.    c
5.    b	10.    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.

 

          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.

 

 

 

 

 

 

 

 

 

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.