Chapter 12Paleozoic Life History: Invertebrates

Historical Geology

Chapter 12Paleozoic 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 12.1Lower Cambrian Shelly Fossils

Figure 12.2Cambrian Predation

Table 12.1The 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 12.3Marine 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 12.4Marine 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 12.5Cambrian Marine Community

Figure 12.6Archaeocyathids

Figure 12.7The Primitive EchinodermHelicoplacus

Figure 12.8Fossils 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.

What are Trilobites? Click here for a more in-depth look:

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 12.9Middle Ordovician Marine Community

Figure 12.10Late Ordovician Acritarchs

Figure 12.11Representative Brachiopods and Graptolites

Figure 12.12Conodonts 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 12.13Middle Devonian Marine Reef Community

Figure 12.14Silurian Brackish Water Community

Figure 12.15Ammonoid Cephalopod

Figure 12.16Late Mississippian Marine Community

Figure 12.17Permian Patch-Reef Marine Community

Figure 12.18Fusulinids

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 12.19Phanerozoic 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)