The Earth Through Time, 8e

Eighth Edition
by Harold L. Levin

Chapter 12 - page 2

Life of the Paleozoic

Diversity in the Paleozoic

The Paleozoic was a time of several adaptive radiations and extinctions.

Almost all of the geologic periods of the Paleozoic began with adaptive radiations, or times of rapid evolution of organisms. Several of the Paleozoic periods ended with extinction events of varying severity.

The extinction event at the end of the Permian Period was the worst mass extinction in the history of life.

Diversity graph
Diversity of marine animals, and extinction events over geologic time.

Origin of Hard Parts
Animals with Shells Proliferate, and So Does Preservation

Multicellular animals evolved in the Precambrian, as seen in the Neoproterozoic Ediacaran fauna.
Soft-bodied Ediacaran-type organisms ranged into the Cambrian period.

Soft-bodied fossils are infrequently preserved.

Preservation improved with the origin of hard parts.

The first animals with shells are called the small shelly fossils.

Geologic time scale across the Precambrian-Cambrian boundary
Geologic time scale across the Precambrian-Cambrian boundary showing the positions of the Ediacaran, Chengjiang, and Burgess Shale faunas.

Small Shelly Fossils

The small shelly fossils are found at the base of the Cambrian, but are also found in the late Neoproterozoic. Most disappeared by the end of the first stage of the Lower Cambrian.

Few mm in size. Many had phosphatic shells. Shells and skeletal remains of primitive molluscs, sponges, and animals of uncertain classification, such as Cloudina, that secreted a calcareous tube.

Late Precambrian and Early Cambrian shell-bearing fossils from Siberia
Late Precambrian and Early Cambrian shell-bearing fossils from Siberia.

Cloudina, an organism with a small (few cm long), tubular shell of calcium carbonate is interpreted as a tube-dwelling worm. Found in the Precambrian of Namibia, Africa, it is the earliest known organism with a CaCO3 shell.

Cambrian Diversity

The initial Paleozoic diversification is known as "the Cambrian explosion", referring to the abrupt appearance of many types of animals about 535 million years ago, and the speed with which they radiated or evolved.

During that episode of explosive evolution, all of the major invertebrate phyla (except the Bryozoa) appeared in the fossil record. Sometimes this event is referred to as "evolution's big bang."

Infaunal, burrowing animals evolved rapidly during the Cambrian, as indicated by trace fossils and bioturbation of sediments. The dramatic change in the character of the seafloor sediments (from undisturbed to highly burrowed) has been called the "Cambrian substrate revolution."

Can you find the Cambrian explosion on the diversity graph, above?

The significance of the Burgess Shale

The extraordinarily well-preserved Middle Cambrian Burgess Shale fauna of Canada provides a window into the past to view the spectacular diversity of the Middle Cambrian. Many soft-bodied organisms are preserved in black shale, along with the soft parts of animals with shells, such as legs and gills of trilobites.

Animals in the Burgess Shale include:

  1. Several groups of arthropods, including trilobites and crustaceans
  2. Sponges
  3. Onycophorans
  4. Crinoids
  5. Molluscs
  6. Corals
  7. Three phyla of worms
  8. Chordates (Pikaia)
  9. Many other species, some of which cannot be placed into known phyla

           Stratigraphic setting of the Cambrian Burgess Shale
           Stratigraphic setting of the Cambrian Burgess Shale.

           Location of the Burgess Shale fauna in British Columbia, Canada
B = Location of the Burgess Shale fauna in British Columbia, Canada, along with examples of some of the organisms with soft parts preserved.
C = An onycophoran, Aysheaia, which appears to be intermediate in evolution between segmented worms and arthropods.
D = Arthropod Leanchoila
E= Arthropod Waptia

           Pikaia, one of the oldest known Chordates.

Pikaia is one of the oldest chordates. Chordates are animals that have a notochord or dorsal stiffening rod associated with a nerve chord, at some stage in their development. Vertebrates are chordates, but this chordate pre-dates the evolution of vertebrae. In vertebrates, the notochord is surrounded by and usually replaced by a vertebral column during embryonic development. It is thought that vertebrates evolved from organisms similar to Pikaia.

Pikaia is a fish-like lower chordate. Modern representatives are called lancelets, such as the genus Amphioxus.

Fossils related to Pikaia, but somewhat older (535 my old) have been found at the Chengjiang fossil site in Yunnan Province, China.

           The giant predator of the Cambrian seas, Anomalocaris, up to 60 cm long.

Predators would have caused selective pressures on prey. The need to avoid being eaten probably encouraged the evolution of hard protective shells. Predation probably also caused an increase in diversity of prey, as they evolved to better survive predation.

Hallucigenia, an onycophoran, which was originally interpreted to walk on the spines as legs, but was later reinterpreted to walk on the "tentacles", after claws were discovered on some of them.

Marrella, one of the most common arthropods in the Burgess Shale. Also called "lace crabs."

The significance of the Burgess Shale is that it records soft-bodied organisms, and the soft parts of organisms with shells. The finely detailed preservation reveals the extraordinary diversity and evolutionary complexity that existed near the beginning of the Paleozoic.

Fossil sites containing abundant fossils with extraordinary preservation are called lagerstatten. Both the Burgess Shale fauna and the Chengjiang fauna are considered to be lagerstatten.

The Chengjiang fauna

In 1984, the Lower Cambrian (535 m.y. old) Chengjiang fossil site was discovered in Yunnan Province, China. More than 100 species of invertebrates have been found, with extraordinary preservation, including many soft bodied forms. The fauna includes:

  1. Jelly fish, with detailed preservation of tentacles, muscles and radial canals
  2. Annelid worms, with segmentation, digestive organs, and other features preserved
  3. Cnidaria
  4. Porifera (sponges)
  5. Brachoipoda
  6. Arthropoda
  7. Early chordates similar to Pikaia (Cathaymyrus and Yunnanozoon)
  8. The world's oldest known fish (Myllokunmingia)
  9. Many other species, some of which cannot be placed into known phyla

oldest fish
The world's oldest known fish (Myllokunmingia)

Ordovician Diversity

Following a slight dip in diversity at the end of the Cambrian, the Ordovician was a time of renewed diversification.
Global diversity TRIPLED.
The number of genera increased rapidly, and the number of families increased from about 160 to 530. This increase was particularly dramatic among trilobites, brachiopods, bivalve molluscs, gastropods, and corals.

An extinction event at the end of the Ordovician led to an abrupt decline in diversity. This extinction event was apparently related to the growth of glaciers in Gondwana, coupled with a reduction in shallow water habitat associated with the lowering of sea level.

Diversity graph
Diversity of marine animals, and extinction events over geologic time.

Silurian Diversity

Diversification of marine animals occurred again at the beginning of the Silurian Period, and the period ended with only a slight drop in diversity.

Devonian Diversity

The Devonian saw continued diversification, but ended with another fairly large extinction event, which extended over about 20 million years. At this time, roughly 70% of the marine invertebrates disappeared. Because of the long duration, the extinction is unlikely to have been caused by a sudden, catastrophic event.

Carboniferous-Permian Diversity

In the early Carboniferous, diversity once again increased. Diversity of marine animals remained fairly constant throughout the Carboniferous and Permian.

The Late Permian is marked by a catastrophic extinction event which resulted in the total disappearance of many animal groups.

Overview of Changes in Diversity Through Time

Some general trends seen in the marine diversity graph above:

  1. Most of the periods of the Paleozoic ended with extinction events
  2. The beginning of most of the periods of Paleozoic were marked by adaptive radiations
  3. Maximum diversity in the Paleozoic seas maintained roughly constant at between 1000 and 1500 genera, according to the graph above
  4. The largest extinction occurred at the end of the Permian
  5. Recovery of diversity in the Mesozoic was slow (Triassic and Jurassic)
  6. Diversity increased rapidly in the Cretaceous
  7. Another mass extinction occurred at the end of the Cretaceous
  8. Diversity increased with extremely rapidly, at unprecedented rates, at the beginning of the Cenozoic
  9. Diversity in the Cretaceous, and Cenozoic Era was much greater than during the Paleozoic

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Document created by: Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA

December 21-22, 2005