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Chapter 12Life of the Paleozoic

Paleozoic overview

Paleozoic Fossil Record

In Paleozoic rocks, we find abundant fossils of multicellular organisms bearing shells.

The fossil record is much improved at the beginning (the base of) Paleozoic strata.

Paleozoic Invertebrates

• Representatives of most major invertebrate phyla were present during Paleozoic, including sponges, corals, bryozoans, brachiopods, molluscs, arthropods, and echinoderms.

• Almost all of the common invertebrate phyla in existence today had appeared by Ordovician.

Paleozoic Vertebrates• Vertebrates evolved during Paleozoic,

including:

– Fishes– Amphibians– Reptiles– Synapsids ("mammal-like reptiles")

• The first vertebrates were jawless fishes, which are found in rocks as old as Cambrian in China.

Paleozoic Vertebrates

• An advanced lineage of fishes with primitive lungs and stout fins gave rise to the four-legged animals or tetrapods.

• The transition from water-dwelling vertebrates to land-dwelling vertebrates depended on the evolution of the amniotic egg.

Paleozoic Plants

• The first primitive land plants appeared near the end of Ordovician.

• Vascular plants expanded across the land, forming great forests during Devonian.

• The plants progressed from seedless, spore-bearing plants to plants with seeds but no flowers (gymnosperms).

Paleozoic Extinctions

• Several mass extinctions occurred during Paleozoic, including the largest extinction of all at the end of Permian.

• Other mass extinctions occurred at the end of Ordovician and Devonian.

Paleozoic Life

Summary of invertebrate phyla

Paleozoic life includes some Precambrian forms, which survived into Paleozoic, as well as more advanced forms:

– Unicellular eukaryotes– Animals

• Invertebrates• Vertebrates

– Plants

Adaptive Radiations and Extinctions

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

• Many geologic periods began with adaptive radiations (times of rapid evolution).

• Several periods ended with extinction events of varying severity.

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

Diversity during Paleozoic

Red arrows mark extinction events

Soft-bodied Animals

• Multicellular animals evolved during Precambrian. Soft-bodied Ediacaran-type organisms ranged into Cambrian.

• Soft-bodied fossils are infrequently preserved.

• Preservation improved with the origin of hard parts.

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

• Small shelly fossils are found at the base of Cambrian, and during Late Neoproterozoic.

• Most disappeared by at the top of the first stage within Lower Cambrian.

Small Shelly Fossils• Many had phosphatic shells, few mm in size. • Shells and skeletal remains of primitive molluscs,

sponges, and animals of uncertain classification, such as Cloudina, that secreted a calcareous tube.

Cambrian Diversification

• The initial Paleozoic diversification is known as "the Cambrian explosion." Abrupt appearance of many types of animals about 535 million years ago, followed by rapid evolution.

• During that episode of explosive evolution, all major invertebrate phyla appeared in the fossil record (except Bryozoa).

Cambrian Substrate Revolution

• Infaunal, burrowing animals evolved rapidly during 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."

Soft-Bodied Fossils in 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 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.

Soft-Bodied Fossils in the Burgess Shale

The significance of the Burgess Shale is that is 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 Paleozoic.

Animals in the Burgess Shale 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 others

Stratigraphic setting of the Cambrian Burgess Shale

• Location of the Burgess Shale fauna in British Columbia, Canada

• C = Onycophoran, Aysheaia, intermediate in evolution between segmented worms and arthropods.D = Arthropod LeanchoilaE= Arthropod Waptia

Pikaia - One of the Oldest Chordates

• Pikaia is a fish-like lower chordate from the Burgess Shale.

• Modern representatives are called lancelets, such as the genus Amphioxus.

Chordates• Chordates have a notochord or dorsal stiffening rod

associated with a nerve chord, at some stage in their development.

• In vertebrates, the notochord is surrounded by and usually replaced by a vertebral column during embryonic development.

• Vertebrates are chordates, but Pikaia pre-dates the evolution of vertebrae.

• It is thought that vertebrates evolved from organisms similar to Pikaia.

Predators in the Cambrian Seas• 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.

Other Burgess Shale Animals• Marrella, a "lace crab," is

common in the Burgess Shale.

• Hallucigenia, an onycophoran, was originally interpreted to walk on its spines, until claws were discovered on its "tentacles."

The Significance of the Burgess Shale

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 Paleozoic.

Exceptional Preservation

• Fossil sites containing abundant fossils with extraordinary preservation are called lagerstätten.

• Both the Burgess Shale fauna and the Chengjiang fauna from China are considered to be lagerstätten.

The Chengjiang fauna

• In 1984, the Lower Cambrian 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 Chengjiang fauna • Jelly fish• Annelid worms• Cnidaria • Porifera (sponges) • Brachiopods • Arthropods• Early chordates similar to Pikaia • The world's oldest known fish (Myllokunmingia) • Other species of unknown phyla

Oldest Known Fish

The world's oldest known fish, Myllokunmingia, from the Maotianshan Shale near Chengjiang, in the Yunnan Province of China.

535 million years old.

Ordovician Diversity • Following a slight dip in diversity at the end of

Cambrian, Ordovician seas experienced renewed diversification.

• Global diversity tripled.• The number of genera increased rapidly, and the

number of families increased from about 160 to 530. • The increase was particularly notable among trilobites,

brachiopods, bivalve molluscs, gastropods, and corals.

Late Ordovician Extinction

• An extinction event at the end of 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 and extinction during Paleozoic

Red arrows mark extinction events

Silurian Diversity

Diversification of marine animals occurred again at the beginning of Silurian.

The period ended with only a slight drop in diversity.

Devonian Diversity

• During Devonian, there was continued diversification, but this ended with another fairly large extinction event, which extended over about 20 million years.

• Roughly 70% of marine invertebrates disappeared.

• Because of the long duration, the extinction is unlikely to have been caused by a sudden, catastrophic event.

Carboniferous-Permian Diversity

• During Early Carboniferous, diversity once again increased.

• Diversity of marine animals remained fairly constant throughout Carboniferous and Permian.

• 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

1. Several Paleozoic periods ended with extinction events

2. The beginning of most Paleozoic periods were marked by adaptive radiations

3. Maximum diversity in Paleozoic seas was maintained roughly constant at between 1000 and 1500 genera

4. The largest extinction occurred at the end of Permian

5. Recovery of diversity during Mesozoic was slow

6. Diversity increased rapidly during Cretaceous

7. Another mass extinction occurred at the end of Cretaceous

8. Diversity increased extremely rapidly, at unprecedented rates, at the beginning of Cenozoic

9. Diversity during Cretaceous and Cenozoic was much greater than during Paleozoic

Diversity during Paleozoic

Red arrows mark extinction events

Unicellular Organisms in the Paleozoic Seas

• The principal groups of Paleozoic unicellular animals with a significant fossils record are the foraminifera and the radiolaria, which belong to Phylum Sarcodina.

• These organisms are unicellular eukaryotic organisms, and belong to Kingdom Protista.

Foraminifera • Name: Foraminifera means "hole bearer." • Chief characteristics:

– Unicellular. – Related to the amoeba, with pseudopods. – Foraminifera build tiny shells (called tests) which

grow by adding chambers. – Some species (called agglutinated foraminifera)

construct tests of tiny particles of sediement. This is the most primitive test.

– Other forams construct tests of calcium carbonate.

Foraminifera

• Geologic range: Cambrian to Holocene.

• Modes of life: – Benthic or benthonic (bottom dwellers) – Planktic or planktonic (floaters).

Fusulinid foraminifera (fusulinids)

• Fusulinids were abundant during Late Paleozoic (primarily Pennsylvanian and Permian).

• Their tests were similar in size and shape to a grain of rice.

• Their internal structure is complex and used to distinguish different species.

• Important guide fossils during Pennsylvanian and Permian because they evolved rapidly, were abundant, and widespread geographically.

Radiolaria • Chief characteristics:

– Unicellular. – Test or shell composed of opaline silica– Ornate lattice-like skeleton – Often spherical or radially symmetrical with spines

• Geologic range: Precambrian or Cambrian to Holocene. Rare during Early Paleozoic. More abundant during Mesozoic and Cenozoic.

• Mode of life: Planktonic. Marine only.

Radiolaria and the Rock Record

• Radiolarians are important constituents of chert at certain times in geologic history.

• Their tests accumulate on the seafloor today to form radiolarian ooze, particularly in deep water, where any calcium carbonate shells would be dissolved.

Marine Invertebrates in the Paleozoic Seas

• The fossils of shell-bearing invertebrates that inhabited shallow seas are common in Paleozoic rocks.

• Archaeocyathids, sponges, corals, bryozoans, trilobites, molluscs, and echinoderms.

• Many were benthic (bottom dwellers), but others, such as graptolites, were planktonic. Currents carried them over wide areas.

• As a result, they are useful index fossils for global stratigraphic correlation.

Phylum Archaeocyatha • Name means "ancient cup" • Chief characteristics: Conical or

vase-shaped skeletons made of calcium carbonate. Double-walled structure with partitions and pores.

• Geologic range: Cambrian only. Extinct.

• Mode of life: Attached to the sea floor. Reef-builders.

Phylum Porifera - The Sponges Name means "pore-bearing." Covered by tiny

pores.

Phylum Porifera - The Sponges Chief characteristics:

– Globular, cylindrical, conical or irregular shape. – Basic structure is vase-like with pores and canals. – Interior may be hollow or filled with branching canals. – Solitary or colonial. – Skeletal elements are called spicules, and they may

be separate or joined. – Composition may be calcareous, siliceous or organic

material called spongin.

Phylum Cnidaria• Corals, sea fans, jellyfish, and

sea anemones. • Name: Cnidaria are named for

stinging cells called cnidoblasts or cnidocytes.

• Many are soft-bodied but only those which form hard skeletal structures are readily preservable as fossils.

Phylum Cnidaria

• Geologic range: Late Precambrian (Proterozoic) to Holocene for the phylum.

• The first corals were the tabulates.

• Mode of life: Corals live attached to the sea floor, primarily in warm, shallow marine environments.

Phylum Cnidaria – Chief Characteristics

1. Radial symmetry2. Mouth at the center of a ring of tentacles.

Phylum Cnidaria – Chief Characteristics

3. Body form may be polyp (attached to the bottom, with tentacles on top) or medusa (free-swimming, jellyfish).

Chief Characteristics of Corals

1. May be solitary or colonial. Colonies are composed of many polyps living together.

2. Hard calcareous skeleton. The skeletal parts formed by polyps are called corallites.

3. The "cup," in which an individual coral polyp sits, is called the theca. Each theca is small, and roughly circular or hexagonal.

4. The theca is divided internally by vertical partitions called septae, arranged in a radial pattern.

Chief Characteristics of Corals

5. Types of corals are distinguished by presence or absence, and number of septae: – Rugose corals (or tetracorals) have

septae arranged in multiples of four. – Tabulate corals lack septae. – Mesozoic and Cenozoic scleractinian

corals (or hexacorals) have septae arranged in multiples of six.

Rugose Corals• Most rugose corals are solitary

and conical (shaped like ice cream cones).

• Septae are visible in the circular opening of the cone.

• Some rugose corals are colonial, having hexagonal corallites with septae (such as Hexagonaria from Devonian of Michigan).

Rugose Corals

• Geologic range: Ordovician to Permian - all extinct.

• Rugose corals were abundant during Devonian and Carboniferous, but became extinct during Late Permian.

Tabulate Corals• Tabulate corals are

colonial and resemble honeycombs or wasp nests.

• They lack septae.• They have horizontal

plates within the theca called tabulae. Tabulae are one of the main features of the tabulate corals.

Tabulate Corals• Geologic range: Ordovician

to Permian - all extinct.• The principal Silurian reef

formers.• They declined after Silurian

and their reef-building role was assumed by the rugose corals.

Modern Corals

• Modern corals are scleractinian corals. Scleractinian corals have septae are arranged in multiples of six, and are sometimes called hexacorals.

• Scleractinian corals did not appear until after Paleozoic

• Geologic range: Triassic to Holocene.