Download pdf - Ch. 13 Early Paleozoicfaculty.chas.uni.edu/~groves/EHCh13lecturept01.pdfCh. 13—Early Paleozoic Cambrian and Ordovician systems • Phanerozoic Eon spans geologic time from end of

Earth History, Ch. 13 1

Ch. 13—Early Paleozoic Cambrian and Ordovician systems

• Phanerozoic Eon spans geologic time from end of Proterozoic Eon up to the present

– Cenozoic Era (65 Ma—present)

– Mesozoic Era (250 Ma—65 Ma)

– Paleozoic Era (543 Ma—250 Ma)

• Cambrian and Ordovician systems comprise the Early Paleozoic

Cambrian

System

Ordovician

System

Early

Early

Middle

Middle

Late

Late

543

495

443


Early Cambrian life

Cambrian

Period

• Conodonts (primitive chordates) appeared in earliest Cambrian time


Conodonts


• Middle part of Early Cambrian is called “Tommotian Stage”

• Tommotian fauna is the earliest diverse “shelly” fauna

– Did shells evolve in response to the earlier evolution of teeth?

– Variety of animals that cannot be related to anything in post-Tommotian time

– First sponges, brachiopods

– First reefs: built by archaeocyathids

Cambrian

Period


Tommotian fauna

Primitive mollusk

Fossils of

unknown biologic

affinity


Tommotian fauna

Archaeocyathid—A calcareous animal

that first appeared in the Tommotian, then

built the earliest true reefs somewhat later

in Early Cambrian time


Cambrian and Ordovician Life

• “Cambrian explosion”: Early and Middle Cambrian appearance of most phyla of invertebrates

• Ordovician was a major time of evolutionary radiation; also appearance of graptolites, rugose corals, tabulate corals, stromatoporoids, land plants (?)

• By late Ordovician, complex marine communities had become established


spore plant cells

Primitive land plants are known from Silurian rocks, but so far only spores and sheets of cells have been

recovered from the Late Ordovician


Ordovician

radiation

Cambrian

explosion


The Cambrian Explosion

The rapid diversification of animals


Geologic time • Age of Earth is 4.6 billion years

• If time were distance and 4.6 billion years

equalled 46 feet, then

– 100 million years = 1 ft

– 1 million years = 0.01 ft

4.6 Ga

today


• 540 Ma (40.6 feet up the ladder)

– earliest evidence of predators

• 570 Ma (40.3 feet up the ladder)

– earliest evidence of metazoans

• 2.1 Ga (25 feet up the ladder)

– fossil eukaryotes


– fossil prokaryotes and stromatolites


– carbon isotope evidence of photosynthesis

Precambrian life


The “Cambrian Explosion”


“Cambrian Explosion”

• Chordates and most phyla of invertebrates originated between 520 Ma and 510 Ma

– 10 My = ~1” on the ladder of time

• Best faunas are Chengjiang (China) and Burgess Shale (British Columbia)

Burgess

ShaleChengjiang

bacteria X X

algae X X

sponges X X

cnidarians X X

ctenophores X X

brachiopods X X

mollusks X absent

hyoliths X X

priapulids X X

annelids X X

lobopods X X

arthropods X X

echinoderms X absent

hemichordates X X

chordates X X


The Burgess Shale

Walcott Quarry:

Protected as a

UNESCO “World

Heritage Site”


Cambrian paleogeography


Middle Cambrian of Laurentia

• Laurentian craton was

ringed by a concentric

pattern of sedimentary

environments

– Nearshore detrital

belt

– Shallow marine

carbonates

– Deep-water deposits


The Burgess Shale

• Location of the Burgess Shale quarry is in the

deep-water belt, but immediately adjacent to the

shallow marine carbonate belt

• Exquisite preservation of Burgess Shale fossils is

attributed to rapid burial in oxygen-free

environment

– Animals probably lived in shallow water carbonate

setting, then were swept into deeper water by turbidity

currents


The Burgess Shale

Mt Stephen,

near Field BC


The Burgess Shale

Charles Doolittle Walcott,

discoverer of the Burgess

Shale (1909)


Walcott’s Bio

• Secretary of Smithsonian Institution

• President, National Academy of Sciences

• Member, National Research Council

• Co-founder, Carnegie Institution

• Co-founder, National Park Service

• Co-founder, Advisory Committee for Aeronautics (now NASA)

• Discovered Burgess Shale in 1909, led quarrying parties in 1910-13, 17, 19, 24

• Died in 1927 at age 77


Walcott’s Woes

• 1st wife died 1911; two sons died 1913,

1917

• Re-married 1914 (at age 64), but father-in-

law would not attend wedding because of

Walcott’s “questionable character”






Trilobite Olenoides,

with delicate appendages

preserved

Bizzare beasts of

the Burgess Shale


Modern onychophoran

Aysheaia

Bizzare beasts of the Burgess Shale


Pikaia

Aysheaia

Bizzare beasts of

the Burgess Shale


Hallucigenia

Opabinia

Canadaspis

Bizzare beasts of

the Burgess Shale


Marella

Sanctacaris

Nectocaris Bizzare beasts of

the Burgess Shale


Burgess seafloor scene


Burgess

seafloor scene


Significance of the Burgess Shale

• Was the Cambrian explosion truly a burst of

evolutionary diversification, or….

• Was there a long record of Precambrian

diversification for which there is no

preserved fossil record?

• Why have there been few new phyla since

Cambrian time?