Early Paleozoic Cambrian-Ordovician

545-440mya

Early Paleozoic Life• Primarily confined to shallow-water marine

environments

• Animals with hard body parts appear at the beginning of the Cambrian = beginning of the Phanerozoic

• Cambrian summary

• Ordovician summary

Cambrian• Two major evolutionary events at the

beginning of the Cambrian:1. Rapid increase in the number and types of Phyla:

Brachiopoda, Mollusca, Arthropoda, Echinodermata, Vertebrata• Most Cambrian animals were herbivores that fed on

algae.• Some argue that Phyla had evolved prior to the

Cambrian • Reason for apparent rapid evolution:

– Preserved in the rock record due to development of hard body parts

– Development of hard body parts allowed new forms to evolve

• Two major evolutionary events at the beginning of the Cambrian:

1. Rapid increase in the number and types of Phyla

2. Development of skeletons and hard body parts. Function:• Support for muscles, etc.• Protection against environment & other

organisms, predators• Aid in locomotion

Trilobites• First group to increase and become dominant (i.e.

abundant) – trilobites• Trilobite means three lobe fossil. • Approx. 50% of Cambrian fossils are trilobites. • Lived by crawling along the ocean floor, taking in

sediment and digesting the organic material.

• Trilobites

• Fossil website

Early Paleozoic Life• Ordovician marked by evolutionary radiation

of life in the seas• Two groups expanded:

– Graptolites http://www.graptolite.net/Graptolites/index.html

– Nautaloids

Nautiloid CephalopodCollected from the Maquoketa Shale (Ordovician) of Dubuque, Iowa

http://web.eps.utk.edu/courses/HistoricalGeo/graptolites.html

Early Paleozoic Life• By late Ordovician, burrowing animals

were on the increase

• Indicates a diversification in worms and other burrowers

• Animals with hard-shells appeared in great numbers for the first time during the Cambrian.  The continents were flooded by shallow seas.  The supercontinent of Gondwana had just formed and was located near the South Pole.

NA=Laurentia

N.Europe=Baltica

• Movie• Gondwana shifts nearer to S. Pole by end of Ordovician; a large glacier

grew there and is thought to have contributed to a global extinction event

Avalonia=exotic terrane

What’s happening in IL?

•Early Ordovician (485Ma)

•Middle Ordovician (470Ma)

•Late Ordovician (450Ma)

Early Paleozoic rocks• Cambrian– concentric pattern of deposition around

margin of Laurentia– What kind of rocks do you think were deposited? Think

about low vs. high energy, near vs. off shore

– Fig. 13-7

• Siliciclastic rocks• Carbonate plateforms• Some reef complexes (more in the Ordovician than

Cambrian)

Late Ordovician glaciation

• Tillites deposited

• Two pulses of extinction, one at onset of glaciation and one at the end– Evidence from isotopes in shells– Disappearance of many fossil groups from the

stratagraphic record

Ordovician mountain building• Taconic orogeny– eastern edge of N. America

– 1st event of the 3 events that built Appalachian Mountains

– Fig. 13-20 A-C

• Exotic terranes– Formed of rocks that are different than the cratonic

rocks – Fossils are also different; don’t match other fossils

Burgess Shale• While eastern edge of N. America was

active, the western edge was passive

(c) 1995 by Andrew MacRae

• The Burgess Shaleis a site of exceptional fossil preservation, or a conservation lagerstätte.

• Its fossils are preserved as dark organic films on thin layers of fine-grained shale; soft parts, such as muscle tissue or poorly calcified skeletons, are preserved in great detail