Life Through Time:
I. Precambrian Time:
Precambrian time lasted from the beginning of the earth (4.6 billion years ago) to about 600
million years ago
First life forms were bacteria-like cyanobacteria – autotrophic
Banded Iron formations (BIF’s) – dissolved iron in ocean combined with oxygen from
photosynthesis forming FeO2 – insoluble iron oxide precipitated out of solution forming layers –
important iron deposits now
Stromatolites – structures formed from concentric layering of algal mats
Ediacaran fauna – about 700 million years old. Found first in Australia then China. These were
the first multicellular animal forms but left few fossils.
Paleozoic Era (ancient life)
Cambrian Period (570 million ago):
The oldest period in the Paleozoic Era is the Cambrian. This period is also called the Cambrian
Explosion because the fossil record shows the remains of so many different life forms.
There may be more evidence of fossil life in the Cambrian period because there were more
organisms with a hard parts such as shells that would fossilize. Also, the Ocean chemistry may have
changed by this time allowing the deposition of more animal remains; also might be due to first true
"croppers" or predators.
Most of the major animal phyla appear during this period. Major forms include: epifaunal filter
feeders, Sponges, Brachiopods, Trilobites, Bivalves, Cnidarians, Nautiloids, and early Chordates.
The Burgess Shale (British Columbia) is an important cambrian fossil deposit, over 35, 000
species have been collected from this shale. 10 other phyla found during the cambrian period are
now extinct.
Ordovician Period:
The Ordovician time period is marked with the arrival of the primitive corals and stalked
echinoderms also called crinoids. All major animal phyla are represented during this time period.
Silurian Period:
During the Silurian time period invertebrates dominated the Oceans. Evidence of the first land
plants and animals.
Devonian Period:
The Devonian time period is known as the “Age of Fishes”
Mississippian & Pennsylvanian Period:
Mississippian and Pennsylvanian periods are known collectively as the Carboniferous period.
Known for coal swamps and evidence of the first amphibians.
Permian Period:
Reptiles first appeared. Formation of Pangaea. Mass extinction at end. Over 80% of all marine
organisms (trilobites, primitive corals, most nautiloids). Often referred to as the “Great Dying”.
Mesozoic Era (Middle life)
A. Triassic Period (245 million): Rebound after mass extinction. Few shallow marine
organisms. Many molluscs (cephalopods). Dinosaurs and marine reptiles appear. Modern corals
toward end Of period
B. Jurassic Period:
Dinosaurs dominate land
-Bird-hipped (Ornithischians) - all plant
eaters. Ex: Stegosaurus, Triceratops
-Lizard-hipped (Saurischians) - plant and
animal eaters. Ex: Apatosaurus, T. rex
Birds make their first appearance. Archeopteryx- a fossil with bird like feathers and a reptile like
skeleton - suggests a close relationship between dinosaurs and birds.
Pterosaurs and pterodactyls (flying reptiles)
C. Cretaceous Period:
During the Cretaceous time period, Diatoms and Dinoflagellates appear along with modern
corals (Scleractinids) and Bivalves- become the major reef formers.
The Cretaceous time period shows evidence of much sea life. The first true mammals appear on
land as well as flowering plants.
At the end of the Cretaceous time period, there is once again a mass extinction of organisms.
Ammonites and all Dinosaurs as well as many Bivalves are lost.
Today only turtles, snakes, lizards, crocodiles and alligators survive among the reptiles.
Cenozoic Era (Recent life)
A. Tertiary Period:
Tertiary seas are dominated by one celled organisms and molluscs.
Insects and modern plants co-evolve.
Mammals dominate the land and the first grasses appeared.
B. Quaternary Period:
During the Quaternary time period, the continents move to their present day positions.
This is a cooling period as demonstrated by several ice ages.
The first primates appear.