by Harold L. Levin
Chapter 6 - page 9
Life on Earth: What do Fossils Reveal?
An Overview of the History of Life
- Oldest evidence of life = remains of prokaryotic cells (cyanobacteria or blue-green algae)
more than 3.5 billion years old. Found in algal mats and stromatolites.
Life was at or below the unicellular level until about 1 billion years ago.
- Trace fossils of first multicellular organisms = about 1 billion years ago.
- First body fossils of multicellular organisms
(such as worms, jellyfish, and arthropods) = about 0.7 billion years ago.
Soft-bodied imprint of Mawsonites, similar to jellyfish, Australia. Photo courtesy of Pamela Gore.
- Invertebrate phyla with hard parts appeared in late Proterozoic or early Paleozoic.
Trilobite Olenellus, Early Cambrian. Photo courtesy of Pamela Gore.
Dominant organisms at various times in the geologic past and general time of appearance
Geologic ranges and relative abundances of selected invertebrates and protists that are
commonly found as fossils.
- Early Paleozoic Era - Cambrian Period
- Most animals were deposit and suspension feeders
- Brachiopods without hinged shells (inarticulates)
- Small cap-shaped molluscs
- Soft-bodied worms
- Chitin-shelled arthropods
- Reef-building archaeocyathids
- Later in the Paleozoic Era
- Articulate (hinged) brachiopods
- Rugose (horn) corals
- Tabulate (honeycomb) corals
- Branching twig-like bryozoans (moss animals)
Colonial rugose coral, Acervularia davidsoni, 365 my from the Devonian of Iowa.
Photo courtesy of Pamela Gore.
- Mesozoic Era
- Cenozoic Era
The history of life has been marked by extinctions.
The five largest extinction events are termed mass extinctions.
These mass extinctions were sudden, global in extent, and very devastating.
Mass extinctions occurred at the ends of the following periods:
- Devonian (roughly 70% of the ocean's invertebrates disappeared)
- Permian (the greatest extinction. More than 90% of all marine species at that time
disappeared or nearly went extinct).
- Cretaceous (affecting the dinosaurs and other animals on land as well as organisms in the sea,
about one fourth of all known families of animals became extinct)
Evolutionary History of Plants
Geologic ranges, abundances, and evolutionary relationships of vascular land plants.
- Earliest photosynthetic organisms were single-celled organisms in the Precambrian.
- Green algae or chlorophytes may be the ancestors of vascular land plants.
- Plants invaded the land in the Ordovician, reproducing with spores.
A fern from the Pennsylvanian Period. Ferns reproduce using spores.
- First plants with seeds appeared in the Devonian. Gymnosperms (such as conifers).
- Carboniferous coal swamps dominated by seedless, spore-bearing scale trees.
Bark of Lepidodendron, a lycopod tree or club moss.
Note leaf scars on the trunk.
(Trees grew to 30 m tall; 90 ft).
Photo courtesy of Pamela Gore.
- Flowering plants appeared in the Cretaceous.
Angiosperms. Dominant plants today.
Fossil angiosperm leaf.
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Document created by: Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA
September 30, 2005