Evolution: The Fossil Record

1. Evolution: The Fossil Record Vanessa Couldridge Richard Knight http://en.wikipedia.org/wiki/Image:Amonite_Cropped.jpg

2. What are Fossils? • Fossils are the petrified or otherwise preserved remains or traces of prehistoric plants, animals and bacteria • They are preserved in rock by burial under layers of sedimentary material • The study of fossils is called paleontology http://en.wikipedia.org/wiki/Image:Asaphiscuswheelerii.jpg http://en.wikipedia.org/wiki/Image:Priscacara-liops.jpg

3. How are Fossils Formed? • Fossils are usually formed when an organism that has died becomes covered with sediment; water infused with minerals then seeps into the remains and gradually replaces the original material, forming a rock-like replica • Most organisms simply decay and never fossilize • Usually only the hard tissues (e.g. bones, teeth, wood, exoskeletons) are preserved as fossils • Other ways of fossilization include embedding of an organism in amber or in ice http://en.wikipedia.org/wiki/Image:Amber.insect.800pix.050203.jpg

4. Kinds of Fossils • Body fossils – complete or partial bodies of organisms • Trace Fossils – marks that record the activity of an organism http://en.wikipedia.org/wiki/Image:ClaytonLakeStateParkDinosaurFootprint.jpg Trace fossil (dinosaur Footprint) • Microfossils – very tiny fossils (micrometer or millimeter range) • Megafossils – fossils in the centimeter or meter range http://www.paleoportal.org/ Microfossil (protist)

5. Fossils and Earth’s History • The Earth’s sedimentary strata are laid down in a chronological sequence • The fossils contained in the strata thus form an historical sequence such that the further down you dig, the older the fossils are http://bros.dyndns.org/~dhorlick/album/utah/Utah%202005-Images/36.jpg • Sequences of fossils in one part of the world can be matched with sequences elsewhere in the world to form a global picture of the history of life on Earth • Radiometric dating is used to put a time frame on the sequence of fossils

6. Progression of Prehistoric Life 3500 MYA Stromatolites 2500 MYA 1500 MYA Microscopic cells with nuclei Soft-bodied invertebrates 500 MYA Skeletal invertebrates; Signs of burrowing

7. Progression of Prehistoric Life 500 MYA First fish-like organisms Vascular plants invaded land 400 MYA Major groups of fish differentiated; Insects and amphibians appear on land Reptiles appear 300 MYA Evolution of gigantic reptiles 200 MYA Appearance of mammals; Birds, flying reptiles 100 MYA Flowering plants appear on land; diatoms appear in water Radiation of mammals Modern humans appear Present

8. Fossils and Evolution • Progressive evolutionary change is captured in the fossil record • It reveals the emergence of new species and groups, the colonization of new habitats, as well as extinctions • Most species exist for a relatively short time period (one to several million years) • Larger units of taxonomy persist for longer periods of time http://www.geosociety.org/graphics/gv/PikesPeak/03FossilWasp.jpg

9. Fossils and Ancient Landscapes • The fossil record provides information on geographical and ecological changes (Paleoecology) • For example, analysis of fossil marine foraminifera (a protozoan group) provides clues on water depth, ocean currents and water temperature • The study of fossil pollen grains provides information on shifting forests and grasslands http://en.wikipedia.org/wiki/Image:Benthic_foraminifera.jpg http://en.wikipedia.org/wiki/Image:Misc_pollen.jpg

10. Fossils and Paleogeography • The fossil record also provides information on the physical environment and geographical barriers to migration • For example: • South America was joined to Africa during the Mesozoic, as demonstrated by their similar reptile fauna at that time • When South America broke away during the mid-Cretaceous period, the mammals that emerged were different to those in other parts of the world • During the Pliocene epoch, South America joined North America and the two different mammalian fauna invaded each others’ territories and competed with each other • The majority of South America’s mammals became extinct http://en.wikipedia.org/wiki/Image:Laurasia-Gondwana.png

11. Major Evolutionary Trends • Phylum • Most phyla arise in the late Precambrian from small, soft-bodied ancestors • No fossil record of intermediate forms • Class • Most marine classes appear early in the Phanerozoic • Fossil record contains intermediates in some phyla • Order • New orders appear throughout the Phanerozoic • Marine fossil record shows nearly constant numbers of animal orders from the Ordovician to the present

12. Major Evolutionary Trends • Family • Comprehensive, accurate data is available for all marine and terrestrial animals • Minor mass extinctions not reflected in families • Genus • Accurate data available for only a few animal groups • Between 25 000 and 40 000 fossil genera known • Species • More than 250 000 fossil animal species described http://www.paleoportal.org/

13. Generalizations • The total number of living species has increased over time • The divergence between groups has continued to increase • There are more differences between contemporary organisms than there were in the past • Increasing diversity has not been maintained at constant rates http://en.wikipedia.org/wiki/Image:Haeckel_arbol_bn.png