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Lecture 20: Evolution of Mammals
Latin word ‘unroll’
Changes through time
Proposed by Charles Darwin
On the Origin of Species
Principal of Natural Selection
Individuals compete for space, food and other resources
Individuals produce more offsprings than can survive
Individuals vary in their traits
Due to limited resources only some offsprings will survive.
Nature chooses variation in traits which assure survival and passes them on
Lamarck’s explanation of evolution as applied to giraffes. Older giraffes stretch to
reach leaves subsequent generations retain this characteristic to reach food.
Did Mammals have an advantage over Dinosaurs?
Smaller size
Retaining more options such as: Flexibility to swim, climb, dig, run, jump Larger
species had to specialize harder to adapt to a changing environment
Mammal Diversity
Less diverse than
Total number of vertebrates dwarfed by numbers of animals such as insects or
even mollusks
Fossil record of mammals
Transition from reptiles to mammals is poorly preserved
Features such as:
Mammary glands
Have poor fossil record
Evolution of Mammals
Origin in Late Triassic
Earliest finds date 225-220 Ma
These barely made it over the Triassic extinction
Tertiary Timescale
Pliocene 2 Mill years
Paleocene 65 Mil years
Evolution of Mammals
Mammals remained small for about 115 Ma
Radiation after mass extinction of reptiles
By Eocene (58 Ma) most of major groups of mammals had evolved
Stages in Mammalian Evolution
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