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Chapter 4 Opener Skeletal remains of the Pliocene hominin Australopithecus afarensis
Evolution and thefossil record
Today: phyleticevolution or anagenesis
Dates to remember
• Age of• Earth and solar system: 4.6 bya• Earliest fossils of living things: 3.5 bya• Earliest fossils of animals: 800 mya
Figure 4.1 Plate tectonic processes
Plate tectonics: provides topographicand geographic heterogeneityBut may eradicate fossils
5-10 cm/yr
Absolute dating fossils• Radiometric dating• based on rates of radioactive decay
– one element into an isotope or into a different element
• 1. Rates constant and independent of environmental factors
• 2. Rates of decay are known• 3. therefore, amount of decay from a parent
element (or isotope) into a daughter element (or isotope) = a geologic clock.
Potassium/Argon clock
• Decay of 40K produces 40Ar
• Igneous rocks; e.g. derived from volcanic activity
• Heat drives off previously accumulated Argon gas
• Sets the “clock” to zero• As rock cools and solidifies, 40K continues to
decay to 40Ar which is trapped inside the rock.
• To date the rock, it is reheated and the amount of 40Ar is measured.
• The ratio of 40K to 40Ar permits dating the rock.• 40K has a half-life of 1.3 billion years.• In 1.3 billion years, 1/2 of the original 40K will
have been converted to 40Ar • In 2.6 billion years, 1/4 of the original 40K will
remain.
1. Determine ratio of parent isotope to daughter isotope.2. Convert ratio to number of half- lives elapsed. 3. Multiply number of elapsed half- lives X number of years it takes for a half-life to elapse 4. This is the age estimate of that rock.
Radiometric Dating
Figure 4.6 (A) Lineage leading from basal sarcopterygian fishes to early tetrapods. (B) Articulated skeleton of Tiktaalik. (C) The pectoral fin, or forelimb, of Tiktaalik
Marjorie Latimer: Curator, East London Natural History Museum, South Africa1938
Latimeria chalumnaeJ. L. B. Smith
Second specimen: 1952
Figure 4.8 Skeletal features of (A) Archaeopteryx, (B) a modern bird, and (C) a dromaeosaurid theropod dinosaur, Deinonychus
Figure 4.10 Skulls of some stages in origin of mammals (Part 2)
Dentary-squamosalarticulation.Last remnantsof an articular-quadratearticulation
A sequence of 60 bases from the beta-casein gene = 60 characters
Characters:(a) informativeCharacter 166
(b) uninformative1. no variatione.g., character 1422. occur only onceautapomorphye.g., character 192
(c) conflictingphylogeneticsignals e.g., 162 and 177
Homoplasy
Genetic resolution?
SINES and LINESRetrotransposableinterspersed elements
RNA intermediateContains info forreverse transcriptase
Resolution of whale phylogeny: