BIOLOGY 1B Lecture Notes - Lecture 11: Recapitulation Theory, Plate Tectonics, Castorocauda
12 Jun 2018
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Lecture 11: Evolution of the Vertebrates
• “A just so story”
o origin stories- explain how something came to be; logical but in scientific framework are not
testable ex. how elephant got its trunk
o fanciful, not testable
o elephants got long trunks by crocodiles pulling on them; true and has evidence because they do get
bit but not testable as a theory
• Big 5 extinctions over the Paleozoic, Mesozoic and Cenozoic
o Dramatic extinctions; biggest was at end of Paleozoic 250 Ma before start of Mesozoic, one before
Cenozoic at around 50 Ma
o History of life in marine and terrestrial world has undergone mass extinctions
• Origin of tetrapods- fishapod
o 4 limbed creatures- we belong to this group, includes amphibians and mammals, their common
ancestor and all descendants
o Transitional forms between fish and amphibians
▪ Tiktaalik
• Famous example (skull) of transitionary fossil organism between fish and
amphibians
• Investigators knew time frame in which this should occur (before 365 Ma),
looked at geological map to look for sediments of same age
o Homologous limbs to us
• Pangea (supercontinent) to 7 continents
o Plate tectonics: continents are riding on mobile plates on earth
▪ Used fossils to explain this; fossils are found in south America and Africa in same spots
that would be if they were together
o Vicariance: splitting of population geographically (physical barrier)
• Permian extinction: biggest extinction
o Dynamic place with many large tetrapods and vertebrates
• Synapsid evolution
o A basal synapsid from the early to middle Permian
o “mammal like reptiles”
o Evolved from basal amniotes like mammals
o Dimetrodon- large vertebrae with blood flowing(maybe for temperature control)
▪ Type of synapsid
▪ Not warm blooded like mammals
• Warm blood by putting fin up to warm up; Cool blood by putting fin down
▪ Maybe fin for deterring predators- create form that is difficult to swallow
▪ Sexual dimorphism
• End Permian Extinction (252 Ma)
o Many basal synapsids went extinct
o Possible mechanisms
▪ Plate tectonics: sea level change, increased aridity, habitat loss
▪ Volcanism: greenhouse affect, ocean anoxia, ozone depletion
▪ Methane blech
o Synapsida suruvied, evolution of mammals?
• Synapsids in Mesozoic extinction
• Mesozoic (age of reptiles/dinosaurs) mammal diversity
o Seeing fur/hair for first time
o Major synapomorphy for group that includes mammals
• Castorocauda- diurnal, semiaquatic carnivore, otter/beaver like
o Fur/hair
• Repenomamus- dinosauraivore
o Ate baby dinosaur
o Mammalian ancestor eating dinosaurs
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