Lecture 19: Fossils and History of Life

Fossil Formation

  • The dinosaur collapses and ides, footprints are left in the mud
  • Flesh
  • Water level rises, sediment barriers the bones and footprints
  • Thick sequence of sediments accumulates


Fossil Types: Replaces Bones of Shells

  • Most fossils are hard like rock, like bones or shells
  • Original material is replaced by more durable minerals
  • Material is already a rock, therefore it is easier to classify in the rock record

Molds and casts - Impression of a shell, or a soft-bodied organisms

  • Sediment gets lithified, Leaving an impression of the harder part of the organism

Trace fossils - Footprints of worm burrows, not the organisms themselves, but the traces of their existence

  • Things like their poop is a trace of their existence, looking for evidence of them being able to move
  • Frozen of dried body fossils: mummified remains, often in dry, arid, or cold regions
  • Relatively young, last 10-20000 years

Amber - Preserved whole organisms in sap or tar, can preserve DNA


Fossil Preservation

Criteria to preserve fossils

  • Death in an oxygen-poor environment,
  • Rapid burial, slows down the decay and gets scavengers away from it
  • Presence of hard parts (often)
  • Exceptional fossils preserve additional detail, like feathers or soft parts


The Archean and Proterozoic

  • First chemical evidence of life: ~3.8 Ga,
  • Oldest bacterial cell fossils: ~3.5 Ga
  • First stromatolites: ~3.2 Ga
  • Stromatolites are one of the earliest origins of life. They change a lot
  • Physical evidence of a fossil, but the problem is that this type of layered origin can be produced by non-biological means
  • Bacterial cells are also difficult to interpret as they are very simple in shape, produced by non-biological means like precipitation of minerals
  • Chemical evidence, relates to organic material being preserved
  • Organisms fractionate, biological organisms use lighter isotopes so scientists use this to date the organisms


The Cambrian Explosion

  • Organisms have structures, not microscopic anymore
  • Animals started becoming dominant in the Cambrian

Ediacaran Fauna - Precambrian fossils that are difficult to classify

  • We don’t know what they are, but believe they are organisms that went extinct because of the fact that they competed with actual animals; extinct lineages
  • Arthropods, Trilobites, Echinoderms:

Burgess shale - Origin of animals comes from rocks, these rocks preserve eyes, guts, tongues, etc

  • Oldest forms of life are trapped in these rocks
  • First time we start seeing large animals, preserved in this rock
  • Most organisms were filter feeders


The Paleozoic

  • Evolution of land, trees, plants
  • Fossils are found in carbonate rocks
  • Cambrian and Ordovician life can be seen in the early Paleozoic, resemble to shells
  • Middle Paleozoic, Silurian (new species of tribolites) and Devonian life (age of fish); fish rapidly became diversified
  • First land animals (insects, scorpions and spiders) and plants (vascular land plants) came into existence, which could transport water through their veins internally
  • First animals to walk on land, and breathe using lungs
  • Late Paleozoic, Carboniferous and Permian life
  • Coal swamps were filled with giant insects; Carboniferous
  • Amphibians came into existence, as reproduction was permitted without water, and man reptiles came into being as well; Permian


The Permo-Triassic Extinction

  • Deep oceans become anoxic which led to 96% marine and 70% terrestrial species to die off, species lost every single member of their population
  • Causes include: asteroid impact, triggers
  • Flood basalts in Siberia, triggers
  • Coal bed fires
  • Disruption of methane clathrates


The Mesozoic

  • Warmer climate
  • Early and middle Cenozioc, Triassic and Jurassic life
  • By end of the Triassic, the first true dinosaurs evolved. Dinosaurs differed from other reptiles by having their legs beneath their bodies (instead of to the side). Some bear evidence of warm-bloodiness
  • The first feathered birds appeared
  • The first ancestors of mammals, resembling small rat-like creatures, appeared in the Triassic. Mammals have three bones in the middle ear and a “tool kit” of different teeth for different tasks.
  • Late Mesozoic, Cretacious life
  • Modern fish, with symmetrical tails, specialized fins, short jaws, and rounded scales, appeared and became dominant. Aquatic reptiles and gigantic turtles swam the seas.
  • Angiosperms (flowering plants) appeared and spread rapidly. Angiosperms produce seeds rapidly and utilize insects to facilitate pollination. Hardwood trees proliferated.


The Cretaceous Tertiary Extinction

  • A large bolide impact terminating the dinosaurs that existed at the time
  • Analysis of the clay revealed that it is highly enriched in iridium (Ir), an element rare on Earth and abundant in meteorites. Geologists began to find iridium enrichment in K-T boundary clay worldwide.
  • Evidence of an impact includes a thin layer of plankton-free clay that separates plankton-rich chalk at the K-T boundary. This suggests that plankton were shut off for a short time.
  • The clay contained other unusual material, including pressure-shocked quartz and tiny glass spheres called microtektites, features found elsewhere associated with impacts.
  • A 10 mile wide asteroid appears to have struck the Yukutan Peninsula in Mexico. The impact may have triggered eruption of the Deccan Traps flood basalts in India
  • Unless… the extinction was really caused by a giant basaltic eruption in India! Eruption of the Deccan Traps would have released CO2 and other nasty particles and gasses into the atmosphere


The Cenozoic

  • Diversification of birds and mammals
  • After the K-T boundary, plant life recovered. Forests of angiosperms and gymnosperms reappeared. The first grasses appeared in the middle Cenozoic. Dinosaur descendants (birds) diversified and spread everywhere.
  • The Cenozoic is known as the Age of Mammals. Mammals rapidly diversified to fill vacated niches after the K-T boundary extinction. By the Middle Cenozoic, mammals had diversified and spread everywhere.
  • Ape-like primates diversified in the Miocene (~20 Ma). The first human-like primate appeared about 4 Ma. The first members of the human genus Homo appeared 2.4 Ma.


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