Chapter 18: Cambrian Explosion and Extinction

Evolution and the Fossil Record

  • 3.9 billion years ago Prokaryotes scavenged off organic molecules, heterophile
  • 1.9 billion years ago Eukaryotes like autotrophic algae, with a bit of specialization of cells (photosynthesized, phototrophic) changed CO2 to O2.
    • The atmosphere suddenly oxygen dominant - O2 cycle, balance
  • 543 million years ago to 495 million years ago: Cambrian Explosion
    • the exact date is debatable
    • 1st multicellular life?
    • Most of the living animals appeared
  • Phanerozoic Eon: 543 million years ago to the present day

The Nature of the Fossil record

  • Fossil: any trace of an organism that lived in the past
  • Fossil record: the complete collection of fossils located in my institutions around the world

5 Categories of Fossils

  • The 5 categories of fossils include: Unaltered remains, permineralized fossils, natural casts and molds, compression and suppression, and trace fossils
Unaltered Remains
  • Unaltered Remains: forms in environments where decomposition, weathering, and scavenging by animals do not occur
  • Acidic peat bogs → where iron age humans are found with flesh and hair still intact
  • Permafrost → Discovery channel had specials on the “Ice Man” and wholly mammoths recovered from such environments
  • Provide the most information
Permineralized Fossils
  • Permineralized Fossils: form when structures are buried in sediments and dissolved minerals such as silica, calcium carbonate, or iron circulate through the enclosing deposits and precipitate in cavities such as bone marrow cavities and canals once occupied by blood vessels or nerves.
  • The resulting structure looks the same as the original but is much harder and more durable
  • Dinosaur fossils are the result of permineralization
  • Permineralization can also occur in plants (Petrified Forest)
Natural Casts and Molds
  • Natural cast and molds: form when remains (bones and shells) decay but leave impressions in the surrounding sediment
  • If the sediment is refilled, a mold is formed
  • If the mold cavity is filled in by mineral matter it forms a cast
  • Preserve information about surface shape, but not internal details
Compression and Impression Fossils
  • Compression and Impression fossils: are formed when water or wind-born sediment buries organic matter, the weight of the sediment on top of the organic matter can push the material into the sediment below, creating an impression
  • Superabundant
Trace Fossils
  • Trace Fossils: the preserved remains or traces of animals, plants, and other organisms
  • Like dinosaur tracks
  • Provide the least amount of information

Life through Time

  • The geological time scale is divided into eons, eras, periods, epochs, and stages
    • Defined by diagnostic fossils

The Cambrian Explosion

  • Cambrian Explosion: (543-495 MYA) almost all of the animal phyla known today originated in the early Cambrian and most of the major vertebrate classes too.
    • This “explosion” was very fast and only found in the Cambrian Period
Fossil Evidence for the Cambrian Explosion
  1. The Ediacaran Biota

   

  1. 1st discovered in the Ediacaran Hills of South Australia in the 1940s
  2. Similar fossils were found at 20 sites worldwide
  3. Dated from 565-544 mya, late Pre-Cambrian Period
  4. Ediacaran Fauna: are soft-bodied organisms (Sponges, jellyfish, comb jellies), small organisms with asymmetrical or radial symmetry
  5. Preserved as compressions and impressions
  6. Primitive bilaterally symmetrical arthropods and mollusks also appear
    1. The Burgess Shale Fauna

   

  1. Discovered near the town of Field in British Columbia
  2. Burgess shale fauna: dated at 505 mya, contains large, complex, and bilaterally symmetrical forms (arthropods, segmented worms, several chordates, at least 2 species of jawless vertebrates resembling extant hagfish and lampreys)
Reasons for the Cambrian Explosion
  • Rising Oxygen Levels
    • The key to the rise of multicellular organisms was the rise of O2 levels in the Proterozoic Oceans.
    • With higher oxygen availability, larger body plans are possible
    • Larger body size is necessary for tissues to evolve and oxygen is needed to power these larger and more mobile body sizes
  • Evolution of Hard Parts
    • Late Pre-Cambrian fossils documents the evolution of shells
    • Fossil evidence indicated that predation provided the selection gradient favoring mineralized shells
    • Predation pressure may have also favored increased body size, new modes of locomotion, and mineralized shells
  • Other environmental factors continental configuration, climate changes, ocean circulation, sea level changes, glaciations/deglaciations, etc

Mass Extinctions

  • Extinction: is the ultimate fate of all species
  • Mass extinction: a large-scale, sudden extinction event that is geographically and taxonomically widespread
  • The Big 5: 5 prominent mass extinction events throughout the Phanerozoic eon
    • Terminal Ordovician: 440 mya
    • Late Devonian: 365 mya
    • End Permian: 250 Mya
    • End Triassic: 215 mya
    • Cretaceous-Tertiary (K-T): 65 mya
  • The 5 mass extinction only account for 4% of all extinction during the Phanerozoic eon
  • The other 96% of extinction are referred to as background extinction
    • Background extinction: extinction that occurred at a normal rate

Mass vs. Background

  • Mass extinctions are: global, involve a wide range of organisms (marine and terrestrial), occur rapidly, and result from extraordinary sudden temperature changes in the environment
  • Background extinctions are: a result of poor adaptations, environmental changes, and or competition

The Permian Extinction - The Biggest of the Big

  • The Permian extinction occurred at the end of the Permian period (~245 mya) and is the most severe mass extinction wiping out 90-95% of marine species and 70% of land species
  • Many theories of what caused this extinction have been considered
    • Glaciation of Gondwana
    • The increased fluctuation of temperatures caused by North and South Glacier events
    • volcanic eruptions in Siberia
  • Effects: increased number of variety of reptiles on both land and in the ocean
  • This made the way for the age of the dinosaurs because it helped wipe out competing species

K-T Extinction

  • K-T: high impact extinction is also known as Cretaceous-Paleogene (K-Pg) extinction
  • Impact hypothesis: a large asteroid hit the surface of the earth
    • Evidence
    • Iridium deposits found at KT boundary that is characteristic of an extraterrestrial object
    • shocked quartz particles
    • microtektites
    • Crater 180 km in diameter found on the cost of the Yucatan Pennisula (~65 mya)
  • K-T event may have been the nail in the coffin for the dinosaurs which appeared to be in a decline before this.
  • 60-80% of all species went extinct during this time
    • Prominent Tax that disappeared included the dinosaurs, pterosaurs, large marine reptiles (ichthyosaurs)
    • Mammals and amphibians not affected
    • Only one order of birds survived