Lecture 25: Major biotic changes through time

Lecture 25: Major biotic changes through time

1. Mass extinctions and evolutionary radiations

   • The "big five"
     • 1) end-Ordovician (most distant)
     • 2) end-Devonian
     • 3) end- Permian
     • 4) end-Triassic
     • 5) end-Cretaceous (most recent)
   • Mass extinction event (or biotic crisis)
     • A rapid, widespread decrease in biodiversity in relation to the rate of speciation.
       • Identified by an abrupt reduction in the diversity and abundance of multicellular organisms.
         • From the background rate of extinction, of 2-5 taxonomic families per million years.
     • Extinction record is biased towards easily observed, easily preserved (e.g. marine fossil records) and biologically complex organisms, rather than microbial life (which comprises most biodiversity and biomass on Earth).
   • Evolutionary adaptive radiation (or speciation rate)
     • A process in which organisms diversity rapidly from an ancestral species into a multitude of new forms (with different morphological and physiological traits).
       • This occurs most frequently when a change in the environment makes new resources available, and creates new challenges or biological niches.
       • Speciation occurs most rapidly when habitats are fragmented (e.g. by suitability or by land features).
   • The "Great Oxidation" event (2.4 Ba)
     • Caused by the evolution of photosynthesizing cyanobacteria.
     • Represented by the first appearance of 'red beds' caused by the oxidation of iron-rich minerals.
     • Was likely a mass extinction event for microbial communities, but is not considered one of the "Big Five" events.

2. The Cambrian metazoan radiation

   • Explosion of biodiversity in multicellular life.
     • Many of these species still exist today.
   • Shown in the Middle Cambrian Burgess Shale (Rocky Mountains, British Colombia, Canada).
     • E.g. hallucigenia.
       • An insane, alien-looking, worm-like creature that prowled the ocean bottoms of planet earth half a billion years ago.
         • With 10 pairs of legs and spines on their back.

3. The terminal Ordovican mass extinction (444 Ma)

   • The 2nd/3rd largest mass extinction event.
   • 85% of all marine species (at this time) went extinct.
     • Particularly affected trilobites, brachiopods, graptolites and bryozoans.
   • Caused by
     • Climate cooling, glaciation, sea-level fall and loss of shallow marine habitat.

4. The terminal Devonian mass extinction (375-360 Ma)

   • Likely a series of 2-3 closely-spaced extinction events that only affected marine life.
     • Particularly affected trilobites, brachiopods, and reef-building organisms (which nearly went extinct).
   • 50% of all genera went extinct.
     • Genera = plural of genus.
   • Caused by
     • Possible oceanic anoxia, sea-level fall, and asteroid impact.

5. The terminal Permian mass extinction (252 Ma)

   • Known as the "Great Dying".
   • The biggest extinction event.
     • 96% of all marine species, and 75% of all terrestrial vertebrates became extinct.
   • Caused by
     • Massive volcanism (from Siberian traps), gas hydrate dissociation (like during the PETM), and asteroid impacts.
       • Massive basaltic volcanism caused the outgassing of CO2 and sulfur dioxide.
         • CO2 accelerated the greenhouse effect.
         • Sulfur dioxide poisoned the atmosphere.

6. Mesozoic marine and terrestrial radiations (252 Ma)

   • Age of the reptiles (dinosaurs)
   • When ammonites and other Jurassic fossils originate from.
   • After an extinction event, the remaining species expand into new habitats (and speciation occurs).

7. The KT boundary event (terminal Cretaceous) (66 Ma)

   • All dinosaurs and large numbers of plants and invertebrates went extinct.
     • With terrestrial environments being impacted more than marine.
   • Caused by
     • Asteroid impact followed by prolonged basaltic volcanism (from Deccan traps, India).
   • There is high 'aridium' concentration at the KT boundary, which is an element found in meteorites.
     • Impact sites identified in Mexico and the Gulf of Guinea (off the S-coast of Nigeria).

8. Tertiary (post-KT boundary event) mammalian radiation

   • Early mammals expanded into a variety of different habitats, triggering a burst of speciation.

9. Pleistocene megafaunal extinctions

   • Definition
     • The catastrophic loss of large vertebrate species (megafauna) since the end of the Last Glaciation (since 24 ka) continuing into the present (the Late Pleistocene).
   • Australia
     • 71% of mammalian megafauna extinct.
   • South America
     • 82% of mammalian megafauna extinct.
       • E.g. giant ground sloth and giant armadillo.
   • North America
     • 74% of mammalian megafauna extinct.
       • E.g. sabre-toothed tiger (California) and Colombian mammoth (South Dakota).
   • Europe
     • 59% of mammalian megafauna extinct.
       • E.g. giant Irish deer and European woolly mammoth.
   • Africa
     • 16% of mammalian megafauna extinct.

10. Climate or human overkill?

    • Only 16% of mammalian megafauna species went extinct in Africa.
      • Because they evolved/adapted alongside humans, unlike species from other continents.
    • Megafauna species existed globally over many ice ages, yet only started going extinct following the last ice age, when humans evolved.
    • We are currently living through the sixth mass extinction event . This started in the Late Pleistocene and the evidence indicates this is the result of human overkill rather than climate change.