BIOTIC DIVERSITY AND PLATE TECTONICS

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A. Diversity Through Time

• Is the fossil record equally complete for all types of organisms?
• land plants
• vertebrates
• shallow water invertebrates
• deep water invertebrates
• plankton
• Which group of the above organisms has a studied fossil record which would best represent changes in the diversity of life through time?
• Would the diversity of all taxonomic levels equally represent actual changes in diversity through time?
• What proportion of all phyla have been discovered?
• What proportion of species have been discovered?
• What taxonomic level constitutes an accurate measure of significant diversity?

• For reasons will will discuss in class the best measure of the diversity of life through time appears to be the fossil record of shallow water invertebrate families.
• This record (shown in the handout) reveals the following:
1. few Vendian families
2. a rise to almost 200 Cambrian families during a Late Cambrian plateau
3. a sharp Ordovician rise to some 450 families
4. a general plateau of ~450-400 families until the Late Permian
5. A Late Permian decline to fewer than 200 families
6. A Mesozoic-Cenozoic rise to 800 families, punctuated by declines at the end of the Triassic and C/T boundary

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B. Influences on Diversity

• Diversity of organisms increases from pole to equator
• e.g. bivalves latitudinal gradient in diversity = 38 to 502
• e.g. bird latitudinal gradient in diversity = 222 to 667
• why?
1. resource stability (e.g. deep-sea food supply small but steady and results in high species diversity)
   
   
• Continental size
• islands and small continents have more equable climates, regardless of latitude (e.g. Britain, Japan)
• large continents have more severe climates for their latitudes
• large continents usually have monsoonal climates around their margins.
• summer monsoons blow in nutrient poor open ocean water to the shelf (low pressure over continent and high pressure over ocean)
• winter monsoons blows surface waters offshore causing upwelling (high pressure over continent and low pressure over ocean)
• greater resource variability for monsoonal coasts vs. those without (e.g. India vs. Philippines)
  
  
• Continents and Provincial Diversity
• today's coastal regions have 31 provincial provinces
• what contributes to this provincial diversity

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C. Tectonics and Diversity

• rifting and sea-level
• large scale rifting of continents creates thermal bulging (up to 2 km) and lowers sea level around rifted margins making climate more continental
• rapid sea floor spreading increases ridge volume and raises sea level
• The Record: Sepkoski diversity fluctuations
1. during the late Proterozoic there was a single super-continent - Pangaea I
2. Pangaea I split and split some more during the Cambrian and Ordovician to leave small continents located near the equator and weighted toward the southern hemisphere.
• initial Cambrian radiation
• major Ordovician expansion
3. continents began an accelerated amalgamation during the Devonian, culminating in the formation of Pangaea II during the Permian
• Permian extinctions
4. Pangaea II breakup in Triassic
• beginning of Triassic diversification
5. Atlantic begins forming during the Jurassic

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D. Examples of the Influence of Tectonics on Evolution

The Great American Interchange

 
• Created by the emergence of the Panamanian land bridge
• Destruction of the Bolivar Trough
• ~3.5-3.0 m.y.a.
• linked N. America with South America
• South America had been an island continent for most of Cenozoic
• some of the South American invaders: anteaters, armadillo, giant armadillo, sloth, giant sloth, raccoons (Procyonidae), capybaras, giant flightless birds, opposum
• some of the North American invaders: elephant -like Gomphotheriidae,tapirs (Tapiridae), skunk, peccary, horses, dogs, cats and saber-tooths, bears, camels, deer, rabbits, squirrels, shrews, pocket gophers
 
• Savanna corridors and "sister" Pliocene dispersal events
• glacial lower of sea level
• glacial shrinkage of wet tropical forests and expansion of dryer savanna
 
• South America invaders
• 38 families true dispersants
• North American invaders
• 47 families true dispersants
• 25 pseudodispersants (evolved in South America from N. American migrants)
• Adjustment for continental size
• North American landmass 60% greater than S. American
• number of true dispersants are proportional to the size of the source areas!!
• Later diversification of true dispersants
• asymmetrical (see above pseudodispersants), why?
1. survivors of numerous other invasions and possessed more adaptive advantages
2. exploited niches and adaptive zones unoccupied
3. imbalance in area of each continent for temperate invaders
4. timely invaders into ecological vacuums created by rapid environmental change
 

Plate Tectonics and Vertebrate Evolution During the Cretaceous-Cenozoic

• Reptiles dominated the Cretaceous
• Mammmals dominate the Cenozoic
• The Cretaceous and Cenozoic are not that dissimilar in length
• How does the diversification of reptiles during the Cretaceous compare with that of the Cenozoic
• If we compare the number of new orders of reptiles during the Cretaceous with the number of new orders of mammals during the Cenozoic we find that the diversification of reptiles was significantly less than that for Cenozoic mammals. Why?
• An answer may be related to the number of nucleation sites (geographical isolated areas) for diversification.
• Greater number of geographically isolated areas during the Cenozoic.
• Weighting diversification per geograhically isolated area we find similar diversification rates for Cretaceous reptiles and Cenozoic mammals.
• Diversity of mammals peaked prior to the Pliocene and has declined since....why?