BIOGEOGRAPHY: Lecture 11
Late Cenozoic Earth History
and Biogeography
1) During the last 5 million years
the continents and ocean basins and the floras and faunas achieved evolutionary grades, diversity, and distribution
much like today.
(Is this true? and how do we know?)
2) Chapter 7 follows most contemporary thinking
in assuming that the Pleistocene--the last 1.7 million years--
had a profound effect on todays biota.
This is a logical assumption
and it is supported by obvious evidence --
that a huge fraction of terrestrial habitat
was made uninhabitable by 2-3 km of ice.
It is widely assumed
that most closely related species groups of plants and animals diverged in the Pleistocene and even in the past 10,000 years because their present habitats were different 18,000 years ago.
Is this more than an assumption?
3) We must grant that today's flora and fauna
are distributed much differently than 18 k.y. ago,
and even much different than 12 k.y. ago.
4) But the question remains--are propositions 1 and 3 mutually exclusive? (Pliocene like today; Pleistocene different; now different)
5) Darwin (1859) thought not. In a paragraph that is possibly more perceptive than much that has been written since, Darwin pointed out that :
the slow advances and retreats of the glaciers
would have moved plant and animal communities slowly--
So slowly that the populations and species
would have remained in their accustomed habitats
even as they were moved hundreds of miles back and forth between the south and the north in North America and Eurasia.
Therefore, Darwin reasoned, the populations would have been subjected to very little natural selection
and undergone little evolution or adaptive radiation.
He further concluded that even the communities
would have remained the same,
so interactions of the same species would have remained stable.
Is all of this true? What parts are not? How can we find out?
6) Crucial research questions at this point are:
What are the rates and the time intervals of
climate change?
glacial advances and retreats?
plant/animal immigration and dispersal?
plant/animal adaptation to changing conditions? and
plant/animal speciation?
7) How do we measure time and rates of change?
How do we get data on paleohabitats and climates?
Magnetic stratigraphy--is a time scale marked by world-wide changes in the polarity of the earth's magnetic field.
Radiogenic dates--provide absolute ages of fossils and therefore certain ranges and conditions in the past.
Biostratigraphy--provides ages based on first and last appearance of organisms in the fossil record. For example, the mammalian time zones, Irvingtonian, Blancan, Hemphillian, Clarendonian, etc.
Fossil organisms--
Marine radiolarians, foraminifera, coccoliths, other phyla,
coral reef inhabitants, mollusks, fishes,
Terrestrial pollen, leaves, wood, seeds, pack rat middens,
mammals, birds, reptiles,
Freshwater mollusks, fishes, diatoms, amphibians, turtles, ostracods, zooplankton
Climate indicators:
Stable isotopes, evaporites, varves, tree rings
Glaciation
Began more than 15 m.y.a. in Antarctica
Montain glaciers began about 3 million years ago in NA
Eastern North America glaciated to Ohio, Kansas
(but not parts of NW Alaska, Wisconsin)
Causes: Milankovitch cycles based on
Eccentricity (elipticity),
obliquity = 22-24.5o tilt,
precession (wander of rotational pole) from Vega to Polaris
Cycle number was at least 23,
frequency was 100,000 years for the major glacial/interglacial,
smaller cycles ca. 20 K y, 2 K y, 200 yr, 7 yr, 1 yr, diurnal
amplitude increased from early to late Pleistocene.
Temperature of the globe fluctuated about 2-3C
local changes were as great as 6C
Pluvials, or wet climates with great lakes, occurred in what are currently desert regions in Utah, Nevada, California, Oregon
from moisture laden westerlies.
Adiabatic warming of glacial winds resulted from 2-3 km drop in elevation (and increase in pressure) near glacial front.
Climatic compression of biotic zones meant that
warm climates were closer to the glacial front
Sea Level Changed by as much as 125 m
Glacial Rebound (up to 275 m, but lagged, slowly)
sea level rise was more rapid, resulting in inland seas,
like the Champlain Sea in the St Lawrence River Basin.