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II. What are some
problems inherent in paleoecology reconstruction?
1. What we don't see may be as important as what we
do: Not every creature was fossilized. Organisms with
soft-body parts are difficult to find. At best you can get a
casting or impression of what was there, but no part of it.
Organisms with shells, skeletons and plants with wood or
high production of pollen will be over-represented and
others under represented. Portions of ecosystems which were
anaerobic vs. aerobic were more likely to be
preserved.
2. Fossil beds are composites of fossils - a shallow
lake make be the burial ground of not only the creatures who
lived there but also those washed in by rivers. Animals
digging through layers of mud or soil may 'contaminate' the
historical record by bringing earlier or later fossils to a
strata.
3.The older the material, the more likely it was
modified, or destroyed by geological events or biological
intrusions.
4. At best you are sampling just a portion of what
existed; what happened to be there & could be captured
at the time that an volcanic explosion or great flood
erupted allowing fossilization.
In spite of this pictures of earlier systems have been constructed
and in some cases, population dynamics have even been construed.
II. Before we start in on the specifics on what
happened in the past you should review plate tectonics . An
excellent place is: http://pubs.usgs.gov/publications/text/dynamic.html
from which the following diagram is taken, and reproduced
just in case you can't get there right now.
Once you
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| Age
of snails, bivalve molluscs, brachipods, primitive corals, sponges
and other sea creatures.Over 150 families evolved. first land plants
evolved also. A colliding microcontinent pushed up the east coast
Taconic Orogney). erosion of these mts lead to a layer of sediment
along the continents edge. Large extinction at the end of the period. |
Sea
levels remained high. First jawed fish, large reef communities, movement
of aquatic organisms into fresh water. First land animals millponds,
mites insects. FIrst vascular land plants. Large reefs in center restricted
water flow: Water evaporated forming thick deposits of slat and gypsum.
Sea level dropped as continents clustered. Baltica rammed Laurentia
and the collision initiated the pulse of Appalachian region mt building. |
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| Fish
developed quickly: primitive sharks, bony fish, lung fish, primitive
amphibians, Large barrier reefs to the west formed. Seedless vascular
plants evolved and land plants covered most of the continent. NEar
the end forests spread : early insects spiders, snails. Global cooling
at the end of this period. Reefs hit hard. Greats mt building period
in the east created the Appalachian region. lots of erosion lead to
the build of sediments to the west of the Appalachians. Ramming out
west lead to Franklin & Antler mts. |
The
continents were closely clustered now. Sea levels rose and a shallow
carbonate sea spread. Shallow but widespread reefs with faster moving
sharks and dish. Debris from all these animals left a layer of limestone
rock. On land, new spore bearing plant arose ( ferns and larger pre-angiosperm
types). Amphibians as large as 20 ft. long. At end seas dropped, many
species with them. Lots of erosion of mts both east and west. |
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| Pennsylvanian:
Sea levels rose and fell with glaciation of Gondwanaland. Gymnosperms
appeared. Earlier plants in swampy areas died to form massive coal
deposits. First reptiles appeared. When the 2 continents came together,
the Alleghian Orogeny resulted, which extended the Appalachians further
south, built up the Ouachita Mt belt and formed the ancestral rocky
mts. |
Permian
Siberia join with e Europe, almost completing Pangea which drifted
northward.Seas restricted to the west covered less of the continent.
Gymnosperms dominated. Reptiles took over the amphibians with eggs,
speed. jaws. Mammal like reptiles therapsids. The climate dried.Dune
built up. Basins dried up leaving salt behind . A microcontinent hit
the west edge and crumpled it. THe climate cooled down and 80% of
the species disappeared. |
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| Triassic:
Pangea was complete. Large reptiles joined fish as top predators in
the reefs. Large crocodiles emerged. The land mass was so large, that
most of the land was far from the sea, and become arid. Gymnosperms
& ferns dominated the land. Thecodonts differentiated into dinosaurs.
Mammals arose from reptile but were small.. Late in the period first
flying vertebrates, frog & turtles appeared. by the end 20% of
specie were eliminated. W. NA hit a subduction zone resulting in mts.
In the e. erosion almost eliminated the Appalachians. NA moved apart
from Pangea, Applachians block and faulted. |
Jurassic:
FIsh became modernized. On land forest of cycads, gymnosperms, ginkgo
common. Dinosaurs diversified. First birds crossed skies. Pangea continued
to fall apart. Gulf of Mexico formed a rift zone which lead to seawater
filling basins with alternating evaporations = Louann Salt deposits.As
sea levels rose several times, interior flooding lead to the Sundance
Sea.At the end of the period the sea was expelled. |
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Cretaceous:
Seas were high mostly. Temperature arose early on. ( GW due to volcanoes
and CO2 increase from the Pangea split). In water teleost fish, on
land first angiosperms.Insects developed relationship with flowering
plants. Dinosaurs still dominated though. Formation of widening Atlantic
Ocean forced Greenland and Eurasia to move apart from NA. Along W.
Canada a microcontinent tagged along causing a major seaway formation
in interior. Laramide Orogeny out west . Major extermination after
massive extraterrestrial body struck earth .Dinosaurs died out. |
Cenozoic
(not pictured) with loss of dinosaurs, mammals filled niches. Mt
building out west formed the Rockies. Glaciers sculpted northern
sections. Humans appeared.
Palogene
(above): Carnivorous mammals took to the sea to form whales.Also
evolved penguins, walruses seals etc. First grasses evolved. New
mammal species. ungulates, elephants, rodents, rabbits, bats dogs,
cats primates.. Earths poles cooled down and new ice caps caused
seas to drop worldwide. Cold dense seawater sunk to bottom and flowed
N. CLimates cooled and dried. forests declined and grasslands spread.TO
the west a bridge of land connecting Alaska to Siberia was above
sea level and erosion cut down the mt out west.
Neogene
( 23 M to present) Climates grew cooler, drier, and more seasonal.
Herbs beat out trees. Animals that ate such prospered including
elephants & ungulates.WIth the west mts eroded, the continent
rebounded raising the modern Rockies and Colorado Plateau. Sierra
Nevada range rose. Lava poured across Columbia river.> The climate
fluctuated creating periods of glaciation, which formed the Great
Lakes.
Modern
Ice Age: 3-2.5 MYA: Major period of glaciations, sometimes
covering as much as 1/3 of the continent. Why these periods and
general cooling?
- Drop in Greenhouse
gases
- Uplifting
of mountains changed climate. Existing alpine glaciers reflected
back light cooling trend expand glaciers.
- Isthmus of
Panama strengthened the Gulf Stream which increased snowfall in
the Arctic which enlarged the ice cap.
- Cycles may
be due by changes in earths axis tilt.
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Once you understand how the geological system works you can get a
better understanding why the flora and fauna fossils that exist are where
they are located:
Since we can't really get a grip on all the eras why don't we just
concentrate on the Pleistocene and the post glacial time period.
Pleistocene extended from 2,000,000 years till post
glacial recent period of 10,000 years ago. It is known as
the 'Ice-age", the time of the first true humans & the
mixture and thinning out of the mammalian fauna. It was a
period of great climatic fluctuations throughout the globe
with 4 major periods of ice sheet advance and retreat in NA.
Between each glaciation period there was an interglacial
period during which conditions changed from cold to
temperate. Thus a successional progression occurred starting
with tundra plants and boreal spruce & fir, replaced by
pine & birch. Eventually the soil would build up again
and temperature warm enough for the more typical oak, beech
and ash
During this period there was also a spatial
discontinuity in climate. Along with the continued drying
and cooling due to the glaciers which led to the
developments of forests dominated by evergreens ( why
evergreens?) in the north there was also the spread of more
arid lands and grasslands in the interior. The trend in
increasing dryness was a result of the uplift of the western
mountains ( remember the climate class). The rain shadow
caused the change from forest to savanna ( grassland with
few trees) to grassland. During these millions of years
animals immigrated &emigrated changing species with the
climate.
The last great ice sheet, the Laurentian reached it s maximum advance
about 18,000 BP during the Wisconsin glaciation period. Canada was under
ice, as you will see in the diagram below. Tundraextended across the US
along its border with a bit of an extension into the Appalachians. West
of Kansas there was basically just desert like areas. As the glacier finally
retreated, the boreal forests disappeared to be replaced by grasslands
The following maps were taken directly from the following
site: http://www.soton.ac.uk/~tjms/adams4.html
A period summary of the global situation at 18,000,
8,000 and 5,000 years ago.
Overwhelmingly, the picture of the world 18,000 radiocarbon years
ago is of a much colder and more arid place. Treeless and barren landscapes
were more extensive almost everywhere, even at low latitudes. The only
notable exception is in the southwestern USA, where there was a greater
area of woodland than at present. The precise degree to which tropical
forests retreated under the glacial-age climate remains far more speculative
than for the higher latitude forests. However, it seems safe to assume
that there was a net reduction in forest cover in all three main tropical
forest regions, relative to the pre-agricultural Holocene.
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Paleoecology
pollen
grain of fossil pine impression of leaf in
