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ANTH-UA 2 Chapter Notes -James Hutton, Mesozoic, Radiocarbon Dating

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Richard Bailey

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Fossils in Geological Context
How to Become a Fossil:
Paleontology = the study of extinct organisms, based on their fossilized remains
Fossils = the preserved remains of once-living things, often buried in the ground
Geology = the study of Earth systems
Very few living things become fossils, and only a small proportion of these fossils are
discovered, collected, and studied fossil record is not entirely representative of the
composition of past biological communities
Taphonomy = the study of what happens to the remains of an animal from the time of
death to the time of discovery; reveals some of the factors that determine whether an
organism becomes a fossil
Part of the organism must be preserved by burial, a natural process in which the
carcass or part of it is covered with sediment
Burial interrupts the biological phase of decomposition, protecting the skeleton
from further ravaging and trampling by biological organisms
Petrification (the process of being turned to stone) takes place when buried
skeletal remains absorb minerals from the environment and use them to replace
the original inorganic tissues
Trace fossils - evidence of an organism, but not the organism itself (ex: tracks)
Coprolites - fossilized feces
The Importance of Context
Strata = layers of rock
Stratigraphy = the study of the order of rock layers and the sequence of events they
Uniformitarianism - suggests that processes operating today are also those that
operated in the past and thus they can explain the fossil and geological record;
advocated by James Hutton and Charles Lyell; four principles:
1. Original horizontality - strata are laid down parallel to the earth’s gravitational
field and thus horizontal to the earth’s surface, at least originally; deformations
are caused by later activity
2. Superposition - with all other factors equal, older layers are laid down first and
then covered by younger (overlying) layers (i.e. fossils found in lower sediments
are older than those found above)
3. Cross-cutting relationships - a geological feature must exist before another
feature can cut across or through it; the thing that is cut is older than the thing
cutting through it
4. Faunal succession - deeper fauna is older; there are predictable sequences of
fauna through strata (i.e. successive layers contain certain types of faunal
communities and types of fossils that follow one another in predictable patterns
through the strata)
Index fossils - fossils that typify a strata
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The Geologic Time Scale
Geologic time scale (GTS) = the categories of time into which Earth’s history is usually
divided by geologists and paleontologists: eras, periods, epochs
Mammals arose in the Mesozoic Era (“middle age of mammals”)
“Age of Reptiles” - abundance of dinosaurs
Lengths of epochs, periods, and eras are not standard in the GTS; boundaries are
placed at points in the time scale where large shifts are evident in the geological column
or the fossils contained within it
How Old Is It:
Relative Dating Techniques
Provenience = the precise location from which fossils come
Geochronology - the field of geology devoted to studying time in the fossil record
Relative dating techniques = tell us how old something is in relation to something else
without applying an actual chronological age
Lithostratigraphy = using the characteristics of the rock layers themselves to correlate
across regions (“litho” = rock)
Tephrostratigraphy = a variation of lithostratigraphy that identifies volcanic ash by its
chemical fingerprints of major, minor, and trace elements (“tephra” = volcanic ashes)
Biostratigraphy = compares fossils from different stratigraphic sequences to estimate
which layers are older and which are younger
Markers that appear and disappear (go extinct) at roughly the same
chronological time in all regions are the most useful
The presence of certain taxa tells how old a site is relative to other sites with
similar or different animals
An absolute age can be assigned only because at other sites with these index
fossils there are also associated chronometric ages
Chemical techniques (analysis of fluorine, uranium, and nitrogen content of fossils) test
associations within sites (different locations have differents sediment makeups and thus
will give different results)
Calibrated Relative Dating Techniques
Calibrated relative dating techniques = include regular or somewhat regular
processes that can be correlated to an absolute chronology to estimate the age of a site
Earth’s magnetic poles move around the globe (~1000 years to switch sides)
As rocks form, their magnetic minerals orient themselves toward magnetic north
Rocks laid down today would have a polarity similar to today’s magnetic field
(normal polarity)
Rocks formed under a reversed field have a reversed polarity
Geomagnetic polarity time scale (GPTS) = records the orientation of sediments from
different intervals; based on a sequence of changes in the magnetism of ancient layers
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