Geoscience Reference
In-Depth Information
Mahuevo were deposited during an interval of time when the earth's
magnetic field was reversed. These results helped to narrow down the
70-to-90-million-year range based on marine fossils preserved in
higher layers of rock and dinosaurs from lower layers of rock. The pres-
ence of rocks at Auca Mahuevo that had formed during an interval
when the earth's magnetic poles were reversed means that the rocks
had to have been formed less than 83 million years ago.
We knew this because the global magnetic calendar documents
that the poles were oriented as they are today between about 100 mil-
lion years ago and 83 million years ago. Because the Auca Mahuevo
rocks were formed when the magnetic poles were reversed, they had
to have been formed after the end of that long interval. In addition,
recent studies of fossil pollen found in rock layers just above the egg-
bearing layers at Auca Mahuevo suggest that the pollen is between 76
and 81 million years old. Although we have yet to sample for pollen
in these same layers at Auca Mahuevo, the same sequence of rusty
mudstone and sandstone layers that contains the eggs is present
where the pollen was found, about one hundred miles to the south of
our site. Since these pollen fossils were higher in the rock sequence
than the egg-bearing layers, the pollen had to be younger than the
eggs and embryos. The magnetic poles of the earth were not reversed
between 76 and 79 million years ago, but they were reversed between
79 and 83 million years ago. We concluded, therefore, that the eggs
and embryos from Auca Mahuevo were probably between 79 and 83
million years old.
The 4-million-year span in our age estimate might still seem like an
eternity. After all, almost the entire history of our human lineage is
contained in the last 4 million years of geologic time. Most fossil sites
from these more recent periods of earth history can be dated more pre-
cisely because the analytical uncertainties in estimating the age are not
as great. Sites containing remains of large extinct dinosaurs are at least
65 million years old, however, so, given the analytical uncertainties, we
would have to be satisfied that the time of death had been refined a
bit through our magnetic analyses. But another large mystery
remained to be solved: What was the cause of death?
