Geoscience Reference
In-Depth Information
Neuquen province from gigantic meat-eaters that are even larger
than
Giganotosaurus.
We knew that the animals at Auca Mahuevo lived between 70 mil-
lion and 90 million years ago, near the end of the Cretaceous. But 20
million years is a long time, and we wanted to pin down the time of
the embryos' death more precisely. This required that we collect
small samples from many of the rock layers at the site. We knew that
the layers had been laid down one on top of the other so that the lower
ones were older than the higher ones. This fundamental geologic prin-
ciple, called superposition, established several centuries ago by early-
students of earth history, allows one to establish the relative age of fos-
sils in a sequence of rock layers. It was, therefore, critical for us to
record which layer or layers contained the fossils at Auca Mahuevo,
because fossils from lower layers were older than fossils from higher
layers. The question was, exactly how old were they?
Figuring out the exact age of the fossils from Auca Mahuevo was
difficult and involved detailed scientific analysis. First we compared
fossil animals at Auca Mahuevo with fossil animals from other local-
ities where the age was known. If the kinds of animals were very sim-
ilar, then the fossils from Auca Mahuevo were assumed to be about
the same age. Using just the dinosaurs, however, it was hard to tell,
because no other rock layers at other sites contained many of the
same kinds of dinosaurs. But some fossils in rock layers higher in the
sequence near our site were of animals that had lived in the ocean,
such as clams, snails, and microscopic plankton. These suggested
that the dinosaurs at Auca Mahuevo were at least 70 million years
old. In addition, rocks that underlie the layers at Auca Mahuevo con-
tain dinosaurs that were estimated to be older than 90 million years.
How was the age of these animals established?
Some rocks containing similar fossils in other parts of the world
also contain ancient layers of weathered volcanic ash, and the vol-
canic ash contains small crystals of minerals that were formed during
the volcanic eruption. Some of these crystals contain atoms, includ-
ing uranium and potassium, that break apart into other atoms at a
constant rate in a process called radioactive decay. The atoms that
break apart are called parent atoms, and the atoms that result are
called daughter atoms. Using sophisticated scientific instruments,
geologists can measure how long it takes for half of the parent atoms
