Geoscience Reference
In-Depth Information
In order to identify the main four egg layers separated one from
another by several feet of rock, we decided to number them in ascend-
ing order. We called the layer containing the isolated clutch Lowell
had discovered twenty-five feet below the lowest layer on the flats egg
layer 1. The lowermost egg layer at the flats was labeled egg layer 2.
The egg layer containing our quarry and all the eggs we had found dur-
ing our 1997 expedition was identified as layer 3. Finally, the upper-
most egg layer that photographer Brooks Walker had first spotted was
called egg layer 4.
That four distinct rock layers contained clusters of sauropod eggs
at Auca Mahuevo provided important evidence about the reproduc-
tive behavior of these huge dinosaurs. Based on the principle of
superposition, these giant sauropods clearly returned to the nesting
site at least four different times to lay their eggs, a behavior called site
fidelity. Since we don't know the precise age of each rock layer, we
don't know whether they returned every year, but that is certainly pos-
sible given the probability that not all of the eggs laid by the sauropods
at the site were preserved as fossils. In years when floods did not bury
the eggs quickly, most of the embryos would have hatched and left the
site. The eggshells would have been broken up, and the shell frag-
ments would probably be dissolved by rain or destroyed by other
natural processes, leaving no evidence of these breeding events in the
fossil record. So, while we cannot be certain exactly how many times
or how often the sauropods used the site, we do know that they used
it at least four or five separate times.
Because the surface ornamentation of the eggs varies, both within
each layer and between different layers, we wondered whether the
same kind of sauropod had laid all the eggs. But after careful study in
the lab, we concluded that the differences in the microstructure are
not so great as to suggest that more than one species of sauropod
nested at Auca Mahuevo. Female dinosaurs, like all other egg-laying
reptiles, laid slightly different kinds of eggs during their lifetime. For
example, eggs laid by modern reptiles, including birds, tend to be
larger in older individuals, and variations may occur depending on the
environmental conditions and food available during a particular nest-
ing season. Recent studies of modern mallard ducks has documented
that females lay larger eggs when they mate with their preferred
male partner. One theory is that the female invests more resources in
