Geoscience Reference
In-Depth Information
Second, although it is difficult to identify single nesting episodes
involving more than one individual with complete certainty, the
densely distributed egg clutches both in the quarry and on the flats
suggest beyond anv reasonable doubt that these sauropods were gre-
garious nesters. The alternative, that the hundreds of egg clutches
exposed on the flats at Auca Mahuevo were laid at widely disparate
times by solitary mothers, seems quite unlikely. Several birds and
crocodiles congregate into groups—often, enormous ones in the case
of birds—during the nesting season. The causes for this are not well
understood, but scientists believe that nesting colonies offer better
protection for the eggs, thus increasing their chances for survival. Per-
haps this same adaptation drove the Auca Mahuevo sauropods to
nest in large numbers.
Third, as we noted earlier, the sauropods returned to Auca Mahuevo
numerous times, undoubtedly over many different breeding seasons,
because at least four different layers of rock contain the same kind of
fossil eggs.
What emerges from our investigations between 1997 and 1999 is a
somewhat fuzzy, almost impressionistic portrait of the sauropods' life
on the ancient floodplain during their breeding season. It is likely that
herds of gravid females lumbered to Auca Mahuevo during many, if
not all, of the breeding seasons in which the nesting site was in use. It
is not clear whether males accompanied them. Upon arrival, each
female laid between fifteen and forty eggs in a nest on the gently slop-
ing, hummocky ground away from the major stream channels. Most
of the nests were spaced five to fifteen feet from one another. Some-
time after laying the eggs, the females probably left the site, leaving
the eggs to incubate, although some adults may have remained in the
area to guard the colony. The embryos grew inside the eggs to a
length of about twelve inches before they were ready to hatch. At the
end of their development inside the eggs, the embryos began to
exercise their jaw muscles and to grind their teeth, in preparation for
eating vegetation when they hatched. Normally, many of the eggs
hatched without incident, and the hatchlings, with their relatively
large heads, strong jaw muscles, and small but fullv developed teeth,
immediately set about consuming any nourishing vegetation that they
could find. Over the next fifteen to twenty years, assuming they sur-
vived, the sauropods would increase in body size more than thirtyfold
