Geoscience Reference
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and maniraptors evolved from the same common ancestor, one that
laid eggs with shells divided into two or more layers. Again, as noted
earlier, the bony structure of the wrist and other skeletal parts, along
with the presence of feathers in some theropods and birds, suggests
the same thing. Perhaps the restriction of the radial section of the
dinosaur prismatic and ornithoid types to the inner portion of the
eggshell is a characteristic that blurs the distinction between these two
types. Alternatively, it may be that dinosaurs that laid these two
types shared a common evolutionary origin that was not shared by the
dinosaurs that laid the dinosauroid spherulitic type of egg. Such
research in the field of dinosaur eggshell structure remains in its
infancy, and further investigations within this relatively new field
may provide significant new insights on the evolutionary history of
dinosaurs.
Returning to our discovery, the eggshell in the Auca Mahuevo
eggs is rather thin, somewhat less than one-tenth of an inch thick.
Although this may seem thick in relation to a chicken's egg, it is indeed
much thinner than the eggshell of other similar eggs that have been
found in Patagonia and Europe. We were lucky in that the microscopic
structure of the eggs we collected was preserved in perfect detail. By
cutting thin slices through the shell and studying the shell structure
with a stereo electron microscope, we could see that our eggs belonged
to the spherulitic type, which suggested that they did not belong to
theropods. This left sauropods and omithischians as possible candi-
dates. The Auca Mahuevo eggs were of a shape, size, and structure
consistent with an egg type referred to as megaloolithid. A variety of
megaloolithid eggs have been found in various late Cretaceous sites
in South America, as well as in India and Europe. Much older mega-
loolithid eggshell has also been found in the late Jurassic of North
America and Europe. Megaloolithid eggs have often been thought to
belong to sauropod dinosaurs, but no such egg had ever been found
with an embryo inside.
Based on evidence from sites found in many different parts of the
world, we know dinosaurs laid their eggs in a variety of different pat-
terns. Some dinosaurs laid their eggs in well-defined nests. Large con-
centric circles with pairs of eggs are typically found in nests of
theropod dinosaurs, such as troodontids and oviraptorids, two kinds
of maniraptors. The troodontids lay their eggs vertically, half-buried
