Geoscience Reference
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ture. Prismatic eggshell appears to be typical of primitive theropods.
The third structural type, called ornithoid, is also composed of an
inner core that supports two or more crystalline zones, but in this
case, the boundary between these crystalline zones is abrupt. The
lower or inner part of the shell is similar to that of the prismatic type,
but the upper or outer part has a scaly appearance. Ornithoid struc-
ture is typical of advanced theropods, including all birds.
This threefold classification based on microscopic structure has
dominated studies of dinosaur eggshells for several decades, but
recent investigations suggest that some revisions are necessary. Just
as the bony structures of dinosaur skeletons can be used to classify
dinosaurs into different evolutionary groups, so can the microscopic
structure of their eggshells. If the cladistic method is used, eggshell
structure might provide important new information about how dif-
ferent groups of dinosaurs evolved from one another, since the struc-
ture of dinosaur eggs evolved right along with the animals that laid
them. Initial research along these lines has suggested that the exist-
ing, threefold system used to categorize dinosaur eggs may be hin-
dering our attempts to glean information about dinosaur evolution
from the structure of their eggshells.
Paleontologists have, therefore, begun to reanalyze and recategorize
different kinds of dinosaur eggshell, and the preliminary results seem
promising. For example, all dinosaur eggshells, including those seen
in bird eggs, grow outward from an inner core often referred to as the
organic core,
which indicates that all the different kinds of dinosaur
eggs evolved from one kind of egg laid by a single common ancestor.
In crocodiles, distant relatives of dinosaurs, the inner organic core is
poorly differentiated if not absent. This is not surprising, since the
structure of the hips in all dinosaurs, including birds, also suggests that
dinosaurs evolved from a single common ancestor, as we discussed in
an earlier chapter. Furthermore, the presence of two or more distinct
structural layers in the eggshells of birds and theropod dinosaurs
provides more evidence to suggest that birds evolved from meat-
eating theropods. The ornithoid type of eggshell found in living birds
is composed of a minimum of two distinct layers. The inner layer is
composed of calcite crystals that radiate out from the core, while the
outer layer(s) have a more scaly appearance. A similar structural
composition is found in some maniraptors, suggesting that birds
