Geoscience Reference
In-Depth Information
composition called calcite (crocodiles, geckos, birds, and other
dinosaurs).
Some dinosaur eggs, including those of birds, fit into the ornithoid
type, but two other categories have been created to accommodate the
rest of the dinosaur eggs that have been found. These are the
dinosauroid spherulitic and the dinosauroid prismatic types, which are
based on the form and structure of the crystalline units that make up
the shell, just as in the case of living animals. To see these features, the
shell is often sliced to create a thin cross section that cuts through the
crystalline units, and the shell structure is then observed through a
polarizing microscope, which utilizes light that vibrates in only two
planes. French scientists first used this traditional method of slicing
the shell into thin sections in the late 1800s, when research on the
microscopic structure of fossil eggshell originated.
This approach allows paleontologists to identify the structural
type of the shell, but a more comprehensive understanding of the
shell's structure can be obtained when this approach is combined with
observations made with a SEM. To be viewed with the SEM, the
pieces of the eggshell often need to be coated with a thin layer of gold.
Some newer SEMs, called environmental SEMs, can observe the
shell without this coating, which is great for viewing unique specimens
that paleontologists cannot risk coating or damaging, but the resolu-
tion of the images is not as good. Another approach is to view the shell
Turtles, crocodiles, and dinosaurs have specific types of eggshell microstruc-
tures. The study of eggshells is an active field of vertebrate paleontology.
