Geoscience Reference
In-Depth Information
from directly above the outer and inner surfaces, rather than in cross
section. Both of these approaches provide information about a shell's
crystalline structure and pore patterns.
Dinosaur eggs come in many shapes: round like a softball, oval like
a football, and elongated like a loaf of French bread. They also range
greatly in size, with those of many extinct Mesozoic dinosaurs being
as large as that of an ostrich. Interestingly, the dinosaur eggs exhibit-
ing the greatest variation in size are those of the dinosaurs that still
live, birds. They can be as tiny as those of a hummingbird or as large
as those of an elephant bird, a large, flightless species that lived in
Madagascar until a thousand years ago and laid eggs ten times the size
of an ostrich egg, much larger than those of any Mesozoic dinosaur.
Our Patagonian eggs are round and relatively large. With an aver-
age diameter of five to six inches, they are about the size of a softball
and have a volume roughly equivalent to that of a dozen chicken
eggs. Today most eggs are preserved in the shape of a disk, probably
a consequence of compaction when they were buried beneath thick
layers of rock for millions of years, but originally, they were probably
almost spherical. Their dark gray surface has a dense ornamentation
of small bumps that sometimes coalesce into ridges shaped like
worms, but the distinctness of this ornamentation varies widely,
probably due to water seeping through the ground and partially dis-
solving the calcite shell. It is not possible to tell whether the dark
gray color we see today was the original color of the egg or whether it
resulted from changes that occurred during fossilization. It is also
impossible to tell whether the eggs originally had a marbled col-
oration, typical of many living birds.
Three different structural types of dinosaur eggshell are typically
recognized, although the boundaries between these categories are
somewhat fuzzy One widespread type is termed spherulitic, in
which an inner core supports stacks of calcite crystals that radiate
out from it. This structural type is usually considered typical of all
dinosaurs except theropods, and the eggs from Auca Mahuevo are of
this type. In the second type, called prismatic, the shell is composed
of an inner core that supports two crystalline layers, but the bound-
ary between them is poorly defined. The inner layer is formed by the
same radiating crystalline structure typical of the external layer in
spherulitic eggshell, while the outer layer has a more prismlike struc-
