Dinosaur egg



Dinosaur eggs are represented today as fossils, in particular trace fossils. Trace fossils of dinosaurs are marks or indicators preserved in the rocks left by the particular vertebrate at a particular time. They represent that product of egg laying activity and can offer clues to how dinosaurs behaved.

Dinosaur eggs are known from about 200 sites around the world, the majority in Asia and mostly in terrestrial (nonmarine) rocks of Cretaceous Age. It may be that thick calcite eggshells evolved during the Cretaceous (145 to 66 million years ago). Most dinosaur eggs have one of two forms of eggshell that are distinct from the shells of related modern animal groups, such as turtles or birds. However, some dinosaur eggs closely resemble bird eggs, particularly the type of eggshells in ostrich eggs.

Size
Dinosaur eggs vary in size, depending on the species. Among the largest are fossilized dinosaur eggs collected in the mid 1990s from Late Cretaceous rocks in China. These eggs are more than 60 cm (2 ft.) long and about 20 cm (8 in.) in diameter.

History
The first real discovery of dinosaur eggshell was in 1859 from southern France, by Jean Jacques Pouech. The French eggs were thought to belong to giant birds at first, because of their large size. More complete eggs were found in 1869 by Matheron. He thought these eggs belonged to a giant crocodile. In 1877 Paul Gervais published the first detailed study of the eggs, and suggested that they could belong to a dinosaur. They are now known to have been laid by the sauropod dinosaur Hypselosaurus.

Types
The egg structure consists of a series of basic vertical units that grow from particular sites on the surface of the shell. The organisation of these units determines the classification scheme, being either spherulitic or prismatic:
 * Spherulitic egg shells show spherical patterns in the crystalline structure, and they are seen in sauropods and hadrosaurs.
 * Prismatic egg shells grow into spherical crystals only in the lower portion of the shell, while crystals in the upper portion are prisms.
 * Ornithoid eggs (also seen in birds) are generally laid by theropods. In this type only the very bottom part of the shell exists as separate or discrete units (mammilae). The upper and mid-portions of the shell consist of a mass of biocrystalline material with a spongy (squamatic) ultrastructure, that comprises a homogeneous layer.