Dinosaur egg

Fossilized Dinosaur eggs displayed at Indroda Dinosaur and Fossil Park.

Dinosaur eggs are represented today as fossils. They represent the product of egg laying activity and can offer clues as to how dinosaurs behaved.[1] In some cases the embryonic dinosaur is preserved within the eggshell, and can be studied.

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.[2]

Contents

History

A Citipati osmolskae egg with preserved embryo, at the AMNH.

The first real discovery of dinosaur eggshell was in 1859 from southern France, by Jean Jacques Pouech. Due to their large size, the French eggs were at first thought to belong to giant birds. More complete eggs were found in 1869 by Matheron. He, in turn, believed these eggs were those of 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.[1]

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.[3]

Types

Therizinosaur nest and eggs in from Dinosaurland, Lyme Regis

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.[1]

The shape of dinosaur eggs may reflect some aspects of their biology. In general, they are more symmetrical than bird eggs, implying that the head was not given more priority than other structures during development. Like birds, the shell was likely an important source of calcium, especially later in development. In modern reptiles, calcium from the shell is mobilized and stored in the yolk sac immediately prior to hatching. A spherical egg would have provided the smallest shell surface compared to the volume of the egg but many dinosaur eggs were elongated. This shape would have increased the total volume of an egg given the diameter of the passage between the pubic bones. It would also have greatly increased the area of calcium-rich shell available to the embryo and may have been important in animals producing precocious young with large well-developed legs and a long bony tail.

Embryos

Dinosaur embryos, the animal inside the eggs, are very rare but useful to understand ontogeny, heterochrony, and dinosaur systematics. The known dinosaurs embryos include:

Oogenera

Oogenera are taxonomic names for types of eggshell. Nearly three dozen oogenera have been named for dinosaur eggs:

  • Paraspheroolithus[17]
  • Phaceloolithus[4]
  • Placoolithus[18]
  • Porituberoolithus
  • Preprismatoolithus[4]
  • Prismatoolithus[19]
  • Protoceratopsidovum[20]
  • Pseudogeckoolithus
  • Shixingoolithus[21]
  • Sphaerovum[4]
  • Spheroolithus[22]
  • Spheruprismatoolithus[20]
  • Spongioolithus[23]
  • Stromatoolithus[4]
  • Subtiliolithus[4]
  • Tacumarembovum[4]
  • Trachoolithus[24]
  • Tristraguloolithus
  • Youngoolithus[4]

References

  • Deeming, D. C. and M. W. J. Ferguson (eds.) 1991. Egg incubation: its effect on embryonic development in birds and reptiles. Cambridge University Press, UK. 448pp.
  • Ricqles, A. DE; Mateus, O.; Antunes, M. Telles ; & Taquet, P. (2001). Histomorphogenesis of embryos of Upper Jurassic Theropods from Lourinhã (Portugal). Comptes rendus de l'Académie des sciences - Série IIa - Sciences de la Terre et des planètes. 332(10): 647-656.
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