Eggs and eggshell are rare but important parts of the vertebrate fossil record. Amniote eggs and eggshells (amniotes are the group of animals including all reptiles, birds, and mammals) are the most common eggshell found in the fossil record. Invertebrates and other vertebrates like fish and amphibians also lay eggs, but these are rarely preserved in the fossil record. Because amniote eggs are crystallized and hardened with calcium carbonate (CaCO3) or aragonite (CaCO3 + Magnesium), they are more likely to be fossilized and preserved. Crocodilians, gecko lizards, turtles, dinosaurs, and birds are among the amniotes with hard eggshell that is more readily preserved. The evolution of the amniote egg marked an important step in the colonization of terrestrial environments by vertebrates as the eggshell structure prevents desiccation of the embryo. This meant that vertebrates were no longer dependent on water for reproduction and could become fully terrestrial and able to exploit all terrestrial niches.
The study of fossil eggshell sheds light on the paleobiology, systematics, and paleoecology of the organisms that laid them, and provides a unique glimpse into reproductive physiology and behavior. Examination of size, shape, surface texture, and the eggshell internal structure helps scientist describe and identify eggshell. However, identification of the animal who laid the egg can only be absolutely determined if embryonic material is preserved in association with the shell. The number of eggs in a clutch, egg size, eggshell structure, and egg arrangement in nests provide paleobiological information on reproductive anatomy (i.e., the number of functioning ovaries) and parental behavior. Embryonic remains associated with eggs aid in understanding embryonic development in extinct animals. Modern eggshell is also used for comparative studies.
The fossil egg figured here is a portion of a bird egg preserved with embryonic bones. The specimen was found in 1940 in the Eocene Willwood Formation of Wyoming. Analysis of the internal eggshell structure by light microscope and scanning electron microscope (SEM), description of the external eggshell characteristics, and study of the embryonic bones preserved are tools used by Museum scientists to describe and identify the egg. The internal structure of the embryonic bone positively identifies the bones and associated eggshell as avian. While the preservation of eggshell in the fossil record is rare, the preservation of embryonic bones is even rarer, making this specimen an important contribution to the study of fossil eggs. Because the eggshell structure can be identified by the embryonic bones, this specimen is a useful tool for identifying other eggshell fragments with no associated bones.
The Paleontology Section of University of Colorado Museum houses the Karl Hirsch Eggshell Collection, a unique and world-class collection of fossil and modern eggshell dating back to the Triassic and composed of specimens collected around the world. Curation of and research on this collection is still ongoing at the Museum through a NSF grant. The collection will be online by 2010.