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Amniotes evolved ~340 million years ago, but the oldest known fossilized amniote eggs are from the Late Triassic, over 200 million years later. Preservation probably played a large role in the hiatus between evolution and preservation, as some kinds of eggshell are more easily fossilized than others. A few finds of flexible-shelled eggs have been reported, but rigid eggshells dominate the fossil eggshell record. The rigid eggshell fossil record includes gecko, turtle, crocodilian, bird, and dinosaur eggs.
Where fossil eggs and eggshells are found
Floodplains, river channels, and lakes are environments where terrestrial and freshwater amniote eggshells are commonly found. Animals frequently nest in these areas so that they are near water and food sources. Also, sediments are more likely to accumulate and bury eggshell in such areas due to seasonal flooding. Organic remains generally need to be covered rapidly in order to be preserved this can prevent destruction due to transport, trampling by other animals, and microbial decay.
The decay of organic material and deposition of volcanic ash can increase the acidity of the depositional environment, leading to partial or complete dissolution of eggshell buried within such sediments (Fig. 1). Sediments can also become acidic thousands or millions of years after deposition due to the introduction of hydrothermal fluids or other acidic ground waters. Other diagenetic processes that affect eggshells include loss of organic material and mineral replacement; shells made of aragonite will often be replaced by more stable calcite. More rarely, shells can become completely replaced by a mineral with a different chemical composition (such as silica), a process that destroys the original eggshell structure.
Despite all the taphonomic processes that can alter or destroy eggshell, in certain deposits fossil eggshell can be relatively abundant. Much information can be recovered from fossil eggshell, both about the organisms that laid the eggs and the depositional environment in which eggshell has been preserved. Internal eggshell structure is often still evident, despite mineralogical alteration. The extent of chemical and physical alteration can be used to interpret the depositional history of fossil eggshell, including the geochemistry of the depositional environment and sedimentological conditions. These interpretations can then be used in paleoenvironmental reconstructions. Although eggshell is subject to alteration, unique features can be preserved. For example, paleontologists have found fossil eggs with what appear to be peck marks from the beak of a bird (Fig. 2).
Hirsch, K.F. 1994. The fossil record of vertebrate eggs. Pp. 269-294 in S.K. Donovan(ed.), The Palaeobiology of Trace Fossils. John Wiley and Sons.
Hirsch, K.F., A.J. Kihm, and D.K. Zelenitsky. 1997. New eggshell of ratite morphotype with predation marks from the Eocene of Colorado. Journal of Vertebrate Paleontology 17(2):360-369.
Tokaryk, T.T., and J. Storer. 1991. Dinosaur eggshell fragments from Saskatchewan, and evaluation of potential distance of eggshell transport. Journal of Vertebrate Paleontology 11(3):58A.
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