IV. Fossil record of amniote eggs

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
Fossil eggs and eggshell have been found on every continent except Antarctica, and in every major geologic period dating back to the Triassic of South America (see Table 1). Eggshell is most often found as small, isolated fragments, but sometimes whole eggs, nest structures, and egg clutches are preserved. Amniote eggshells can be found in both terrestrial and marine deposits, depending on the habitat of the egg-laying animals, how much transport was involved before deposition, and the rate of burial. Amniote eggs found in marine deposits were transported there from terrestrial habitats, as an embryo in an amniotic egg would be unable to survive under water. The quality and amount of material preserved determines how much paleobiological information can be extracted about the eggshell and egg-layer.

Period/epoch     Country Late Triassic South America Late Jurassic Great Britain, Portugal, Switzerland, USA (Colorado, Utah, Wyoming) Early Cretaceous Korea, Mongolia, USA (Idaho, Montana, Utah) Late Cretaceous Argentina, Canada (Alberta), China, Columbia, France, India, Kazakhstan, Peru, Romania, Spain, Uruguay, USA (Montana, New Mexico, South Dakota, Utah, Wyoming) Paleocene France, USA (Montana) Eocene Algeria, France, Germany, Mongolia, USA (Colorado, Wyoming) Oligocene USA (Colorado, Montana, Nebraska, South Dakota, Wyoming) Miocene Australia, Chile, France, Greece, Kenya, Spain, USA (Nebraska, Colorado) Pliocene Ethiopia, Kenya, USA (Arizona, Florida) Pleistocene Libya, Madagascar, South Africa

Table 1. Geographic and temporal extent of the Karl Hirsch Eggshell Collection now housed at the University of Colorado Museum of Natural History.

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.

Eggshell taphonomy

Click any photo on this page to see an enlargement.

Figure 1. Thin section of fossil eggshell with a highly irregular outer surface denoting weathering of the calcite. The specimen is Oviraptor eggshell from the Late Cretaceous of Inner Mongolia. Specimen UCM 815. Photomicrograph taken under polarized light with a gypsum plate for color. Figure 2. Fossil eggshell from the Eocene of Colorado with a probable peck mark from the beak of a bird. Inset shows a close-up. Specimen UCM 109-10.

Amniote eggs and eggshells are affected by the same preservational biases that affect other vertebrate fossil material, but eggshell is more susceptible to some processes of transport and destruction due to its physical and chemical composition. The thinness of eggshell increases its susceptibility to physical destruction — imagine a chicken egg in a river; it would quickly be broken and dispersed. Consequently, eggshell fragments are more commonly found in the fossil record than whole eggs. Additionally, calcium carbonate eggshell can dissolve in an acidic environment — bone is composed of calcium phosphate and is more resistant.

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).

Additional readings
Hayward, J.L., K.F. Hirsch, and T.C. Robertson. 1991. Rapid dissolution of avian eggshells buried by Mount St. Helens ash. Palaios 6:174-178.

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.

Fossil eggshell home		Karl Hirsch and the Hirsch Eggshell Collection		Case studies		Interactive map

 
Figures 1 and 2 courtesy of the University of Colorado Museum.