I. Introduction to eggshells

Evolution of the amniote egg

Figure 1. Generalized geologic time scale showing some of the definitive events in vertebrate, especially amniote, evolution.

The first amniotes evolved from amphibian-like animals during the Carboniferous Period, ~359-299 million years ago (Fig. 1). Invertebrate and non-amniote vertebrate animals had been laying eggs for millions of years prior to the Carboniferous, but the amniotes did things differently. For one thing, amniotes reproduce through internal fertilization and lack a larval stage in their development. But what really sets amniotes apart is their semi-permeable egg. Amniote eggs are larger and the yolk and albumin ("egg white") provide nutrients and water, respectively, to the embryo. One or more membrane layers cover these components and permit the transfer of gases between the embryo and the environment (i.e., oxygen passes into, and carbon dioxide passes out of, the egg). Additionally, the mineralized shell protects the embryo from damage and desiccation, while still permitting gas transfer. This evolutionary innovation opened the door to new habitats. Amniotes did not have to lay their eggs in water like their ancestors — they could become fully terrestrial and exploit all terrestrial niches.

Amniotes include reptiles, birds, and mammals. It may seem surprising that mammals are amniotes — we don't typically think of them as egg-layers. However, early mammals laid eggs, and some modern mammals called monotremes (like the duck-billed platypus and echidna) still do. All other mammals (the placentals and marsupials) and some reptiles have lost the calcified shell and the female retains the egg during development, resulting in live birth. Most egg-laying amniotes produce eggshell calcium carbonate in the form of calcite (CaCO₃); however, turtles, unlike all other amniotes, build their eggs out of aragonite (CaCO₃ + magnesium). Organic material is deposited simultaneously with the calcium carbonate (Fig. 2). Pore canals run through the shell and permit gas exchange between the embryo and atmosphere.

Click on Figures 2 through 6 to see enlargements.
Figure 2. SEM (Scanning Electron Microscope) photomicrograph showing eggshell membrane, organic matter, and calcite crystals in a recent bird eggshell (Northern Gannet, Morus bassanus). Organic matter is also woven in through the calcite crystals. Organic material is not typically preserved in fossil eggshells, resulting in a loss of environmental and physiological information during fossilization. Specimen UCM 853, photomicrograph G245-238. Figure 3. SEM photomicrograph of a recent bird eggshell (White-Bellied Go-Away Bird, Corythaixoides leucogaster) annotated to show the major features of eggshell microstructure. The continuous layer can be replaced by columnar or wedge crystalline organization. Specimen UCM 872, photomicrograph G251-2.

The identity of the egg-laying animal can only be absolutely known if embryonic remains are preserved within an egg. Because shell unit structures are unique among taxa, they can be used to help in fossil eggshell identification and classification.

Physical properties of the amniote egg
Eggs can be divided into three general categories based on the physical properties of the eggshell: soft, flexible, or rigid. Eggshell rigidity is determined by the proportion of inorganic to organic matter. Soft and flexible eggshell contains more organic matter than calcareous crystalline material; conversely, rigid eggshell has more calcium carbonate (calcite or aragonite) than organic matter incorporated into its structure.

• Soft eggshell (Fig. 4): Most lizards, snakes, and tuataras lay soft eggs composed of an organic framework and poorly organized calcite crystals. These eggs collapse and shrivel after the animal hatches, and are therefore unlikely to be identified or even preserved in the fossil record.

• Flexible eggshell (Fig. 5): Many amniotes, including some lizards, snakes, and turtles, lay eggs with flexible shells. These shells differ from soft shells because of their higher mineral content. Nevertheless, preservation of flexible eggs is also rare in the fossil record.

• Rigid eggshell (Fig. 6): Some turtles and geckos, and all crocodilians, dinosaurs, and birds lay eggs with rigid eggshell. The calcite crystals form a relatively thick eggshell of interlocking shell units. Fossilization is more likely to occur in rigid eggshell because the crystalline calcium carbonate (calcite or aragonite) layer is stronger, more durable, and does not shrivel upon hatching.

Figure 4. SEM photomicrograph of recent tuatara eggshell (Sphenodon punctatus) in radial view. Note the poorly organized calcite shell layer and no distinct membrane layer. Specimen UCM 194-4R, photomicrograph 502. Figure 5. SEM photomicrograph of a recent turtle eggshell (Trionyx spiniferus). Note the radial organization of aragonite crystals and slightly discernable separation of eggshell membrane and crystalline layer. Specimen UCM 192, photomicrograph 1698. Figure 6. SEM photomicrograph of recent bird eggshell (Malleefowl, Leipoa ocellata) in radial view. Note the well-organized shell units, thin membrane, and distinct division between calcite and membrane. Specimen UCM 563, photomicrograph 241-234.

Additional reading
Carpenter, K. 1999. Eggs, Nests, and Baby Dinosaurs: A Look at Dinosaur Reproduction. Indiana University Press. 338 pp.

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.

Mikhailov, K.E. 1997. Fossil and recent eggshell in amniotic vertebrates: Fine structure, comparative morphology and classification. Special Papers in Palaeontology (56):1-80.

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

 
Geologic time scale by Laura Wilson; Figures 2-6 courtesy of the University of Colorado Museum.