||I. Introduction to eggshells
Evolution of the amniote egg
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.
Figure 1. Generalized geologic time scale showing some of the definitive events in vertebrate, especially amniote, evolution.
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
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.
The calcareous shell unit is the most basic building block of an amniote egg (Fig. 3). The genetically controlled characteristics of a shell's organic material and calcium carbonate determine a shell's physical properties (structure, rigidity, breaking strength). Many characteristics of the calcium carbonate structure are preserved in fossilized eggshell, but organic matter is not commonly preserved because it decays easily and is usually lost during fossilization. However, in the past few decades, scientists have begun to identify organic matter from eggshell in the fossil record.
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.
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.