Dinosaur Eggs

For reptiles and most of their descendants, which includes dinosaurs and birds, an egg is an enclosed yet porous mineralized or organic structure that contained or contains an amnion (a fluid-filled sac) surrounding a developing embryo (Fig. 6.4).

An egg serves as a form of protection for an embryo that also keeps its nutrients in a restricted space while allowing the inflow of oxygen and exit of waste products (such as carbon dioxide) from the egg environment through its pores.

Dinosaur eggs and all other fossil eggs are body fossils. Although some paleontologists used to classify them as trace fossils, their explicit physiological function makes eggs distinctive from dinosaur trace fossils such as tracks, nests, toothmarks, coprolites, or gastroliths (Chapter 14). An eggshell secreted by a mother dinosaur was an integral, connected part of a developing embryo. This makes it an extra body part, analogous to the exoskeleton of an invertebrate, that was essential for survival of that embryo. The occasional inclusion of embryonic dinosaur remains within an egg provides a complete picture of an egg as a body fossil (Chapters 9 to 11).

In a modern reptilian or avian egg, the amnion forms around the embryo soon after cleavage. The enclosed fluid of the amnion suspends and thus protects the embryo from concussions or desiccation. The mother then secretes eggshell around the amnion, further protecting the embryo, and a second sac (allantois) develops between the eggshell and amnion. The allantois serves as a respiratory organ for the embryo, bringing in oxygen and giving off carbon dioxide. This type of egg is a cleidoic egg, which means that it provides a food supply (through the yolk sac) and a membrane for respiration, temperature maintenance, and waste disposal.

In oviparous animals, the egg is retained in the mother's body until a sufficiently protective layer for the developing embryo is secreted, which normally involves some biomineralization (formation of minerals by an organism). It is then laid outside of the mother's body for further development. Subsequent growth of the embryo within the enclosed environment of an egg for weeks afterward is made possible through the large yolk sac in the egg, which provides food. The formation of cartilage and bones happens during this time within the egg, in which the eggshell supplies calcium. Microscopic pits in the inner surface of an eggshell show where the embryo absorbed the calcium; such pitting has been described in some dinosaur eggs. Development of all other organs and muscles that are needed for an animal to hatch and move after hatching also occurs within the egg.

Pathological conditions brought on by environmental stresses, such as dehydration, are reflected by eggshell abnormalities. For example, a multilayered eggshell is a symptom of stress suffered by a mother. This condition develops when a mother retains an egg for a longer period of time than normal, such as during environmental stress, such as a drought. The physiological response of the mother is to form another membrane and shell layer on the previously complete egg. A few dinosaur eggs also show evidence of this paleopathologic condition (Chapter 7), which illustrates some of the reproductive problems faced by dinosaurian mothers, but physiological adaptations had already evolved in archosaurs.

The mineral material composing an eggshell is CaCO3, either in the form of ara-gonite, found in turtles, or calcite, found in eggs of other reptiles, birds, and dinosaurs. Some organic materials, such as amino acids, also form in eggs and are documented from dinosaur eggshells. Eggs composed mostly of organic material are sometimes described as "leathery," a common descriptor for eggs from modern sea turtles and a few other reptiles, such as some crocodilians and lizards. However, modern bird eggs are noticeably calcified. Excellent examples of this mineralization are seen in chicken or ostrich eggs.

Dinosaur eggs are preserved in a variety of shapes. Some are nearly spherical, whereas others are ellipsoidal or semiconical (Fig. 8.2). Sizes range from a few centimeters to slightly more than 30 cm long, with comparable widths depending on the degree of egg sphericity. Approximate egg volume can be calculated by using formulas appropriate for the shape of the egg. For example, the volume of a spherical egg can be calculated simply by using the formula for the volume of a sphere (see Eqn 7.7). In contrast, an egg shape that deviates from a sphere, in that one or more of its axes may be unequal, is termed an ellipsoid (a body where all plane sections are either circles or ellipses).

A typical ellipsoid describing most eggs is a prolate spheroid, resembling a sphere that is elongated ("stretched") in a single axis.

Sphere Prolate Oblata Semiconical

Spheroid Spheroid

FIGURE 8.2 Shapes, sizes, and dimensions of dinosaur eggs. From left to right, sphere, prolate spheroid, oblate spheroid, semiconical; axes and measurements associated with prolate spheroid.

Sphere Prolate Oblata Semiconical

Spheroid Spheroid

FIGURE 8.2 Shapes, sizes, and dimensions of dinosaur eggs. From left to right, sphere, prolate spheroid, oblate spheroid, semiconical; axes and measurements associated with prolate spheroid.

The volume of a prolate spheroid is: V = 4/3(na2c)

where the a and b axes are equal and c is the long axis. Some dinosaur eggs were prolate spheroids, but others were oblate spheroids, which are spheres shortened ("squashed") in the c axis in relation to the a and b axes. The volume is still calculated with the same formula in Eqn 8.1, assuming that axis a = b.

For example, a dinosaur egg that is 13 cm long and 10 cm wide in its other two dimensions (hence it is a prolate spheroid) has a volume of

For perspective, this volume is more than ten times that of a chicken egg (also a prolate spheroid), which is typically 6 cm long and 4.5 cm wide:

This calculated difference means that one dinosaur egg could have been substituted for more than half a dozen chicken eggs in an omelet.

One of the qualitative characteristics described in dinosaur eggs is surface texture, which is evident in some eggs as a slight bumpiness or microrelief. The regularity of the texture is better defined with higher magnifications, revealing the distinctive shell microstructures. These microstructures relate to the functional morphology of an egg as determined through biomineralization, which began as either aragonite or calcite crystals that grew outward and perpendicular to a shell membrane surface (Fig. 8.3). The inner shell membrane, which is organic, is also called the eisospherite layer, whereas the crystalline exterior is the exospherite layer. In modern eggs, crystals are intimately interlocked with organic material throughout growth to form a lightweight but strong and flexible structure. Eggshells are also porous, with pores developing perpendicular to the shell membrane and more or less parallel to the crystals. These pores allow gas exchange, facilitated by the allantois, between the developing embryo and its outside environment.

FIGURE 8.3 Cross-section of eggshell microstructure, showing (from inside to outside) eisospherite layer (with shell membrane) and exospherite layer (with mammillary layer, column layer, and cuticle layer).

FIGURE 8.3 Cross-section of eggshell microstructure, showing (from inside to outside) eisospherite layer (with shell membrane) and exospherite layer (with mammillary layer, column layer, and cuticle layer).

Structure Egg Shell

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