||II. Eggshell morphology and structure|
Researchers employ a variety of analytical methods to learn about eggshell morphology and structure. Two of the most common techniques are microscopic analysis of thin sections and use of scanning electron microscopy, which renders a three-dimensional image. Cathodoluminescence can be used to determine patterns of mineral replacement in fossil eggshell (Figs. 1-3), and other geochemical analyses such as isotopic analysis of carbonate and trace elements are used for paleoenvironmental analysis.
Thin sections of fossil eggshell can be examined under a cathodoluminescence (or CL) microscope. Exposed to a beam of electrons, the minerals in the thin section emit visible light, providing composition information and revealing structures that can not normally be seen. Figure 1. Thin section of fossil eggshell from the Jurassic of Colorado. Specimen UCM 462-2-1. Figure 2. A CL microscope setup at the University of Toronto. Figure 3. Photomicrograph of same specimen as in Figure 1 (UCM 462-2-1) viewed with cathodoluminescence. The bright coloration denotes places of mineral replacement.
The study of fossil eggshell includes describing the general morphology and microstructure of the eggshell. Combinations of these characteristics help researchers classify fossil eggshell types and sometimes provide clues about the identity of the egg-layer.
General morphology includes the external physical characteristics such as egg shape and size, eggshell thickness, pore distribution, and ornamentation on the outer shell surface.
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Figure 4. Drawing showing the basic components of the shell unit (based on avian eggshell), including the membrane, mammilla, prismatic layer, and external layer as seen under SEM. The prismatic layer and external layer make up the upper shell unit, which is distinctly different from the mammilla layer. Figure based on similar images from Mikhailov (1991), Hirsch (1994), and images from the Hirsch Eggshell Collection.
Figure 5. Examples of different eggshell textures: A. Eggshell with a smooth outer surface (from Troodon), Specimen UCM 378; B. Eggshell with ridges (probably hadrosaurian), Specimen UCM 346-2-5; C. Eggshell with nodes (unidentified egg-layer), Specimen UCM 346-4. All eggshell fragments are from the Late Cretaceous of Montana.
Figure 6. Eggs come in different shapes from (A) spherical to (B) elliptical. A. Recent turtle (Trionyx spiniferus) eggshell, Specimen UCM 192. B. Incomplete Late Cretaceous theropod dinosaur (Troodon formosus) eggshell. Specimen UCM 371-1, photograph KH 90.06A.
Ultrastructure is a term referring to the finely detailed crystalline organization present in an eggshell such as radial and tabular crystal structure, squamatic texture, and basal plate group. This may include the transition between different zones of organization (ultrastructure zones) within the shell unit. Ultrastructure describes features produced by the interaction of the organic and inorganic components of the shell unit, but many researchers now include these features in descriptions of the microstructure.
Our knowledge of modern eggshell is used as the basis for the description, classification, and identification of fossil eggshell. Fossil eggs can only be definitely identified when the egg contains identifiable embryonic remains or is preserved within the body cavity of the adult, both of which are rare in the fossil record. However, comparisons between the structures of fossil and recent eggshell can sometimes be used to classify and even identify fossil eggs at higher taxonomic levels such as turtle, bird, or crocodilian.
Eggshells display different types of surface ornamentation some are smooth, whereas others have distinctive nodes or ridges. Sometimes the surface texture is an artifact of geologic processes such as physical and chemical weathering. Microstructure analysis can sometimes determine which processes caused these surface textures, but often the surface ornamentation is significantly altered. Fossil dinosaur eggs often show various sculpture patterns, but the function of this ornamentation is not clearly understood at this point, in part because modern eggs do not show the same patterns; most modern eggshell has a relatively smooth external surface. (Fig. 5)
Animals that lay eggs come in a variety of sizes, and so do their eggs. However, the size of the egg is not necessarily correlated with the size of the egg-layer. For example, the long-necked sauropods were among the largest terrestrial animals to inhabit the Earth, with some weighing up to 100 tons. However, sauropod eggs from Argentina that contain fossilized embryos are only about 15 cm in diameter. This is smaller than the eggs of some living birds!
Egg shape is usually fairly consistent within a given species, but eggs from different species can vary in roundness, and include spherical, subspherical, and elongate shapes with the ends either equal or asymmetric. In modern amniotes, egg shape appears to have evolved for different reasons: to reduce the amount of pressure at any one point on the outer surface of the shell, to increase surface area available for gas exchange in large clutches, for closer packing in the oviduct in small species, and even to prevent eggs from rolling off of cliffs. The curvature of the inside of the egg also allows the hatching animal to break out of the shell with relative ease. Egg shape can also be tied to the mother's reproductive physiology. Birds and some theropod dinosaurs produce asymmetric eggs, a shape that results from having only one egg in the oviduct at a time, as opposed to the mass egg production by reptiles. (Fig. 6)
Eggshell thickness varies among amniotes based on the size of the eggs, reproductive behavior of the adult, and the general shell morphology. For example, a large egg typically requires a thick eggshell due to its greater weight. In some cases, the shape and orientation of the egg in the sediments has also been tied to breaking strength and brooding behavior of the adult. Extremely large eggs may also show calcite structural features that provide additional strength. In modern animals, the day gecko (Phelsuma madagascariensis) and the recently extinct elephant bird Aepyornis represent opposite ends of the eggshell thickness spectrum, with thicknesses of ~0.15 mm and ~3.5 mm, respectively! Some dinosaur eggs from Argentina, however, are about 5 mm thick.
Surface pore pattern
Pores are important features of amniote eggs. They allow the exchange of gases and water between the developing embryo and the environment. Since pores are open to the atmosphere for gas exchange, they are visible on the eggshell surface with, and sometimes without, a microscope. Pore arrangement, density, and relationship with surface texture can aid in eggshell description and classification. Shell thickness and pore area can also be used to calculate the rate at which gasses enter and exit the egg in modern and fossil eggs. These calculations from fossil eggs provide information about ancient incubation environments relative to those of modern reptiles and birds. Some researchers have also tied pore patterns to environmental conditions such as aridity (see "Paleobiology and eggs").
To study the microstructure of eggshell, researchers examine cross sections of eggshell (in thin sections) and use scanning electron microscopy (SEM) to observe and describe the internal (versus surface) structure and orientation of the calcium carbonate crystals and shell units. The shell unit is the basic component of eggshell microstructure, and the crystalline structure of shell units differs in various amniote eggs (Fig. 4). The eggshell membrane that covers the egg contents (embryo, yolk, albumin) has organic centers on the outer surface (relative to the embryo) that serve as nuclei for deposition of the calcite crystals that form the shell. In birds and theropod dinosaurs, the cone-shaped mammillae form the base of the shell units and are distinctly different from the overlying layer. This overlying layer consists of prisms that are visible throughout the eggshell or may be obscured by "squamatic" texture, depending on the species. (The "squamatic" layer derives its name from an unusual texture that typically obscures the internal crystalline structure; on the other hand, prisms in a prismatic layer are easily recognized.) The transition between the mammillary and overlying layer may be gradual or abrupt, depending on the egg type. Although rare in non-avian theropod dinosaurs, many bird eggs include a third layer called the external layer. Where present, the transition to an external layer at the outer surface of the egg may also be gradual or abrupt. Figure 4 shows an idealized diagram of an avian eggshell with three distinct layers: the mammillary layer, the prismatic or palisade layer, and the external layer. The structural characteristics of the eggshell vary according to species. For example, in contrast to avian and non-avian theropod eggs, herbivorous dinosaurs usually produce eggs with interlocking shell units that typically consist of a single layer.
Pore pattern is important in the classification of eggshell; however, because several types of eggshell have similar morphology, the pore structure should be used in conjunction with other characteristics. Orientation (to both inner and outer eggshell surfaces), size, and pore shape are considered in classification. Dinosaur eggshells display several different pore types and their morphology may be linked not only to the environment of incubation but to climatic conditions as well.
Ultrastructure has been used to refer to the finer internal texture of the eggshell and the arrangement of aragonite or calcite crystals and organic matter within different zones of the shell unit. Types of ultrastructural zones found in fossil and modern eggshell include: an arrangement of basal plate groups, aragonite radial ultrastructure, calcite radial ultrastructure, tabular ultrastructure, squamatic ultrastructure, radial-tabular ultrastructure, and external ultrastructure.
Uses of morphological and structural descriptions
Eggshell morphologies and microstructural patterns can be useful for identification because they are genetically controlled and because combinations of structures and organizational patterns may be unique to certain amniotes. Several classification schemes have been used in the past, including the designation of "basic types" and "structural morphotypes" that in some cases attempt to tie eggshell structures to specific organisms. Parataxonomy is a classification system used to identify eggshells based solely on shell morphology and microstructure (independent of taxonomic association). Today, research is trending towards a cladistic approach that analyzes discrete eggshell characters in an evolutionary framework. To learn more about past and future trends in eggshell identification, check out the next page "Eggshell identification: Who laid the egg?".
Carpenter, K. 1999. Eggs, Nests, and Baby Dinosaurs: A Look at Dinosaur Reproduction. Indiana University Press. 338 pp. Chapter 8 (pp. 135-144), "How to study a fossil egg," focuses on eggshell classification.
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.
Jackson, F.D., D.J. Varricchio, R.A. Jackson, B. Vila, and L.M. Chiappe. 2008. Comparison of water vapor conductance in a titanosaur egg from the Upper Cretaceous of Argentina and a Megaloolithus siruguei egg from Spain. Paleobiology 34(2):229-246.
Mikhailov, K.E. 1991. Classification of fossil eggshells of amniotic vertebrates. Acta Palaeontologica Polonica 36(2):193-238.
Mikhailov, K.E., E.S. Bray, and K.F. Hirsch. 1996. Parataxonomy of fossil egg remains (Veterovata): Principles and applications. Journal of Vertebrate Paleontology 16(4):763-769.
Mikhailov, K.E. 1997. Fossil and recent eggshell in amniotic vertebrates: Fine structure, comparative morphology and classification. Special Papers in Palaeontology (56):1-80.
Seymour, R.S. 1979. Dinosaur eggs: Gas conductance through the shell, water loss during incubation and clutch size. Paleobiology 5(1):1-11.
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Figures 1-3 © R. Neuser and Lumic; shell unit components graphic by Laura Wilson; Figures 5 and 6 courtesy of the University of Colorado Museum.