Evolutionary Time Capsules

Dinosaur eggs have been found on all continents except Antarctica. Among the oldest are those associated with the tiny prosauropod hatchlings of Mussasaurus from the late Triassic of South America. The few known late Jurassic sites are restricted to North America, Europe, and Tanzania. Eggs from the early Cretaceous are far more abundant, but with the exception of one site from southern Australia, all are restricted to the Northern Hemisphere. Although a large number of late Cretaceous sites yielding dinosaur eggs have been found across North America, Europe, and Asia, relatively few have been discovered in the Southern Hemisphere. Only South America and India have provided evidence of dinosaur nesting sites in the southern continents at the end of the Mesozoic, probably due to the limited amount of paleontological exploration of the continents that occupied the Southern Hemisphere at that time (including India).

An egg for an embryo is like a space suit for an astronaut. A dinosaur egg has numerous special structures to nourish the embryo growing inside and provide protection from the hostile environment outside. The most obvious is the shell. A few reptiles, such as sea turtles, some lizards, and snakes, lay flexible or soft-shelled eggs, but in dinosaurs, as well as in most other reptiles, the shell is primarily composed of hard mineral crystals called calcium carbonate —the same basic material that makes up limestone and cement. These crystals, which are arranged into distinctive structural units that fit tightly together and occasionally interlock, form a hard shell that cre ates a formidable barrier against bacteria, fungi, and other organisms that can cause disease. However, the shell cannot be too hard, or the embryo cannot break out of it when it is ready to hatch. In actuality, even hard eggshell is quite porous, with microscopic holes that allow gases and water vapor to pass in and out. Oxygen penetrates through the pores into the egg so the embryo can breathe, and carbon dioxide passes out through the pores into the atmosphere.

Inside the egg, a flexible container composed primarily of proteins and fat called the yolk sac (the yellow part of a chicken egg) contains food for the growing embryo and antibodies to help protect the embryo from disease. Another membranous sac, the allantois, serves as a receptacle for waste products. The yolk and allantois are surrounded by albumen, a water-saturated gel that absorbs whatever shocks might jostle the embryo developing at the center of the yolk. The albumen also contains more chemical components to help fend off dangerous microbes. All these structures work together to keep the embryo at a constant temperature inside a fluid environment that cushions it from the extremes and threats of the world outside.

Eggs are typically classified based on visible characteristics of the egg and the structure of its shell, including size, shape, distribution

Main sites where dinosaur eggs have been found.

and number of pores, and ornamentation on the shell's surface. At the microscopic level, eggs are classified based on the characteristics of crystalline units that form the shell, the structure of the pore canals, and the chemical composition of the crystalline layers that make up the shell. Such detailed study requires microscopes that analyze polarized light and scanning electron microscopes, also known as SEMs, which magnify the features of the shell tens or hundreds of thousands of times.

Based on the characteristics of the shell units, hard-shelled eggs of living animals are divided into four basic categories: testudoid (some turtles), geckonoid (geckos), crocodiloid (crocodiles), and omithoid (birds). These eggs come in two main structural types: those made of calcium carbonate crystals called aragonite (some turtles), and those made of calcium carbonate crystals with a slightly different chemical

Chemical Composition Eggs
Main functions of an egg.

composition called calcite (crocodiles, geckos, birds, and other dinosaurs).

Some dinosaur eggs, including those of birds, fit into the ornithoid type, but two other categories have been created to accommodate the rest of the dinosaur eggs that have been found. These are the dinosauroid spherulitic and the dinosauroid prismatic types, which are based on the form and structure of the crystalline units that make up the shell, just as in the case of living animals. To see these features, the shell is often sliced to create a thin cross section that cuts through the crystalline units, and the shell structure is then observed through a polarizing microscope, which utilizes light that vibrates in only two planes. French scientists first used this traditional method of slicing the shell into thin sections in the late 1800s, when research on the microscopic structure of fossil eggshell originated.

This approach allows paleontologists to identify the structural type of the shell, but a more comprehensive understanding of the shell's structure can be obtained when this approach is combined with observations made with a SEM. To be viewed with the SEM, the pieces of the eggshell often need to be coated with a thin layer of gold. Some newer SEMs, called environmental SEMs, can observe the shell without this coating, which is great for viewing unique specimens that paleontologists cannot risk coating or damaging, but the resolution of the images is not as good. Another approach is to view the shell

Gecko Eggshell Sem
Turtles, crocodiles, and dinosaurs have specific types of eggshell microstructures. The study of eggshells is an active field of vertebrate paleontology.

from directly above the outer and inner surfaces, rather than in cross section. Both of these approaches provide information about a shell's crystalline structure and pore patterns.

Dinosaur eggs come in many shapes: round like a softball, oval like a football, and elongated like a loaf of French bread. They also range greatly in size, with those of many extinct Mesozoic dinosaurs being as large as that of an ostrich. Interestingly, the dinosaur eggs exhibiting the greatest variation in size are those of the dinosaurs that still live, birds. They can be as tiny as those of a hummingbird or as large as those of an elephant bird, a large, flightless species that lived in Madagascar until a thousand years ago and laid eggs ten times the size of an ostrich egg, much larger than those of any Mesozoic dinosaur.

Our Patagonian eggs are round and relatively large. With an average diameter of five to six inches, they are about the size of a softball and have a volume roughly equivalent to that of a dozen chicken eggs. Today most eggs are preserved in the shape of a disk, probably a consequence of compaction when they were buried beneath thick layers of rock for millions of years, but originally, they were probably almost spherical. Their dark gray surface has a dense ornamentation of small bumps that sometimes coalesce into ridges shaped like worms, but the distinctness of this ornamentation varies widely, probably due to water seeping through the ground and partially dissolving the calcite shell. It is not possible to tell whether the dark gray color we see today was the original color of the egg or whether it resulted from changes that occurred during fossilization. It is also impossible to tell whether the eggs originally had a marbled coloration, typical of many living birds.

Three different structural types of dinosaur eggshell are typically recognized, although the boundaries between these categories are somewhat fuzzy One widespread type is termed spherulitic, in which an inner core supports stacks of calcite crystals that radiate out from it. This structural type is usually considered typical of all dinosaurs except theropods, and the eggs from Auca Mahuevo are of this type. In the second type, called prismatic, the shell is composed of an inner core that supports two crystalline layers, but the boundary between them is poorly defined. The inner layer is formed by the same radiating crystalline structure typical of the external layer in spherulitic eggshell, while the outer layer has a more prismlike struc-

Shapes And Sizes Dinosaur Eggs

Dinosaurs laid eggs of various shapes and sizes. The round eggs of the Auca Mahuevo sauropods (center right) greatly differ from the elongate eggs of carnivores such as therizinosaurs (top left, about 12 inches long) and ovirap-torids (bottom left, about 8 inches long). Bird eggs (bottom right) are also dinosaur eggs.

ture. Prismatic eggshell appears to be typical of primitive theropods. The third structural type, called ornithoid, is also composed of an inner core that supports two or more crystalline zones, but in this case, the boundary between these crystalline zones is abrupt. The lower or inner part of the shell is similar to that of the prismatic type, but the upper or outer part has a scaly appearance. Ornithoid structure is typical of advanced theropods, including all birds.

This threefold classification based on microscopic structure has dominated studies of dinosaur eggshells for several decades, but recent investigations suggest that some revisions are necessary. Just as the bony structures of dinosaur skeletons can be used to classify dinosaurs into different evolutionary groups, so can the microscopic structure of their eggshells. If the cladistic method is used, eggshell structure might provide important new information about how different groups of dinosaurs evolved from one another, since the structure of dinosaur eggs evolved right along with the animals that laid them. Initial research along these lines has suggested that the existing, threefold system used to categorize dinosaur eggs may be hindering our attempts to glean information about dinosaur evolution from the structure of their eggshells.

Paleontologists have, therefore, begun to reanalyze and recategorize different kinds of dinosaur eggshell, and the preliminary results seem promising. For example, all dinosaur eggshells, including those seen in bird eggs, grow outward from an inner core often referred to as the organic core, which indicates that all the different kinds of dinosaur eggs evolved from one kind of egg laid by a single common ancestor. In crocodiles, distant relatives of dinosaurs, the inner organic core is poorly differentiated if not absent. This is not surprising, since the structure of the hips in all dinosaurs, including birds, also suggests that dinosaurs evolved from a single common ancestor, as we discussed in an earlier chapter. Furthermore, the presence of two or more distinct structural layers in the eggshells of birds and theropod dinosaurs provides more evidence to suggest that birds evolved from meat-eating theropods. The ornithoid type of eggshell found in living birds is composed of a minimum of two distinct layers. The inner layer is composed of calcite crystals that radiate out from the core, while the outer layer(s) have a more scaly appearance. A similar structural composition is found in some maniraptors, suggesting that birds and maniraptors evolved from the same common ancestor, one that laid eggs with shells divided into two or more layers. Again, as noted earlier, the bony structure of the wrist and other skeletal parts, along with the presence of feathers in some theropods and birds, suggests the same thing. Perhaps the restriction of the radial section of the dinosaur prismatic and ornithoid types to the inner portion of the eggshell is a characteristic that blurs the distinction between these two types. Alternatively, it may be that dinosaurs that laid these two types shared a common evolutionary origin that was not shared by the dinosaurs that laid the dinosauroid spherulitic type of egg. Such research in the field of dinosaur eggshell structure remains in its infancy, and further investigations within this relatively new field may provide significant new insights on the evolutionary history of dinosaurs.

Returning to our discovery, the eggshell in the Auca Mahuevo eggs is rather thin, somewhat less than one-tenth of an inch thick. Although this may seem thick in relation to a chicken's egg, it is indeed much thinner than the eggshell of other similar eggs that have been found in Patagonia and Europe. We were lucky in that the microscopic structure of the eggs we collected was preserved in perfect detail. By cutting thin slices through the shell and studying the shell structure with a stereo electron microscope, we could see that our eggs belonged to the spherulitic type, which suggested that they did not belong to theropods. This left sauropods and omithischians as possible candidates. The Auca Mahuevo eggs were of a shape, size, and structure consistent with an egg type referred to as megaloolithid. A variety of megaloolithid eggs have been found in various late Cretaceous sites in South America, as well as in India and Europe. Much older megaloolithid eggshell has also been found in the late Jurassic of North America and Europe. Megaloolithid eggs have often been thought to belong to sauropod dinosaurs, but no such egg had ever been found with an embryo inside.

Based on evidence from sites found in many different parts of the world, we know dinosaurs laid their eggs in a variety of different patterns. Some dinosaurs laid their eggs in well-defined nests. Large concentric circles with pairs of eggs are typically found in nests of theropod dinosaurs, such as troodontids and oviraptorids, two kinds of maniraptors. The troodontids lay their eggs vertically, half-buried in the substrate, whereas oviraptorids lay them horizontally facing the center. Other dinosaurs laid their eggs in a spiral pattern within the nest. Still others laid their eggs in rather poorly defined patterns within a nest, and some laid them randomly across an area without a nest. These two latter patterns are typically the way in which mega-loolithid eggs are found. In some Patagonian sites, megaloolithid eggs have been found in close clusters of six to ten eggs, whereas at a site in southern France, megaloolithid eggs were laid in what appears to be large semicircular arcs that do not seem to represent what we typically think of as a nest.

The distribution of the eggs at Auca Mahuevo was intriguing. In some places, the eggs appeared to be scattered over large areas, almost like a carpet of eggs. In other areas, they appeared to be more clustered, suggesting that nests may have been present. Clarifying the pattern of their distribution would clearly involve a lot of work and time. We were so busy just trying to collect egg fragments on the surface that contained fossilized patches of skin and quarrying more complete specimens of eggs that appeared to contain embryonic bones that we decided that studying the distribution of the eggs would have to wait until another season. Back in the laboratory, our top priority was to prepare the embryonic bones and see if they came from an ornithischian or a sauropod.

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  • fethawit
    What are dinosaur capule eggs made of?
    7 years ago

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