Taphonomy
The formation of fossil eggs begins with the original egg itself. Not all eggs that end up fossilizing experience the death of their embryo beforehand. Fossil eggs with open tops are common and could result from the preservation of eggs that hatched successfully, although in other cases the egg was opened by predators attempting to feed on it. Specimens left by eggs that didnot hatch sometimes contain fossil fecal pellets left by insect larvae inside them, which implies insects like flies laid their eggs there for their larvae to scavenge upon the dead embryo. Modern reptile and bird eggs are commonly killed by overheating, drowning, dehydration, asphyxiation from being buried too deep, disease, or congenital problems. In the case of eggs dying by asphyxiation if they're buried deep enough insects are unlikely to scavenge them. However, even in the absence of insects, fungi, bacteria, and other microscopic life participate in decomposition. Whether or not hatching was successful water and wind would fill the egg with sediment through any large openings. Not all fossil egg specimens are of complete specimens. Individual pieces of eggshell are much more robust than the entire egg. This strength comes from the organic matter that cements the eggshell's calcite crystals together. Simple experiments have demonstrated that under certain conditions eggshell can be transported for 68 kilometers or 42 miles with little loss of size. The durability of eggshell under transport means that pieces of fossil eggshell aren't necessarily discovered in deposits geographically close to the nest they originated from.
More complete eggs in the process of fossilization are gradually buried until the weight of the sediment overtop them causes them to crack. These cracks allow even more sediments to fill the eggs. Sometimes, though, fossilization can begin fast enough to prevent the eggs from being cracked. If the watertable is high enough dissolved minerals can percolate through the pores of the eggshell, filling the egg with minerals until its sturdy enough to withstand the weight of the overlying sediments. Calcite is the primary mineral responsible for this quick fossilization and makes its way to the egg from calcium carbonate dissolved by naturally occurring acids in rainwater. These naturally occurring acids include carbonic acid formed by interactions between rainwater and carbon dioxide in the atmosphere as well as acids formed by plant decomposition in the soil.
When the egg is buried deeply enough, the bacteria decomposing the egg are cut off from oxygen and begin to power their metabolisms using different energy sources. These processes can result in certain minerals being precipitated out of the groundwater selectively. The bacteria also dispose of metabolic waste using ions. Ken Carpenter observes one common method links Co2 to Ca2+ ions to produce calcium carbonate. Some of the bacteria involved in the decay process use the conversion ofnitrogento ammonia to power their metabolisms. When this ammonia leaks out of the egg it raises the surrounding pH into the alkaline and can change what minerals will precipitate out of the water. The organic material in the eggshell itself can lead to the precipitation of calcite out of solution.This often causes fossil eggshell to be covered in a thin layer of calcite that complicates the identification process. However, since eggshells are usually chiefly composed of calcite anyway the eggshell itself is mostly composed of the original calcite it had in life. The lack of change in composition and structure despite undergoing fossilization allows scientists to study the original structure of the shell.
However, eggs can also sometimes be chemically altered after burial. This process is called diagenesis. One common form of diagenesis affecting fossil turtle eggs is the conversion of aragonite, which is unstable, into the regular calcite more typical of reptile eggs. This reaction is triggered by heat when the egg is buried deep enough. In other eggs the pressure from being buried imposes a cross-hatched pattern on the calcite when viewed through a microscope. In more extreme cases the eggshell's internal structure can be obliterated completely. Silica is another substance that can precipitate out of solutions under alkaline conditions and therefore sometimes is present in fossil eggs. When silica is incorporated into fossil eggshell the internal structure is severely damaged due to the difference in size between silica molecules and the calcite molecules in the shell. In addition to calcite and silica. other minerals can be present in small amounts in fossil eggs. The most important of these is iron. Iron sometime tints eggshell black, possibly when the iron is in the form of iron sulfide or pyrite. Iron can also tint eggs a reddish color when in the form of iron oxide, or hematite.
Read more about this topic: Dinosaur Egg