percolating through pores or cracks in the shell. Throughout the fossilization process the calcium carbonate composing the eggshell generally remains unchanged, allowing scientists to study its original structure. However, egg fossils buried under sediments at great depth can be subjected to heat, pressure and chemical processes that can alter the structure of its shell through a process called diagenesis
.
Laying and burial
The formation of fossil eggs begins with the original egg itself. Not all eggs that end up fossilizing experience the death of their
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.[3]
Preservation
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. This process involves
precipitate out of solution. If enough minerals make it inside the egg can become sturdy enough to withstand the weight of the overlying sediments.[2]
When the egg is buried deeply enough, the bacteria decomposing the egg are cut off from
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.[4]
Depositional environments
sauropod dinosaurs who chose the site based on how easy beach sand would be to dig nests into and because such sands would absorb enough heat to help incubate the eggs.[5]
mudstones deposited in ancient floodplain environments as one of the best places to find fossils of dinosaur eggs. Dinosaurs would bury their eggs on the floodplain where the periodic floodwaters would carry the sediments that would bury and preserve the egg.[3]
Sand dunes: Many dinosaur eggs have been preserved in sandstone formed from the sands of ancient deserts in modern Mongolia and northern China.[6] The presence of Oviraptor preserved in their life brooding position suggests that sandstorms may have been the primary way that the eggs found in the deposits were buried.[5]
gravidfemales being washed out to sea and releasing its eggs when by splitting open, carried out to sea by floods, or drifting out on mats of vegetation.[1]
volcanic debris deposits although not lava deposits. At least two clutched of hard-shelled turtle eggs preserved in this manner are known from the Canary Islands. These eggs were likely buried in the debris by large tortoises. Not all volcanic debris deposits are capable of preserving eggs, however, because acids in these sediments can dissolve eggshell.[8]
Alteration
After burial eggs can still be altered. This process is called
Silica can be incorporated into fossil eggshells, but this process is damaging to the shell's internal structure due to the difference in size between silica molecules and the calcite molecules. In addition to calcite and silica other minerals can be present in small amounts in fossil eggs, especially iron. Iron sometimes 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.[10]
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