Dinosaur eggs contain a wealth of useful biogeochemical information pertinent to how dinosaurs took elements into their bodies and used them. Relevant chemical constituents of eggshells include:
Calcite, which provides information about the degree of mineralization of the egg as well as the effects of diagenesis (Chapter 7). Stable isotopes for oxygen, which can indicate the temperature of the original environment inhabited by the egglayer; and for carbon, which can reflect dietary choices by the egglayer.
Trace elements, which may give clues about paleopathology, paleoenviron-mental conditions, and diagenesis (Chapter 7).
Amino acids and proteins, whose presence and proportions can be compared to those in modern eggshells, which in turn can be used for interpreting the relatedness of dinosaurs to extant animals, perhaps indicating a similar physiology.
Once a dinosaur eggshell is determined as relatively unaltered, a geochemical analysis may begin with calculations of stable isotope ratios.
Calcite was first recognized as a constituent in dinosaur eggs in 1923, and in these and other fossils it is normally re-crystallized. Original, non-recrystallized calcite is diagnosed by preservation of fine-scale structures in the eggshell and open pores. The opposite characteristics indicate at least one episode of recrystallization. A paleontologist or geochemist considering an elemental analysis of dinosaur eggs always tries to determine the degree of recrystallization first, because a recrystallized eggshell may contain chemicals that were more recently acquired.
Stable isotopes differ from radioactive isotopes (Chapter 4) as they do not undergo radioactive decay. Oxygen isotopes, 16O and 18O, are the most commonly used stable isotopes. Their ubiquitous presence in eggshell material as a component of CaCO3 makes them a natural choice for study. The principle behind using them is that changes in the ratio of 18O to 16O can be a direct consequence of temperature changes in waters of ancient environments. Because 18O is a heavier isotope than 16O, it tends to remain in water undergoing evaporation. Hence, higher temperatures cause higher rates of evaporation. This results in a depletion of 18O in the evaporated water and an
18O/16O ratio lower than that in the original water source. Conversely, glaciation of an area would cause an enrichment of 16O in the ice. This condition causes a higher ratio than in tropical water. The fractional change in the amount of stable isotopes, where one becomes depleted while the other is enriched, is called fractionation. The value derived from the ratio of 18O to 16O is called the S18O value.
Organisms use water to make shells or bones. Consequently, the 518O value of an organism's unaltered bones or eggshell should reflect the temperature of the water used in the precipitation of the calcite or dahllite. This method has been used, with calcite from fossil unicellular organisms and some mollusks, to successfully interpret paleotemperatures. Unfortunately, its use with dinosaur eggshells is still controversial because of the high likelihood that most eggshells have been recrys-tallized. These eggs would have 818O values that represent the temperature of the water used in the recrystallization process during diagenesis, rather than the temperatures of water used by a dinosaurian mother. However, once more unaltered dinosaur eggshells are discovered, the method will have important paleoecological applications. Oxygen isotopes have also been used to interpret body-temperature regulation in dinosaurs, as discussed later. Nevertheless, this is not without dispute, again because of the same effects of diagenesis.
Oxygen isotopes provide information about paleotemperatures, but carbon isotopes are instructive about what dinosaurs ate or drank. With carbon isotopes, 12C is enriched relative to 13C through photosynthesis. As a result, plants have lower 813C values than dahllite in its mineral form. Modern herbivores can affect the carbon isotope ratio by eating plant material, which is enriched in 12C; this manifests as a low 813C value in the herbivores' bones. The same principle has been extended
Was this article helpful?