The organic phase in dinosaur eggshell: analysis, models of preservation, and implications for the evolution of bird reproduction

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It has long been recognized that in addition to their shape, eggs and eggshells also preserve their histological structure, giving rise to the field of eggshell paleohistology. Eggshell is a biocomposite material just like bone, but it is mineralized to a much higher degree. In addition, eggshell represents a more promising matrix for the preservation of endogenous biomolecules than bone because it is less affected by dissolution-remineralization processes during fossilization. This stable, crystalline matrix contains minor amounts of organic constituents, the organic phase (OP), which plays a role in embryo development, strengthening of the eggshell, and egg coloration. Composite systems of biominerals and organic constituents are known to form a local chemical environment that allows the almost unmodified preservation of its organic constituents over geological time. The applicants tested this assumption on eggshell color pigments in fossilized dinosaur eggs. Based on the extensive fossil eggshell collections and fruitful Chinese collaborations of the Steinmann Institute, we detected fossil egg pigments successfully and in significant amounts for the first time. Key to these discoveries were the shared expertise from the fields of natural product and metabolite extraction from biological matrices (lab of Prof. Christa Müller) and organochemical high-end analytical techniques (lab of Dr. Marianne Engeser), e.g., liquid chromatography (HPLC) ESI Q-TOF mass spectrometry. Specifically, we detected the eggshell metabolites biliverdin and protoporphyrin in eggs of the oviraptorid dinosaur Heyuanni huangi. These compounds color bird eggs, serving in detection of parasitism and in egg crypsis. Because the uterine de novo synthesis of such pigments and their incorporation into eggshell was believed to be restricted to extant birds, this research has important evolutionary implications, including aspects of sensory and behavioral ecology. The general goal of the proposed project is thus to understand the material nature of dinosaur and Mesozoic bird eggshell. The focus will be on the organic phase (which remains largely unstudied as opposed to the mineral phase), exploring the range of biomolecular preservation in eggshell. From a theoretical point of view, eggshell offers an ideal matrix for biomolecular preservation, corroborated by our preliminary work. In addition, the eggshell pigments are cyclic or linear tetrapyrroles that form a stable class of molecules. We intend not only to detect, locate, and identify degradation products of preserved pigments, but there is potential for the preservation of the protein compounds that build up the squamatic scaffold in eggshell. Our research is set against the background of the evolution of avian reproductive traits for which a refined understanding of the fossil egg record is crucial.