Abstract
The cuticle layer consisting mainly of lipids and hydroxyapatite (HAp) atop the mineralized avian eggshell is a protective structure that prevents the egg from dehydration and microbial invasions. Previous ornithological studies have revealed that the cuticle layer is also involved in modulating the reflectance of eggshells in addition to pigments (protoporphyrin and biliverdin). Thus, the cuticle layer represents a crucial trait that delivers ecological signals. While present in most modern birds, direct evidence for cuticle preservation in stem birds and non-avian dinosaurs is yet missing. Here we present the first direct and chemical evidence for the preservation of the cuticle layer on dinosaur eggshells. We analyze several theropod eggshells from various localities, including oviraptorid Macroolithus yaotunensis eggshells from the Late Cretaceous deposits of Henan, Jiangxi, and Guangdong in China and alvarezsaurid Triprismatoolithus eggshell from the Two Medicine Formation of Montana, United States, with the scanning electron microscope (SEM), electron probe micro-analysis (EPMA), and Raman spectroscopy (RS). The elemental analysis with EPMA shows high concentration of phosphorus at the boundary between the eggshell and sediment, representing the hydroxyapatitic cuticle layer (HAp). Depletion of phosphorus in sediment excludes the allochthonous origin of the phosphorus in these eggshells. The chemometric analysis of Raman spectra collected from fossil and extant eggs provides further supportive evidence for the cuticle preservation in oviraptorid and probable alvarezsaurid eggshells. In accordance with our previous discovery of pigments preserved in Cretaceous oviraptorid dinosaur eggshells, we validate the cuticle preservation on dinosaur eggshells through deep time and offer a yet unexplored resource for chemical studies targeting the evolution of dinosaur nesting ecology. Our study also suggests that the cuticle structure can be traced far back to maniraptoran dinosaurs and enhance their reproductive success in a warm and mesic habitat such as Montana and southern China during the Late Cretaceous.
Highlights
Cuticle structures and functionsAn avian egg is an evolutionary invention that protects the developing embryo against mechanical damage, dehydration, and microbial invasion (Romanoff & Romanoff, 1949; Board & Fuller, 1974)
On the outside of Macroolithus yaotunensis and Triprismatoolithus stephensi eggshell fragments, we found peaks in these spectral regions (Fig. 2A)
The chemometric analysis using ChemoSpec suggests that both chicken and dinosaur eggshells show similar spectral pattern between 800 cm−1 and 1,200 cm−1 (Fig. 2B)
Summary
An avian egg is an evolutionary invention that protects the developing embryo against mechanical damage, dehydration, and microbial invasion (Romanoff & Romanoff, 1949; Board & Fuller, 1974). This multi-functionality is contributed to by both mineralized and non-mineralized organic layers, which exhibit unique biomaterial properties. The final layer to be deposited, the eggshell cuticle, represents the interface between the embryo inside and its outside environment. The cuticle layer is often described as ‘‘waxy’’ due to its high amount of lipids, which keep the internal fluids from evaporating and protect the encased embryo from desiccation. The avian cuticle contains proteins, polysaccharides, calcium carbonate (vaterite), calcium phosphates (hydroxyapatite, HAp), and pigments (Wedral, Vadehra & Baker, 1974; Nys et al, 1991; Packard & DeMarco, 1991; Dennis et al, 1996; Mikhailov & British Ornithologists’ Club, 1997; Fraser, Bain & Solomon, 1999; Cusack, Fraser & Stachel, 2003; Igic et al, 2015)
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