Microstructural description of the maniraptoran egg Protoceratopsidovum

Abstract

Since their discovery in the 1920s, some asymmetric, elongated dinosaur eggs from the Upper Cretaceous of Mongolia have been interpreted as ceratopsian eggs. However, recent views support a maniraptoran affinity mainly based on macroscopic features. Technical advancements in palaeontology provide a novel approach to diagnose maniraptoran eggs, and the discovery of soft ceratopsian eggs makes the debate a timely issue worth revisiting. Here, we analysed Protoceratopsidovum eggshell from southern Mongolia with electron backscatter diffraction and the results were compared with East Asian and North American maniraptoran and ornithischian eggs. The microstructure and crystallography of Protoceratopsidovum confirms the maniraptoran hypothesis, given that it shows diagnostic features of the clade absent from ornithischian (e.g. hadrosaur) eggs. Additional characters, such as egg shape, ornamentation, egg pairing and clutch structure also support a maniraptoran affinity. Protoceratopsidovum has both elongatoolithid (oviraptorosaur egg) and prismatoolithid (troodontid egg) features, and it may bridge the morphological gap between the two established oofamilies. Considering the similarity between Protoceratopsidovum and potential dromaeosaur eggs, at least some Protoceratopsidovum may be deinonychosaur eggs. Fossil localities that yield both deinonychosaurs and Protoceratopsidovum may yield conclusive evidence for the true affinity of this ootaxon. Future discovery of the egg‐laying taxon of Protoceratopsidovum will not only improve our understanding of maniraptoran reproductive biology but also make Protoceratopsidovum a reliable indicator of the palaeobiogeography of that maniraptoran clade.

Microstructural and crystallographic evolution of palaeognath (Aves) eggshells

“…tinamou, kiwi, and Lithornis eggshells) is represented by needle-like ML, splaying SqZ, and massive EZ (but see below). Noticeable qualitative features (Figure 13) of palaeognath eggshells are: (i) calcite grains have strong vertical c -axis alignment (hence, mostly reddish in IPF Y mappings), which may be homologous to that of Mesozoic maniraptoran eggshells (Moreno-Azanza et al, 2013; Choi et al, 2019, 2020, 2022); (ii) ML is mostly composed of wedge-like calcite but needle-like calcite is present or dominant in tinamou-style eggshells (Zelenitsky and Modesto, 2003; Grellet-Tinner and Dyke, 2005; Grellet-Tinner, 2006; Appendix 1—figure 3); (iii) tinamou-style eggshells have round (or barrel-shaped) ML (Grellet-Tinner, 2006); thick moa eggshell appears to have round ML (Appendix 1—figure 3); (iv) ML and SqZ are easily differentiated due to grain shape differences except for in ostrich-style eggshells where the boundary between the two layers is unclear; (v) calcite in SqZ are mostly ‘splaying’ ( sensu Panhéleux et al, 1999). However, calcite in SqZ of ostrich-style eggshells are nearly prismatic; (vi) EZ exists in all palaeognath eggshells; (vii) cassowary and emu eggshells have peculiar ornamentation on the outer surface (= ‘granular layer’ sensu Mikhailov, 1997a) and very porous outer EZ (see Appendix 1—figure 2).…”

“…Non-avian maniraptoran eggshells (see Appendix 1—Figures 9–12 for selected examples) have strong vertical c- axis alignment as in palaeognath eggshells (Moreno-Azanza et al, 2013; Choi and Lee, 2019; Choi et al, 2019, 2020, 2022). Shell unit structure of oviraptorosaur eggshells is similar to that of rhea-style palaeognath eggshells except for needle-like ML, whereas shell unit structure of troodontid eggshell is strikingly similar to ostrich-style palaeognath eggshell.…”

“…The eggshells of three species were presented in the main text (Figure 13 carlylei sensu Simon et al, 2018). Prismatoolithus levis is an ootaxon of Troodon formosus (Troodontidae;Varricchio et al, 2002) and the materials are from an egg that contains an embryo (MOR 246;Horner and Weishampel, 1988;Varricchio et al, 2002;Choi et al, 2022).…”

“…Three oospecies (parataxonomic classification of fossil eggshell) are presented in the main text: Prismatoolithus levis, Elongatoolithus oosp ., and Macroelongatoolithus xixiaensis (or M. carlylei sensu Simon et al, 2018). Prismatoolithus levis is an ootaxon of Troodon formosus (Troodontidae; Varricchio et al, 2002) and the materials are from an egg that contains an embryo (MOR 246; Horner and Weishampel, 1988; Varricchio et al, 2002; Choi et al, 2022). Elongatoolithus (MPC-D 100/1047) and Macroelongatoolithus (SNUVP 201801) are oviraptorosaur eggshells (Norell et al, 1994; Bi et al, 2021; Xing et al, 2022) and Macroelongatoolithus was laid by a giant oviraptorosaur (Pu et al, 2017).…”

“…The former was acquired by selecting the adjacent pairs of grains and the angle between them are calculated, whilst the latter was calculated by using the randomly selected (hence, usually distant) grains. We selected 1000 randomly chosen grains in constructing the random-pair misorientation to be consistent with former studies that deals with misorientation of fossil and modern eggshells (Moreno-Azanza et al 2013; Choi and Lee 2019; Choi et al 2019, 2020, 2022). Supplementary Text 3.…”