Comparative crystallography suggests maniraptoran theropod affinities for latest Cretaceous European ‘geckoid’ eggshell

Abstract

Thin fossil eggshell from Upper Cretaceous deposits of Europe, characterized by nodular ornamentation similar to modern gekkotan eggshell, has mostly been interpreted as gekkotan (=‘geckoid’) in origin. However, in some cases, as for the oogenus Pseudogeckoolithus, a theropod affinity has also been suggested. The true affinity of these fossil ‘geckoid’ eggshells has remained controversial due to the absence of analytical methods for identifying genuine gecko eggshell in the fossil record. In this study, we apply electron backscatter diffraction (EBSD) analysis to latest Cretaceous European ‘geckoid’ (including Pseudogeckoolithus) eggshell, in comparison with modern gekkotan and theropod (avian) eggshell. We found that Pseudogeckoolithus has a definite theropod eggshell‐like crystallographic configuration, in clear contrast to that seen in modern ‘gecko eggshells’. Furthermore, the crystallography of the nodular ornamentation in Pseudogeckoolithus is similar to that seen in megapode eggshell, but different from that of gecko eggshell, despite superficial morphological similarity. The remarkable morphological similarities between Pseudogeckoolithus and modern gecko eggshells are thus convergent, and the ‘gekkotan affinity’ hypothesis can be dismissed for Pseudogeckoolithus. This study provides a template for differentiating true gekkotan from dinosaurian eggshells in the fossil record. The potential functional significance of eggshell ornamentation, lost in most modern birds, requires further study, and experimental zoological approach may shed light on this issue. Finally, the present results suggest caution about the dangers of using potentially homoplastic eggshell characters in eggshell parataxonomy.

Raman Spectroscopy Detects Amorphous Carbon in an Enigmatic Egg From the Upper Cretaceous Wido Volcanics of South Korea

“…Except for this ornamentation, the external surface is smooth (see PLM observation above) and uniform in thickness. Considering the fact that non-ornamented plain parts of the eggshell have consistent thickness (e.g., Choi et al, 2018Choi et al, , 2020, it also supports the non-taphonomic origin of ornamentation structure described herein.…”

“…In the ornamentation, the grains constituting the ornamentation are continuous with respect to the underlying main body of the eggshell (Figures 7D,E). This observation supports the biogenicity of the ornamentation unless it was made by the epitaxial growth of calcites (i.e., diagenetic overgrowth; Moreno-Azanza et al, 2016;Choi et al, , 2020Kim et al, 2019). However, the epitaxy is highly unlikely considering the presence of larger vesicles inside the ornamentation (Figures 7D-F; see SEM observation above).…”

“…In summary, although similar in their position, the inner dark bands of A. vesicularis and extant Caiman latirostris (Crocodylia) eggshell have completely different backgrounds in their origin: dark bands of A. vesicularis is attributable to amorphous carbon (Figures 8, 9), while those of (at least inner one) modern crocodylian eggshell would be caused by porous calcite (Figure 10 and Table 3). It implies that the morphological or optical similarity alone should not be used as a diagnostic character before the reason behind the phenomenon is resolved by independent approach (e.g., Choi et al, 2020). For these reasons, despite crystallographic similarity, A. vesicularis cannot be safely ascribed to crocodyliform eggshell T = −445 X R2 + 641 1 R2: 0.722 320 T = (RA1-0.3758)/0.0008 2 RA1: 0.659 354 T = (RA2-0.27)/0.0045 3 RA2: 1.93 369…”

“…Although Hirsch (1996) suggested the gekkotan affinity for G. columnaris, as admitted in his paper, the diagnosis of the G. columnaris was provisional and based on a morphological similarity with extant gekkotan eggshells. Recently, Choi et al (2018) provided a more rigorous criterion to diagnose gekkotan eggshells in fossil record using the crystallographic configuration of eggshells (see also Choi et al, 2020). When the crystallography of G. columnaris is examined using EBSD and if it shows strong crystallographic similarity to modern gekkotan eggshells, G. columnaris can be concretely identified as a fossil squamate ootaxon.…”

“…Even though there have been many reports on eggshells with dispersituberculate ornamentation, these eggshells are usually thicker than 200 µm (Choi et al, 2020; Table 1) and many of them would be highly probable maniraptoran eggshells because at least two-layered crystallographic microstructure of maniraptoran dinosaur eggshells are observed (Choi et al, 2020) that is absent in A. vesicularis (Figure 7).…”