A comparative study of eggshells of Gekkota with morphological, chemical compositional and crystallographic approaches and its evolutionary implications

The Gekkota is an important clade in the evolution of calcified eggshells in that some of its families lay rigid eggshells like archosaurs. However, the fundamental differences and similarities between the mechanism of rigid eggshell formation of the Gekkota and Archosauria have not been investigated thoroughly due to the lack of knowledge of gekkotan eggshells. Here, we report for the first time a comprehensive analysis of morphological, chemical compositional, and crystallographic features of rigid and soft gekkotan eggshells. Exhaustive morphological description provided common characters for gekkotan eggshells, as well as unique features of each species. We found that elemental distribution of rigid gekkotan eggshells is different from that of avian eggshells, especially in the case of Mg and P. In addition, the crystallographic features (size, shape, and alignment of calcite grains) of gekkotan eggshells are completely different from those of archosaur eggshells. The result of this study suggests that soft gekkotan eggshells are morphologically more similar to tuatara eggshells rather than soft eggshells of derived squamates. The chemical compositional analysis suggests that the eggshell may act as a mineral reservoir for P and F as well as Ca. More importantly, all chemical compositions and crystallographic features imply that the gekkotan eggshell formation may begin at the outer surface and growing down to the inner surface, which is opposite to the direction of the archosaur eggshell formation. This character would be crucial for identifying fossil gekkotan eggs, which are poorly known in paleontology. All these lines of evidence support that soft gekkotan and tuatara eggshells share the primitive characters of all lepidosaurid eggshells. Finally, gekkotan and archosaur rigid eggshells represent a typical example of convergent evolution in the lineage of the Sauropsida.壁虎下目（Gekkota）是钙化蛋壳演化历程中的重要演化支，其部分类群可产下与主龙类（Archosauria）相仿的硬质蛋壳。然而，受限于当前对壁虎类蛋壳的认知不足，学界尚未深入探究壁虎下目与主龙类硬质蛋壳形成机制间的根本异同。本研究首次针对硬质与软质壁虎类蛋壳的形态学、化学组成及晶体学特征开展全面分析。详尽的形态学描述不仅明确了壁虎类蛋壳的共有性状，同时揭示了各物种的独有特征。研究发现，壁虎类硬质蛋壳的元素分布模式与鸟类蛋壳存在显著差异，尤以镁（Mg）和磷（P）元素为甚。此外，壁虎类蛋壳的晶体学特征（方解石晶粒的尺寸、形态与排布方式）与主龙类蛋壳完全不同。本研究结果表明，软质壁虎类蛋壳在形态上更接近喙头蜥（tuatara）蛋壳，而非衍生鳞类（squamates）的软质蛋壳。化学组成分析显示，壁虎类蛋壳不仅可作为钙（Ca）的矿物储备库，同时也能储存磷（P）与氟（F）。更关键的是，所有化学组成与晶体学特征均表明，壁虎类蛋壳的形成过程始于蛋壳外表面，并向内表面延伸生长——这与主龙类蛋壳的形成方向完全相反。该特征对于识别古生物学中记录匮乏的壁虎类化石蛋壳具有重要参考价值。上述所有证据均表明，软质壁虎类蛋壳与喙头蜥蛋壳均保留了鳞龙类（lepidosaurid）蛋壳的原始性状。最后，壁虎下目与主龙类的硬质蛋壳是蜥形纲（Sauropsida）演化支中趋同演化的典型案例。