Evidence of Egg Diversity in Squamate Evolution from Cretaceous Anguimorph Embryos

Lizards are remarkable amongst amniotes, for they display a unique mosaic of reproduction modes ranging from egg-laying to live-bearing. Within this patchwork, geckoes are believed to represent the only group to ever have produced fully calcified rigid-shelled eggs, contrasting with the ubiquitous parchment shelled-eggs observed in other lineages. However, this hypothesis relies only on observations of modern taxa and fossilised gecko-like eggshells which have never been found in association with any embryonic or parental remains. We report here the first attested fossil eggs of lizards from the Early Cretaceous of Thailand, combining hard eggshells with exquisitely preserved embryos of anguimoph (e.g. Komodo dragons, mosasaurs). These fossils shed light on an apparently rare reproduction strategy of squamates, demonstrate that the evolution of rigid-shelled eggs are not an exclusive specialization of geckoes, and suggest a high plasticity in the reproductive organs mineralizing eggshells.

在羊膜动物（amniotes）中，蜥蜴类堪称极具特色的类群，它们展现出从卵生至胎生的独特繁殖模式镶嵌分布格局。在这一镶嵌式的繁殖体系中，学界此前认为壁虎类（geckoes）是唯一能够产下完全钙化硬质卵壳的类群，而其他蜥蜴支系普遍产出革质卵壳。然而，这一假说仅基于现生类群的观测结果，以及此前从未发现与胚胎或亲体遗骸共存的、类似壁虎类的卵壳化石。本研究报道了产自泰国早白垩世（Early Cretaceous）的首个经证实的蜥蜴类化石卵，其卵壳坚硬，且伴存有保存极为精美的蛇蜥亚目（anguimorph）胚胎（例如科莫多巨蜥、沧龙所属类群）。这些化石为我们揭示了有鳞类（squamates）一种此前看似罕见的繁殖策略，证明硬质卵壳的演化并非壁虎类独有的特化特征，并表明参与卵壳矿化的生殖器官具有极高的可塑性。