Calcified eggshells protect developing embryos against environmental stress and contribute to reproductive success 1 . As modern crocodilians and birds lay hard-shelled eggs, this eggshell type has been inferred for non-avian dinosaurs. Known dinosaur eggshells are characterized by an innermost membrane, an overlying protein matrix containing calcite, and an outermost waxy cuticle 2 – 7 . The calcitic eggshell consists of one or more ultrastructural layers that differ markedly among the three major dinosaur clades, as do the configurations of respiratory pores. So far, only hadrosaurid, a few sauropodomorph and tetanuran eggshells have been discovered; the paucity of the fossil record and the lack of intermediate eggshell types challenge efforts to homologize eggshell structures across all dinosaurs 8 – 18 . Here we present mineralogical, organochemical and ultrastructural evidence for an originally non-biomineralized, soft-shelled nature of exceptionally preserved ornithischian Protoceratops and basal sauropodomorph Mussaurus eggs. Statistical evaluation of in situ Raman spectra obtained for a representative set of hard- and soft-shelled, fossil and extant diapsid eggshells clusters the originally organic but secondarily phosphatized Protoceratops and the organic Mussaurus eggshells with soft, non-biomineralized eggshells. Histology corroborates the organic composition of these soft-shelled dinosaur eggs, revealing a stratified arrangement resembling turtle soft eggshell. Through an ancestral-state reconstruction of composition and ultrastructure, we compare eggshells from Protoceratops and Mussaurus with those from other diapsids, revealing that the first dinosaur egg was soft-shelled. The calcified, hard-shelled dinosaur egg evolved independently at least three times throughout the Mesozoic era, explaining the bias towards eggshells of derived dinosaurs in the fossil record. Molecular analyses of newly discovered, embryo-bearing ornithischian and sauropod dinosaur eggs suggest that the ancestral dinosaur egg was soft-shelled, and that hard-shelled eggs evolved independently at least three times in the major dinosaur lineages.

中文翻译：

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第一个恐龙蛋是软的

钙化蛋壳保护发育中的胚胎免受环境压力，并有助于繁殖成功 1。随着现代鳄鱼和鸟类产下硬壳蛋，这种蛋壳类型已被推断为非鸟类恐龙。已知的恐龙蛋壳的特征在于最内层的膜、含有方解石的上覆蛋白质基质和最外层的蜡质角质层 2-7 。方解石蛋壳由一个或多个超微结构层组成，这些超微结构层在三种主要恐龙进化枝之间存在显着差异，呼吸孔的构造也是如此。迄今为止，只发现了鸭嘴龙类、少数蜥脚类恐龙和破伤风龙的蛋壳；化石记录的缺乏和中间蛋壳类型的缺乏挑战了对所有恐龙的蛋壳结构进行同源化的努力 8 - 18 。在这里，我们介绍矿物学，有机化学和超微结构证据表明保存完好的鸟臀目原角龙和基础蜥脚类恐龙蛋具有原始非生物矿化、软壳性质。对一组具有代表性的硬壳和软壳、化石和现存的双孔类蛋壳获得的原位拉曼光谱的统计评估将最初有机但二次磷酸化的原角龙和有机Mussaurus 蛋壳与柔软的非生物矿化蛋壳聚集在一起。组织学证实了这些软壳恐龙蛋的有机成分，揭示了类似于乌龟软蛋壳的分层排列。通过成分和超微结构的祖先状态重建，我们将原角龙和 Mussaurus 的蛋壳与其他双孔目动物的蛋壳进行比较，揭示了第一个恐龙蛋是软壳的。钙化的硬壳恐龙蛋在整个中生代至少独立进化了 3 次，这解释了化石记录中对衍生恐龙蛋壳的偏向。对新发现的、带有胚胎的鸟臀目和蜥脚类恐龙蛋的分子分析表明，祖先的恐龙蛋是软壳的，而硬壳蛋在主要的恐龙谱系中至少独立进化了 3 次。