Abstract
One of the fossil record’s most puzzling features is the absence of preserved eggs or eggshell for the first third of the known 315 million year history of amniote evolution. Our meagre understanding of the origin and evolution of calcareous eggshell and amniotic eggs in general, is largely based on Middle Jurassic to Late Cretaceous fossils. For dinosaurs, the most parsimonious inference yields a thick, hard shelled egg, so richly represented in the Late Cretaceous fossil record. Here, we show that a thin calcareous layer (≤100 µm) with interlocking units of radiating crystals (mammillae) and a thick shell membrane already characterize the oldest known amniote eggs, belonging to three coeval, but widely distributed Early Jurassic basal sauropodomorph dinosaurs. This thin shell layer strongly contrasts with the considerably thicker calcareous shells of Late Jurassic dinosaurs. Phylogenetic analyses and their Sinemurian age indicate that the thin eggshell of basal sauropodomorphs represents a major evolutionary innovation at the base of Dinosauria and that the much thicker eggshell of sauropods, theropods, and ornithischian dinosaurs evolved independently. Advanced mineralization of amniote eggshell (≥150 µm in thickness) in general occurred not earlier than Middle Jurassic and may correspond with a global trend of increase in atmospheric oxygen.
Highlights
The origin of the amniote egg is a topic of great significance because it represents one of the major evolutionary innovations in vertebrate evolution, allowing the group to complete their invasion of the terrestrial landscape and sever their reproductive cycle from the aquatic medium[1]
Mussaurus eggshell has to our knowledge never been described in a formal publication. These remains are the earliest confirmed amniote eggshells recorded in the fossil record
The very thin crystalline layer (~10 μm) topping the eggshell units is phosphatic in nature (Figs S1 and S2), and looks scalloped with shallow pits and low ridges, not necessarily matching eggshell unit borders. These surface irregularities or tubercles are of such small dimensions that the surface of the calcareous layer looks smooth (Fig. S2a), and it remains unclear if they match the ornamentations seen in younger dinosaur eggshells
Summary
The origin of the amniote egg is a topic of great significance because it represents one of the major evolutionary innovations in vertebrate evolution, allowing the group to complete their invasion of the terrestrial landscape and sever their reproductive cycle from the aquatic medium[1]. In a study on prenatal remains of Lufengosaurus, some of the authors of the current study previously provided a very brief description of its eggshell, and noted its extreme thinness[12]. Mussaurus eggshell has to our knowledge never been described in a formal publication. These remains are the earliest confirmed amniote eggshells recorded in the fossil record. Due to their rarity, fragmentary nature, and great geographic distance from each other, they were never studied from the perspective of the evolution of amniote eggshell. This study contributes to our understanding of the evolution of rigid shelled eggs; a key trait in the evolutionary success of archosaurs
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