duction: live births, parchment eggs that are held within the mother for extended periods of time, parchment eggs that are laid soon after formation within the mother, and calcareous eggs. Following birth there is also a series of possibilities: the eggs are buried or not, and when not buried, the eggs can be cared for by the parent or not. The advantages of each of these and the time they first appeared are unknown.
And we have a second mystery. Most known dinosaur eggs are from the Cretaceous (mainly late Cretaceous) and are calcified. Also, the appearance of burial behavior in dinosaurs is also characteristic of the late Cretaceous. But what of the pre-Cretaceous dinosaurs? While there are eggs from sauropods and bipedal saurischians from the late Jurassic, most spectacularly from deposits in Portugal where the eggs contain the bones of embryos, earlier rocks are nearly barren of dinosaur eggs and/or nests. Only a single confirmed egg is known from the Triassic.
When these various kinds of eggs first evolved thus remains a mystery. In 2005, Nicholas Geist and John Ruben published an abstract in which they proposed that calcareous eggs first appeared at the end of the Permian as an adaptation to avoid desiccation in the increasingly dry late Permian through Triassic global climates. Unfortunately, there is no fossil evidence to support this: there are no accepted Permian eggs despite the existence at that time of anapsids, diapsids, and synapsids, and only a small number of late Triassic eggs that may have been from dinosaurs. Eggs commonly preserved in Cretaceous sediments, in the same kind of sedimentary environments that can be found in Permian and Triassic rocks. In all likelihood, if archosaurs used hard eggs in the Permian or Triassic, they would already have been found. The absence of evidence is always a dangerous tool, though; all too well known, and deservedly so, is the hoary adage that the absence of evidence is not necessarily evidence of absence.
In their wonderful 1997 summary of dinosaur eggs, Darla Zelenitsky and the late Karl Hirsch recognized only two dinosaur egg shapes (rounded and elongated) but seven different patterns of crystal arrangement. This diversity of egg-wall morphology would be surprising if all dinosaurs had evolved from a single, egg-laying