nearest living relatives, the birds and reptiles. After lung type and metabolic type, some of the most interesting questions relate to reproductive strategy, and this too can be examined in the light of changing oxygen levels.
Birds show little variation in their reproduction. Extant birds, our best window to the dinosaurs, all lay eggs with a porous calcareous shell. There are no live births in birds, in contrast to extant reptiles, which have many lineages using live birth.
There is also great variation in egg morphology between birds and some reptiles. While the eggshell in birds and reptiles consists of two layers, an inner organic membrane overlain by an outer crystalline layer, the amount of crystalline material varies greatly, from a thick, calcium carbonate layer like that in birds to almost no crystalline material at all, so that the outer layer is a leathery and flexible membrane. Even the mineralogy of the crystalline layer varies, from calcite in birds, crocodiles, and lizards to aragonite (a different crystal form of calcium carbonate) in turtles. Eggs are thus divided into two main types: hard or crystalline and soft or parchment (some scientists further subdivide the parchment eggs into flexible [used by some turtles and some lizards] and soft [parchment, used by most snakes and lizards]). Not surprisingly, the fossilization potential of these different hardness categories of eggs differs markedly. There are numerous fossil hard eggs known (many from dinosaurs), a few flexible eggs, and no undisputed soft eggs preserved.
Because of the great interest in dinosaurs there has been much speculation about their reproductive habits (the thought of two gigantic Seismosaurus mating rather boggles the imagination), and there are still many mysteries. One of the seminal discoveries about dinosaurs was that they laid large calcareous eggs, with calcite crystals making up the mineral layer, a finding from the first expedition to the Gobi Desert by an American Museum of Natural History expedition in the 1920s. Since then, thousands of Cretaceous dinosaur eggs have been found around the world, and even the nesting patterns have been discovered, the most notable being the nest discoveries in Montana by Jack Horner.
But are these Cretaceous finds characteristic of dinosaurs as a whole? This question remains unresolved and controversial. While most scientists assume that all dinosaurs laid hard-shelled eggs, this is