far from proven, and, as we shall see below, there is indirect evidence that some early dinosaurs may have utilized parchment eggs or even live births.
What is the egg record for dinosaurs? Almost all dinosaur eggs come from the Cretaceous, and the nature of their crystal form and size, as well as the number and pattern of pores in the egg, show a wide variety. There are certainly plenty of eggs found from the Cretaceous, but, while known, there are far fewer Jurassic dinosaur eggs and almost none known from the Triassic.
There are several possibilities for this. Perhaps there is some preservation bias, with pre-Cretaceous eggs as common as those of the Cretaceous, but the lesser extent of Triassic and Jurassic dinosaur beds compared to the vast expanse of Cretaceous-aged beds has caused this difference. Another possibility is that pre-Cretaceous eggs fossilized much less readily than those from the Cretaceous. This would certainly be the case if pre-Cretaceous eggs were leathery like those of extant reptiles, rather than calcified like birds. And if, like the marine ichthyosaurs, some dinosaurs utilized live birth rather than egg laying there would certainly be fewer eggs to find. As in so many other aspects of the history of life, the level of atmospheric oxygen may have played a major role in dictating mode of reproduction.
Fossil eggs from Cretaceous deposits attributed to dinosaurs (what else could have laid such large eggs?) have a calcium carbonate covering like a chicken egg (but thicker). But unlike chicken eggs, which are smooth, the dinosaur eggs were usually ornamented with either longitudinal ridges or nodular ornamentation. Presumably, ornamentation allowed the eggs to be buried after emerging from the female, with the ornament allowing airflow between the eggs and the burial material. The ability to bury eggs may have aided their fossil preservation potential and perhaps helps explain why there are so many Cretaceous eggs and so few other kinds. The heavy calcification would also help the eggs withstand the overpressure of burial in soil or sand. Also the complex behavior involved in nest making and orienting the eggs in burial mounds is now known for the late Cretaceous—but not before.
What are the advantages of calcareous eggs? They are strong, harder for predators to break into, and aid in development. As the embryo grows inside the egg, some of the calcium carbonate is dissolved