then buried, was a reproductive strategy that was not viable in the late Permian through middle Jurassic environments of lower atmospheric oxygen.

So, let me more formally pose this:

Hypothesis 9.3: The low-oxygen and high-heat conditions of the late Permian into the Triassic stimulated the evolution of live birth and of soft eggs that would have been effective at allowing oxygen movement into eggs and carbon dioxide out. On the other hand, the higher-oxygen levels (and continued high temperatures) of the late Jurassic-Cretaceous interval stimulated the evolution of rigid dinosaur eggs and egg burial in complex nests.

Only time will tell if new discoveries from late Triassic through middle Jurassic strata will add significant new information to the topic of dinosaur (and mammalian!) birth strategy. Like characteristic metabolisms, the contrasting patterns of live births versus egg laying are fundamentally important—and ones that have received surprisingly scant attention by evolutionary biologists. Solving this problem—by learning the time of origin and the distribution of one kind of birth strategy or the other—should be a major research topic of the near future but, sadly, may prove to be intractable because of the non-preservation of parchment eggs.

JURASSIC-CRETACEOUS IN THE SEAS

Let’s now move from land to sea. The Jurassic and Cretaceous oceans would have been dangerous places to swim in. The major Triassic marine predators just increased in number in the Jurassic and added a new and efficient kind of fish- and cephalopod-eater, plesiosaurs. In the Cretaceous yet another kind of tetrapod predator appeared as well: mosasaurs displaced plesiosaurs and ichthyosaurs as the top carnivores in the sea. Ammonites continued to flourish but evolved large numbers of uncoiled shell shapes in addition to the traditional planispiral shapes of the Jurassic and lower Cretaceous. A diverse calcareous plankton including coccoliths and foraminifera changed the nature of



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