Reading this. As I understand it, dinosaurs were small beer during the Triassic, outclassed by bigger, more dominant competitors, like pseudosuchians and amphibians. Then Pangea split about 201 million years ago, venting vast amounts of magma and noxious gas, initiating catastrophic climate change and triggering the Tr-J extinction event. Dinosaurs fared better than other families, becoming dominant in the Jurassic. There's no consensus as to why.
It sorta stands to reason that dinosaurs' ascendancy has something to do with their adaptability in response to the Tr-J extinction event. But what, exactly? Did the changes to the climate suit their physiology better? Did the mass deaths of other species simply open niches that dinosaurs were then free to fill? There's a lot of room for speculation, but little definitive evidence on which to eliminate competing theories, AFAIK.
The section on the adaptations that allowed sauropods to reach the upper limits of body mass is pretty fascinating.
Some of these sauropods are clearly just giant legsnakes.
One thing this book does well is provide a logic for the diversity of sauropods— Brachiosaurus, Brontosaurus, Diplodocus, Camarasaurus, etc— a branch of dinosaur evolution that often gets boiled down into a slurry of long necks and flat teeth. The notion is that different species of sauropod are specially adapted to eat certain types of plants, and the traits that differentiate them are, to a large degree, bound to their differences in diet.
"The sauropods weren't competing for the same plants, but dividing the resources among themselves. The scientific term for this is niche partitioning—when coexisting species avoid competing with each other by behaving or feeding in slightly different ways. The Morrison world was highly partitioned, which is a sign of how successful these dinosaurs were. They were carving up almost every square inch of the ecosystem, a dizzying array of species flourishing alongside each other…"
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