Rich and Strange Aeons

I'm currently reading The Rise and Fall of the Dinosaurs by Steve Brusatte, a 2018 book on dinosaur history by a professional paleontologist, obviously way more up to date than my childhood reading. I've learned a lot, not all about dinosaurs. Supplemented by some Wiki reading about periods:


* Land animals of the Carboniferous period was dominated by large arthropods and amphibians, some of the latter 6 meters long; reptiles evolved too, and came to dominate the subsequent Permian, in all sorts of categories I've barely heard of: paraeisaurs, gorgonopsians (thank yoU Dresden Codak), dicynodonts (some elephant-sized), cynodonts (ancestors of mammals). All sprawling.

* Early in the Triassic archosaurs take over, upright (or limbs tucked under body) with two big lineages: crocodile-line (despite modern crocs being sprawlers) and bird-line (pterosaurs and dinosaurs), with the latter giving rise to the dinosauromorphs -- proto-dinosaurs (and a somewhat arbitrary distinction.) And then, dinosaurs. But not dominant yet:

overshadowed by the larger and more diverse amphibians, mammal cousins, and crocodile relatives that they lived alongside on those dry, occasionally flooded plains of the Triassic.

Giant amphibians like Metoposaurus were leading actors in the story of Triassic Pangea. They prowled the shores of rivers and lakes over much of the supercontinent, particularly the subtropical arid regions and midlatitude humid belts. If you were a frail little primitive dinosaur like Eoraptor, you would want to avoid the shorelines at all costs. It was enemy territory. Metoposaurus was there waiting, lurking in the shallows, ready to ambush anything that ventured too close to the water. Its head was the size of a coffee table, and its jaws were studded with hundreds of piercing teeth. Its big, broad, almost flat upper and lower jaws were hinged together at the back and could snap shut like a toilet seat to gobble up whatever it wanted. It would only take a few bites to finish off a delicious dinosaur supper.
Salamanders bigger than humans seem like a mad hallucination. As bizarre as they were, though, Metoposaurus and its kin were not aliens. These terrifying predators were the ancestors of today’s frogs, toads, newts, and salamanders. Their DNA flows through the veins of the frog hopping around your garden or the one you dissected in high school biology class. As a matter of fact, many of today’s most recognizable animals can be traced back to the Triassic. The very first turtles, lizards, crocodiles, and even mammals came into the world during this time.

* Triassic dinosaurs seem mostly limited to temperate humid areas, only a minority even there, and excluded from arid ones. What did dominate were 'crocodiles' -- which had convergently evolved to be like the dinosaurs (or vice versa), but more diverse and numerous.

Then there are proper crocodiles, but nothing like the ones we’re familiar with today. These primitive Triassic species—the ancestral breed that modern crocs evolved from—looked like greyhounds: they were about the same size, stood on four legs, had the emaciated build of a supermodel, and could sprint like champions. They fed on bugs and lizards and were most certainly not top predators. That title went to the rauisuchians, a ferocious bunch that grew up to twenty-five feet long, bigger than the largest saltwater crocodiles today. We met one of them previously, Saurosuchus, the top gun in the Ischigualasto ecosystem that would have haunted the nightmares of the very first dinosaurs. Imagine a slightly smaller version of a T. rex walking around on four legs, with a muscular skull and neck, railroad-spike teeth, and a bone-breaking bite.

* But the Triassic came in with one volcanic mass extinction, the end of the Permian, and went out with another, which somehow wiped out most of the supercrocs and let the dinosaurs slip into first place.

* So the Jurassic really was a "Jurassic Park", the first age of dinosaur dominance, with many of the classic sauropods (Brontosaurus, Brachiosaurus, Diplodocus) and allosaurs and stegosaurs and anklylosaurs. And we can talk about the whole Earth being dominated by them, because Pangaea was just breaking up, and big animals could get everywhere.

* Some periods are separated by mass extinctions. The K-T boundary has a literal line of iridium marking it off. But other periods are from geologists looking at two points in time, and going "well, this is different enough from that that we should draw a line somewhere." The Jurassic-Cretaceous are like that, with the big change being that Pangaea finished breaking up into pieces, and even its pieces were breaking up. In the Cretaceous we have continents, and need to be more precise about where dinosaurs are. The classic T. Rex and Triceratops, for example, were confined to what is now western North America (cut off from the east by an intrusive sea), in the last 20 million years, with not a sauropod in sight. Those still existed -- not the classics, but new titanosaurs -- in the southern fragments of Gondwanaland.

* Romania used to be an island, with dwarf dinosaurs on it.