Trachodon—well, I was just talking about it, so let's start there. In every old dinosaur book I ever read as a kid, Trachodon was the "boring" duck-bill, and he'd be sitting there on the lakeshore along with Lambeosaurus, Corythosaurus and Parasaurolophus. They always hung out together on the beach. And whenever Tyrannosaurus rex came along, they'd make a scramble for the water, which T. rex was so afraid of that he wouldn't even let his feet get a tiny bit wet in a few inches of water, it seems.
This is of course nonsense, as only Edmontosaurus lived at the same as as T. rex; the other three were all part of Judithian or Edmontonian faunas. None of them lived at the same time as each other either. And while it's true that these hadrosaurs all seem to have favored wetlands as habitat, they weren't lake-dwellers, and T. rex hunted those same terrains without worrying too much about it. There is also a theory that the transition from the Judithian to the Lancian fauna came as sea levels decreased and generalists from the more arid uplands flourished, while wetlands specialists declined. This is more marked in the south, with the Alamosaurus-Quetzalcoatlus guys. Migration from Asia and possibly South America as sea levels fell is another theory. In any case, nothing about the environment or habits of Trachodon turns out to be true, so maybe it's not such a bad idea that the name is closely associated with the debunked paradigm of dinosaurs anyway.
Monoclonius—While some still say that this might be a valid animal, most think that Monoclonius was actually a fragmentary juvenile Centrosaurus these days. As with the duckbills, horned dinosaurs in old books were often thrown together regardless of whether they were from Judithian, Edmontonian or Lancian faunas—although at least they were more or less geographically close (much of the fossils of the southern province of Laramidia hadn't been found yet, with names like Bravoceratops, Eotriceratops, Arrhinoceratops, etc.) In those old books, a hungry T. rex almost always found the duckbills—who ran into the lakes, thus frustrating him, then he stumbled across an Ankylosaurus, which he couldn't do anything with because of its armor, and then he went looking for the horned dinosaurs, which oddly enough were always believed to be out on the plains like scaly buffalo. Monoclonius and Styracosaurus were usually pictured together, but for whatever reason, T. rex didn't try to eat them, always looking instead for the Triceratops. Often these other two were pictured forming musk-ox like circles around the young. Triceratops, on the other hand, were always the big bad loners.
|Complete with Triceratops in the distance|
Curiously, Brontosaurus has made a return; some specialists now believe that Apatosaurus deserves to be split into two genera, and Brontosaurus was the obvious choice. There are now two to three species of both Apatosaurus and Brontosaurus specifically (probably separated in time, so that there was only one of each in the Morrison at a time, most of the time.)
|Classic square-headed Brontosaurus trying to escape by going to the lake.|
Very interesting. Listening to the stuff about halfway through (a few minutes after one hour) they talk about sauropods being really big all of the time; that clearly isn't true, though. He does talk about island dwarfs briefly, but the fact is, there really are a number of modest sized sauropods, and much of the latest Cretaceous sauropods are modestly sized.
He also talks about the polyphyly of Sauropoda. This is not something that he really believes (in fact, he specifically says that) but it's still an interesting idea.