Saw noted dinosaur paleontologist Paul Sereno speak last night. Learned a few things about dinos, and I thought I'd pass them on:
1. Dinosaurs evolved from small (~4ft long) bipedal ancestors. The primitive dinos looked like little Tyrannosaurs (your generic short arms, stocky legs, long tail body type). Apparently, quadrupedism (I may have just made that word up) is a derived trait among dinos. That's different from mammals, where most species are quadrupeds and only a few derived body types (i.e., humans) can be described as truly bipedal.
2. All true dinosaurs inhabited terrestrial habitats. There were no aquatic dinos, no air borne dinos (other than birds), and no arboreal dinos. Sereno believes that dinos' large size limits them from occupying these habitats. For instance, mammals evolved from a much smaller ancestor (some sort of small rodent-like creature) and therefore had more flexibility in where they could live. Since almost no (or none?) dinos were as small as a cat, they did not have the evolutionary potential to evolve into airborne (until birds), sea living, or tree living creatures.
3. He discovered the skull of a "supercroc." Extrapolating based on modern crocodile skull to body size ratios, he approximates that this giant beast was about 40 feet long. I'd like to see Steve Erwin wrestle that fella'.
I'm still not clear as to what those sea living reptiles that I remember from my childhood dino books are, but apparently they ain't dinosaurs.