They were among the largest and strongest creatures to inhabit Earth but, as nature would have it, some were also among the strangest.
Many of the dinosaurs that lived millions of years ago were so fantastical-looking, it's almost hard to believe that they were actually of this Earth.
Arms ostensibly too short to use, necks that seemed to stretch forever, and spikes that were likely more of a hindrance than help in combat.
But paleontologists say that these seemingly strange appendages and body parts developed for a reason, although in some cases, it's still not clear exactly what some of those reasons were.
"These dinosaurs are not just icons for extinction, they are really evolutionary successful, innovative creatures," Kristy Curry Rogers, a paleontologist at Macalester College, in St. Paul Minn., says on "Bizarre Dinosaurs," a National Geographic program airing Sunday night.
"They evolved horns, frills, spikes, plates, long necks, long tails, sharp claws, big teeth," said Rogers. "They do all of these things and they're wildly successful organisms on the planet from 228 million years ago to 65 million years ago. They're the kings and queens of the Earth at that time."
"Bizarre Dinosaurs" features some of the most interesting members of the dinosaur family. Here are five of our favorites.
Spinosaurus Is Example of 'Evolution Gone Wild'
About 60 feet long, the Spinosaurus was the largest of the meat-eating dinosaurs, bigger even than the fearsome Tyrannosaurus Rex.
But paleontologists remain puzzled about one of its distinguishing body parts: a 5-foot fin attached to its back.
"I think it would have made it heavier. It would have made it catch wind in a strong breeze. There's no good advantage to it that you can think of except showing that, 'Hey, I can grow this 5-foot sail and I'm healthy and I'm bigger than you," said Paul C. Sereno, a paleontologist at the University of Chicago.