Oct. 11, 2009 -- 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.
He and others believe the purpose of the giant fin was to help establish its territory and attract mates.
"We have never really seen anything quite like it in the dinosaur world or thereafter," said Sereno. "They're an example of evolution gone wild."
What Happened to Carnotaurus' Arms?
The fierce-looking Carnotaurus is another dino that leaves paleontologists scratching their heads.
Intimidating in its appearance, the 25-foot carnivore has bull-like horns on its head and a box-like jaw overflowing with teeth. But though it has enormous shoulder blades, its arms are incredibly small.
"It's got a very shortened skull. It looks like someone has gone up to the dinosaur and smashed it in the face with a frying pan. But one thing that really strikes me, though, is this enormous shoulder blade -- it is absolutely vast. At the end of the shoulder blade you'd expect to see enormous arms. But you don't -- they're absolutely tiny," said Phil Manning, a paleontologist at the U.K.'s University of Manchester. "Carnotaurus is one weird-looking dinosaur."
Mamenchiasaurus's Long Neck Was for Mating
The giraffe of the dinosaur family, the plant-eating Mamenchiasaurus had a neck and tail twice as long as its body.
Although many think that long necks in animals, including giraffes, are driven by the need to reach food at the tops of high trees, paleontologists say neck length is driven by mate selection.
Giraffes have a ritual in which they wrap their necks around each other and push, Sereno said.
"I'd say probably that's what [the Mamenchiasuruses] were doing," he added.
Parasaurolophus's Elaborate Head Crest Puzzles
One of the dinosaurs to have a cameo in "Jurassic Park," the movie, the Parasaurolophus, has one of the most elaborate head crests in the dinosaur kingdom.
"Parasaurolophus is just truly weird. It's got this enormous expansion of skull bones, which projects into this banana-like lump coming out of the back of its skull," said Manning.
This "banana-like lump" has sparked much discussion among scientists. Some think it might have aided the animal's sense of smell or helped regulate body temperature. Others think the crest helped it make communication sounds.
Why Did the Triceratops Have Horns?
Distinguished by three horns atop its head, the Triceratops has also long been a subject of debate among scientists.
Traditionally, scientists thought the animal's horns were meant to protect its neck or deliver blows in combat. But now many believe that the horns were meant for display in courtship or to establish dominance.
"Bizarre Dinosaurs" will air Sunday, Oct. 11 at 8 p.m. ET on the National Geographic channel. For more information, click here.