Parasaurolophus

Type Species: Parasaurolophus walkeri
Classification: Dinosauria - Ornithischia - Ornithopoda - Hadrosauridae - Lambeosaurinae - Parasaurolophini
Time Period: Late Cretaceous
Location: North America
Diet: Herbivore

What better way to kick off "Dinosaurs of 2015" with the sexiest dinosaur of them all? Parasaurolophus is my all-time favorite dinosaur, and she's had cameos in The Land Before TIme and the Jurassic Park movies. She was a big bitch, too:

and that's for a full-grown adult. She'd look MUCH bigger to me.

Her name means "like Saurolophus," since both dinosaurs had trombone-like crests protruding from the backs of their heads. But whereas Saurolophus' crest was solid bone, Parasaurolophus' crest was hollow and lined with looping nasal cavities running from the nostril on the crest to its throat. You can get a sense of this dinosaur's size (as well as the size of its swooping trombone-like crest) in this clip from The Lost World:

As to the purpose of the crest, paleontologists have offered lots of theories (none testable, of course). There's the theory that it was a snorkel for underwater swimming, that it could've been the "entrance" to an air container so that Parasaurolophus could submerge beneath the waves. I don't think this is too likely: despite the proliferation of movies presenting this dinosaur in the water, research within the last decades has shown how the water pressure wrapping around these titanic dinosaurs when submerged would've been strong enough to crush their ribs. Others theories include its intensification of scent, it was an area used to "cool off" the brain, or that it was a horn used for social display, identifying its age and sex and herd membership. 

The most widely-accepted theory is that the crests were used to emit sounds. These noises would warn of impending danger, attract potential mates, or signal to other members of the herd. The nasal loops within the crest lend credit to this theory. A vocal organ such as a voice-box would make the sounds, and the sound would then be pushed through the hollow tubes within the crest, giving the creature a deep, bellowing rumble. Scientists have mapped the nasal loops and have run simulations to determine what the sounds would've been like:

That's the trumpeting of a dinosaur.

A cry out of place in this world.

A howl very, very alien.