Back when I was a kid reading about sharks, there were "more than 200" living species. Then 300, then 350, and now it's over 500. In addition to being numerous and of global distribution, the shark clan also boasts creatures so weird they could (and do) create nightmares. Prehistoric sharks, were, if anything, stranger.
In the oceans today swim the 15-meter whale shark, the 7-meter great white, the blimplike megamouth, and several species of the bizarre hammerhead. In the oceans of yesterday swam the 15-18m Megalodon, the bizarre "ironing board shark" Stethacanthus, and Helicopreon, which had a "wheel" of teeth" - it looks like it tried to swallow a (vertical) power saw blade.
Helicopreon (artist unknown, license marked "Free to Share and Use" by Bing)
Stethacanthus (artist unknown, license marked "Free to Share and Use" by Bing)
We have new species of present and past sharks, and they continue the weirdness.
From the modern sea, far far below the Pacific near New Zealand, comes the 1.8m kitefin shark, the largest luminous vertebrate in the world. I always thought a glowing shark like the ones in the Meg novels wouldn't be able to find prey down there (it's like swimming around with a big neon sign saying "SHARK!"). Apparently, though, enough inhabitants of its realm are just too curious about light sources.
From the past we have Aquilolamna milarcae. Thanks to pectoral fins that rolled out like glider wings, its 1.8m "finspan" made it longer than it is wide. This Cretaceous resident was more a drifter than a hunter-killer.
The sharks... still full of surprises.