Sharks

Sharks are fucking ridiculous. They are basically nature's Russian Roulette players, attempting an almost retarded level of adaptations that often come up with changes that are so strange, there is no reason for them to have survived.

While they are older than trees, they are also the source of thousands of evolutionary dead ends. Seriously, if you think of putting teeth on anything out there, including bicycles, eye glasses, pry bars, or whatever your imagination may cook up, Sharks have been there and done that.

Sharks have had "circular saw" teeth with a one-sided, centralized jaw. They have had variations where the animal has 200 rows of teeth. They have adapted to all climates and food sources. It is a wonder the things didn't evolve wings, psionics, spell casting, nuclear physics, politics, and opposable thumbs. I would mention stealth technology, but I am pretty sure they invented that.

These days, they seem to be pretty laid-back, compared to the olden times.

Interaction With Mankind

We make soup out of their fins and then just toss them back into the ocean, killing them.

Resilience

Sharks belong to a group of creatures known as cartilaginous fishes, because most of their skeleton is made from cartilage rather than bone. The only part of their skeleton not made from this soft, flexible tissue is their teeth.

The group includes the more famous animals such as whale sharks and great whites, but also all rays, skates and the little-known chimaeras (also known as ratfish, rabbit fish or ghost sharks).

While often referred to as living fossils, sharks have evolved many different guises over the hundreds of millions of years that they have been swimming the oceans. This is probably why they have survived 5 mass extinction events.