(CNN)Sharks are some of the ocean's top predators. In fact, sharks and their relatives were the first vertebrate predators on Earth.
How sharks outlived dinosaurs and adapted to suit their environment
Shark fossils date back more than 400 million years -- that means sharks managed to outlive the dinosaurs, survive mass extinctions, and continue to serve an important role near the top of underwater food chains.
After millions of years of adaptations, more than 500 species of sharks swim the planet's oceans today, and sharks are found in almost every type of ocean habitat. So how have sharks evolved to suit their environments?
To understand how modern sharks adapted and evolved, we first have to look back through the fossil record of their ancestors.
Originating from a time before dinosaurs walked the earth, the earliest shark scales date back about 425 million years, and the earliest shark teeth are from the Devonian Period, about 410 million years ago. And some fossils of shark-like chondrichthyans scales (from a group of fish including sharks, rays, and their relatives) date as far back as 440 million years.
Because shark skeletons are made of soft cartilage, which doesn't fossilize well, most of what scientists know about ancient sharks comes from teeth, scales and fin spine fossils. But the cartilage of early sharks would also be similar to shark cartilage today, which distinguishes sharks from most fish that have heavier skeletons made of bone. Having a skeleton made of lightweight cartilage allows sharks to conserve energy and swim long distances.