Sharks are usually thought of as primitive creatures, sometimes called "living fossils." But a new study of a 325-million-year-old shark fossil — the most complete of its kind — suggests modern sharks have evolved significantly from their bony ancestors.

The ancient fossil has characteristics of both bony fishes and modern sharks. But its gill structures more closely resemble those of bony fishes, challenging the notion that modern sharks have remained unchanged over evolutionary time.

Image: The exceptionally well-preserved fossil of Ozarcus mapesae from two different lateral views. The scale bar is 10 millimeters AMNH / F. Ippolito  

To read the original article: Modern Sharks May Not Be "Living Fossils" After All - Scientific American