Many people consider sharks to be these living fossils that haven’t changed much, but this isn’t necessarily the case!
Before we dive into shark evolution in particular, we’re going to look at the history of fish. As seen in the cladogram below, the superclass Gnathostomata (jawed vertebrates) broke into three classes: Placodermi (extinct armored fishes), Chondrichthyes (cartilaginous fishes – sharks, skates, rays, and chimearas), and Osteichthyes (bony fishes) about 400 million years ago.
Since both types of fish have had the same amount of time to evolve, why are there only around 1200 species of cartilaginous fish and around 30,000 species of bony fish? One major key is their skeletal structure. Chondrichthyans have a simple neurocranium, an essentially one-piece skull, whereas bony fish have many many pieces. Think of it like building blocks – the more pieces you have, the more things you’re able to build!
A living fossil, as the name describes, describes an animal living now that looks just like it did millions of years ago. If sharks were living fossils, they would still be swimming around with spiral jaws like Helipricon and anvil shaped dorsal fins like Stethacanthus. Sharks have come a long way in their evolution, especially when considering the first known shark: Cladoselache from the group known as Cladodonts. Cladodonts are known for their terminal mouths – ones directly in front of their bodies (think Pacman!). Modern sharks have evolved to have sub-terminal mouths (still in front, but closer to the bottom of the body).
Do you think sharks are living fossils?
TFUI Founder Melissa C. Marquez is author of all animal bios and "Behind the Fins" segments.
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