JOHN MAISEY (AXELROD RESEARCH CURATOR, DIVISION OF PALEONTOLOGY): The fossil record is dominated by shark’s teeth. Shark’s teeth are among one of the most common vertebrate fossils that you can find, but the skeletons are exceptionally rare.
Sharks and their relatives don’t have lots of bones covering the head and the body like a fish you buy at the supermarket.
The most bony things are, in fact, the teeth. They’re made of dentine and enamel-like tissue, just like our teeth, but the rest of the skeleton is just soft cartilage coated with this hard calcium phosphate layer.
It’s not bone, but it consist of literally hundreds of thousands of little, tiny crystals or fragments of calcite that are held together by collagen fibers.
And they all gradually get bigger and bigger as the fish grows, but they’re not solid bone.
So one of the things about a shark skeleton is after the shark dies, very often when the collagen fibers disintegrate—because they’re organic and they decompose—the skeleton just falls apart. And that’s not good for fossils. The fossils often just break up as a result.
The only time you ever find fossils—really complete fossils of shark-like fishes—is when they’ve been buried very rapidly in the sediment and the mud and they’ve been removed from any scavengers and they haven’t decomposed and things don’t get moved around by currents and so forth.
There are no solid bones to hold together, so the whole thing is very vulnerable to just collapsing and falling apart.
So when somebody does find a fossil shark–and especially one with a skeleton that’s preserved in three dimensions–it’s a really big deal because they are so rare and so fragile.
