About 310 million years ago 鈥� and now we know how one of those sharks, called Edestus, fed. The 鈥渟aw blade鈥� in its lower jaw glided backwards and forwards like the blade on some modern power tools, allowing the shark to cut through soft prey like fish.
We know that Edestus was a very odd shark that grew to the size of a modern great white. It had what look a lot like two saw blades in its mouth 鈥� one in the upper and one in the lower jaw. The two blades, which could each be 40 centimetres long but just 3 cm wide, seem to have locked together when the shark closed its mouth, a bit like the blades on a pair of serrated scissors.
But exactly how the two blades worked together to cut through flesh has been unclear. While 贰诲别蝉迟耻蝉鈥檚 saw blade teeth were likely to have contained hard layers of calcium phosphate that meant it fossilised well, the rest of the shark鈥檚 skeleton usually didn鈥檛 because it was made of cartilage rather than bone.
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Now, Leif Tapanila at Idaho State University and his colleagues have solved the mystery by examining a 310-million-year-old fossil that, unusually, included the crushed remains of a near-complete Edestus skull. 鈥淸It鈥檚] the most complete skull known for the animal,鈥� says Tapanila.
A careful analysis shows that it had a distinctive hinge between the lower jaw and the rest of its skull. This allowed the lower jaw 鈥� and its saw blade 鈥� to slide back and forth relative to the upper blade, which stayed fixed in place. Tapanila says the lower jaw worked a bit like the blade on a jigsaw power tool. 鈥淚t pulled backwards during the bite. This raked the upper and lower teeth past the food, slicing and splitting it in half.鈥�
Strange though Edestus was, some of its relatives were even odder. A few years ago, Tapanila and his colleagues analysed a related extinct shark called Helicoprion that had just one toothed saw blade in its lower jaw that grew into a spiral. This made it look like it had a circular saw blade mounted vertically in its mouth. Tapanila鈥檚 team showed that when Helicoprion snapped its jaws shut, the , which would have helped pull soft flesh snagged on the teeth towards the shark鈥檚 throat for easy swallowing.
Journal reference: Anatomical Record, DOI: 10.1002/ar.24046
