To make the most of their largest and sharpest teeth, prehistoric sharks that lived 300 to 400 million years ago dropped their lower jaws down and rotated them outwards when opening their mouths to catch prey, new research shows.
Many modern sharks have row upon row of formidable sharp teeth that constantly regrow and can easily be seen if their mouths are just slightly opened. But this was not always the case.
The ancestors of today’s cartilaginous fish (chondrichthyan), which include sharks, rays, and chimaeras replaced their teeth more slowly. With mouths closed, the older, smaller, and worn out teeth of sharks stood upright on the jaw, while the younger and larger teeth pointed towards the tongue and were therefore invisible when the mouth was closed.
As reported in Communications Biology, paleontologists examined the structure and function of this peculiar jaw construction based on a 370-million-year-old chondrichthyan from Morocco. They used computed tomography scans to not only reconstruct the jaw, but also print it out as a 3D model. That allowed them to simulate and test the jaw’s mechanics.
They discovered that, unlike in humans, the two sides of the lower jaw were not fused in the middle, which enabled the animals to not only drop the jaw halves downward but at the same automatically rotate both outwards.
“Through this rotation, the younger, larger and sharper teeth, which usually pointed toward the inside of the mouth, were brought into an upright position. This made it easier for animals to impale their prey,” says first author Linda Frey of the University of Zurich. “Through an inward rotation, the teeth then pushed the prey deeper into the buccal space when the jaws closed.”
This mechanism not only made sure the sharks could use their larger, inward-facing teeth, but also enabled the animals to engage in what is known as suction-feeding. “In combination with the outward movement, the opening of the jaws causes sea water to rush into the oral cavity, while closing them results in a mechanical pull that entraps and immobilizes the prey.”
Since cartilaginous skeletons are barely mineralized and generally not that well preserved as fossils, this jaw construction has evaded researchers for a long time.
“The excellently preserved fossil we’ve examined is a unique specimen,” says paleontologist and last author Christian Klug. He and his team believe that the described type of jaw joint played an important role in the Paleozoic era. With increasingly frequent tooth replacement, however, it became obsolete over time and was replaced by the often peculiar and more complex jaws of modern-day sharks and rays.
Additional coauthors are from the University of Chicago.
Source: University of Zurich