As a species, sharks pre-date even the earliest dinosaurs, so it's not surprising to hear that they've changed a bit over the years. It was recently discovered, for instance, that they were once able to rotate the two sides of their jaw outwards, helping them to feed.
Many readers will already know that modern sharks have successive rows of teeth within their mouths. Whenever one of the teeth in the front row falls out, the corresponding tooth in the row behind moves forward to take its place.
Prehistoric sharks likewise had multiple rows of teeth, with the front row standing upright, and the rows behind angled back in toward the tongue. Over time, the exposed front teeth would get worn down, while the sheltered teeth behind would remain larger and sharper.
In a recent study – which included paleontologists from the University of Zurich, the University of Chicago and the Netherlands' Naturalis Biodiversity Center – it was discovered that those back teeth were also put to use.
The scientists started by conducting CT (computed tomography) scans of the fossilized jaws of a 370-million-year-old shark known as Ferromirum oukherbouchi. Although the cartilaginous skeletons of sharks typically don't fossilize well, that wasn't the case with this particular specimen.
"Shark cartilage is exceptional material – it has a fibrous jacket knitting together a coat of tiny calcified tiles," U Chicago's Dr. Michael Coates tells us. "Interior to the jacket, the cartilage isn't so different from the cartilage in our outer ears or the tip of your nose. Elastic, springy protein-rich gel. But that stuff doesn't preserve. The calcified skin does, but only rarely, and in most instances, it collapses. Hence the importance of the 3D undistorted preservation of these jaws."
Based on their scans, the researchers proceeded to 3D-print a physical replica of the jaws. Among other things, it was noted that the two sides of the lower jaw were not fused together in the middle, so they could move independently.
Mechanical testing of the 3D-printed jaws subsequently indicated that when ancient sharks such as Ferromirum fed, they both dropped their lower jaw and rotated its sides outward, away from one another. This action not only helped to suck water (and prey) into their mouths, but it also angled the bigger, pointier rear teeth upwards, allowing them to bite into the prey. And when the two sides of the jaw moved back together as the mouth closed, the action helped pull the impaled prey inside.
As sharks evolved over the millennia, they developed the ability to replace lost teeth much more quickly. As a result, it is believed that the jaw-rotating mechanism became obsolete and ultimately disappeared.
A paper on the research, which was co-led by U Zurich's Prof. Christian Klug, was recently published in the journal Communications Biology.
Source: University of Zurich
