It has been suggested by an old fish fossil that teeth did not develop within the mouth. There are two hypotheses about the genesis of teeth: either they emerged from internal sources (the inside-out hypothesis) or exterior scales (the outside-in theory).
Photo by Kamel15 via Wikimedia Commons
Outside-In Theory
More proof for the outside-in theory has been discovered by researchers looking at a fossil of the extinct sawfish species Ischyrhiza mira, which inhabited North America 65 to 100 million years ago.
Animals' teeth play a crucial part in their daily lives. They aid in food digestion and chewing.
The traditional hypotheses, generally known as the "outside-in" model, postulate that the odontode-competent tissue layer traveled from the body surface into the mouth cavity of jawed vertebrates, where teeth eventually developed. Although they do not grade into one another, and there is no continuous transition between the two structures, this viewpoint is mainly based on the physical similarity of shark skin denticles to teeth.
The animal featured sharp spikes around its nose to help fight off predators and scavenge for food, just like the sawfishes and sharks of today. These spikes, known as rostral denticles, are assumed to be altered copies of the scales on the rest of the body.
SYDNEY, AUSTRALIA - JULY 25: The teeth and jaw of a Great White Shark are displayed after research into the biological mechanics of the predator July 25, 2007 in Sydney, Australia. Researchers at the University of New South Wales (UNSW) including Dan Huber from the the University of Tampa, Florida in the USA, plan to remove muscles from the head of several sharks in order to create a digital shark, to help determine it's bio mechanics and potential "bite force".Photo by Ian Waldie/Getty Images
The researchers examined the hard enameloid outer layer of the snout spikes to investigate the link between rostral denticles and scales. However, what they discovered was very different from what they had anticipated.
The rostral denticle enameloid of I. mira was surprisingly anything but straightforward, according to vertebrate paleontologist Todd Cook of Pennsylvania State University. Compared to the enameloid of body scales, it was far more complicated.
The overall structure of the enameloid in an ancient sawfish mirrored that of a well-characterized enameloid in an existing shark tooth.
Similar to Modern-Day Sharks
The enameloid on the fossil's tooth is particularly similar to the enameloid on contemporary shark teeth in that both are composed of bundles of fluorapatite microcrystals that are grouped neatly on the tooth's surface, more haphazardly below.
Packed microcrystals that are parallel to the surface of the tooth run through these layers. Shark teeth are strong and resistant to stress because of their many orientations, and the I appears no different. mira.
Cook claims that it is possible that the enameloid of I. mira's rostral denticles' bundle-like microcrystal arrangement also functioned as a means of withstanding mechanical stresses.
It's not unlikely that these scales and teeth acquired their packed microstructures separately, but the outside-in idea regarding the origin of teeth makes more sense if one arose after the other.
An Unexpected Discovery
Interestingly, when the researchers started examining rostral denticles, they had no intention of studying the development of teeth. Still, their conclusions may have a big influence on future research in this area.
The likelihood that the teeth in our mouths are highly developed fish scales is increasing as more and more parallels between the exteriors of marine animals and those of humans are discovered.
According to Cook, this discovery reveals that scales can develop a sophisticated enameloid that resembles a tooth outside of the mouth, providing direct proof for the outside-in concept.
