Fish Scales And The Origins Of Teeth
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As an explorer I have been moderately exposed to a lot of scientific fields related to natural sciences, the environment and the evolution of life, like palaeontology; however, at best I could be described as an amateur palaeontologist only. But despite that I was recently thinking about the characteristic scales of the Thelodonti: those little things made out of dentine that even had a pulp under their lozenge-shaped surface, much like the teeth of all animals, including us.
We should remember that the Thelodonti were agnathans, that is, jawless fishes that lived during the late Silurian. These very primitive animals preceded the gnathostomata, the fishes with jaws and bones, to which sharks and stingrays belong as the oldest members. Primitive sharks like 'Cladoselache' have been found fossilised with dentine scales surrounding the eyes and mouth, and similar in structure to those of the Thelodonti, albeit it has been established that the sharks, already sporting jaws and teeth evolved around the late Devonian, dozens of millions of years after the Thelodonti, and despite the fact that the latter are difficult to classify among agnathans, it is pretty obvious that the structures that they evolved are prior to the appearance of teeth. Their dentine scales, complete with a pulp similar to what we can find in any tooth are more ancient than anything that forms part of an articulated mouth.
Thus, if 'Cladoselache' fossil specimens present these tiny scales around the mouth opening, and we consider that mandibles seem to have evolved as a modification or adaptation of the anterior gill arches of agnathans slowly changing into a more advanced form of animals, it could be the case that the scaled skin covering the perimeter of the arches could also have folded inwards, into the mouth, at least partially. In that way, very early gnathostomes would have had - if not proper teeth - at least a gripping surface with which to grasp their small or soft prey before crushing and swallowing it.
So, perhaps the same mechanism that generated the growth of cartilaginous structures in order to form the jaws, could also have led to the growth of the mouth scales into thicker units, giving way to true teeth.
This is just an idea, but it would be nice to develop it into a proper study.
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