Wednesday, 24 June 2015

On The Origin Of Teeth

A skull of Romundina, a transitional form
between jawed and jawless vertebrates
Jaws transformed the vertebrate kingdom. In just a few tens of millions of years, they went from small bottom dwellers to the dominant group of marine animals. Some attained great size whilst others took to entirely new ecological niches. As a result the hunt for the origin of the jaw has been a major goal in palaeontology. In 2014 light was shed on the problem by a fossil fish known as Romundina. Its morphology and anatomy was a halfway house between jawless and jawed vertebrates. Still older fossils have illuminated the anatomical precursor to the vertebrate jaw itself. Yet jaws are nothing without teeth.

Teeth allow the specialisation of an animal's food sources. In many cases single teeth can be used to characterise entirely new species due to their unique morphology. Evidence for the origin of teeth has come from examinations of the earliest group of jawed vertebrates, the shark-like acanthodians. Their fossils suggest that they were derived from scales which formed on the outside of the jaw and gradually migrated into the mouth over the course of millions of years. However, this did not explain the presence of tooth-like structures in older, non shark groups. Instead a new explanation for the origin of teeth has come from further examinations of Romundina.

A 3D x-ray scan of a tooth plate from Romundina. The oldest 'pioneer' tooth is coloured
gold. The subunits are increasingly younger as you go further out from the centre
Dr Phil Donoghue, along with Martin R├╝cklin from the Naturalis Biodiversity Center in Leiden, conducted 3D x-ray scans of Romundina tooth plates collected from 400 million year old rocks on the Prince of Wales Island in Canada. The tooth plates were composed of a series of microscopic bumps which created a sandpaper-like surface designed to crush and shred food. The 3D x-ray scans made it clear, however, that the tooth plates did not form all at once. The resolution of the scans allowed structures as small as 10 micrometres across to be visualised. They showed that the tooth plates developed gradually.

The process started with a single central 'pioneer' tooth around which additional tooth-like structures agglomerated, the overall tooth plate steadily growing outwards. Each subunit possessed a distinct core of dentine and an enamel covering much like our own teeth, but had failed to differentiate from the overall tooth plate. In time these subunits did eventually differentiate to form individual teeth capable of their own development. Mammals would take this a step further with each tooth gaining a distinct identity, for example as a flat, textured molar or a pointed canine. Yet this heritage may ultimately lie in the tooth plates of Romundina and its kin.