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First teeth grew outside of the body

Devonian fish

Cheek scales on prehistoric ischnacanthid acanthodian fish like this one, appear to have evolved into teeth, say researchers (Source: Mark Wilson/University of Alberta)

First teeth The world's first teeth grew outside of the mouth before later moving into the oral cavity, new research on Early Devonian fish suggests.

The study, published in the Journal of Vertebrate Paleontology, supports what is known as the "outside-in" hypothesis of tooth evolution.

"The first smile would probably have been a prickly one, with many tiny teeth that looked like pointy cheek scales, and other small tooth-like scales wrapping around the lips onto the outside of the head," says researcher Dr Mark Wilson, a professor in the Department of Biological Sciences at the University of Alberta.

For the study, Wilson and colleagues studied animals called ischnacanthid acanthodians, an extinct group of fish that resembled sharks. They lived during the Early Devonian period, which lasted from 416 to 397 million years ago.

The researchers determined that head scales from these fish were in transition, evolving from scales to teeth. The pointy structures were identified on the lips of the fish. This discovery helps to negate the "inside out" theory of tooth evolution, which holds that the first teeth emerged from structures in the pharynx progressing into the mouth.

Project leader Stephanie Blais, a University of Alberta researcher, says that "our findings support the idea that teeth evolved from modified pointed scales on the mouth margins [lips] as we see in obtusacanthus," one of the prehistoric fish included in the study.

All of the analysed fish specimens were excavated at the Man on the Hill site in the Mackenzie Mountains of Canada.

As to why teeth first evolved, Blais says they "would have conferred a major advantage in terms of food acquisition".

"Pointed scales near the margins of their mouths would have helped them grasp prey and hang on to it until they could swallow it whole."

Such prey consisted of "probably whatever they could swallow," says co-author Lindsay MacKenzie of the University of Montana's Department of Geosciences. Based on fossilised stomach contents and other evidence, their primary prey probably consisted of arthropods, including crustaceans, as well as a variety of soft-bodied creatures and fish.

Evolutionary arms race

Blais says jaws, which must have evolved earlier, and the tooth-like formations "allowed fishes to change from a filter-feeding or mud-grubbing more passive lifestyle to one of active predation",

The world's first aggressive conflicts also may have arisen at this point, since the move from passive feeding to hunting led to what Blais terms "the very first evolutionary arms race" among vertebrates, with some becoming predators and others becoming prey.

The first teeth also probably first arose in sets, and not just as a single tooth here and there.

MacKenzie says that "scales and teeth exist in developmental fields in which similar developmental processes generate many similar tooth or scale elements".

Since the first teeth likely arose from these scales, they then to some extent mirrored the prior scale groupings, emerging as sets of teeth.

There is a connection between human teeth and these first fish teeth.

"Because teeth would have allowed vertebrates to become more efficient predators, and eventually more efficient herbivores, they were retained and passed down through generations in most groups," says Blais.

While humans and other animals with teeth share this initial fish connection, a lot of evolution, as well as inter-species battles, occurred over the millions of years.

"Interactions between predators and prey through time only increased their [teeth's] importance, leading to highly specialised teeth, such as those in mammals," says Blais.

Tags: palaeontology, fish