The evolution of fish: the head before the body

Until now, scientists thought that the diversification of fish began with their bodies, following the extinctions of Devonian and Cretaceous. But a new study showed that the head  would have been the first in the evolution, driven by their diet.

The work of Lauren Sallan of the University of Chicago and Matt Friedman from that of Oxford have challenged the theories regarding of the phenotypic changes of species after an explosion radiative (strong increase in biodiversity following a mass extinction ). According to the research, published in Proceedings of the Royal Society B , the main anatomical variations were first held in the head, and only then, the rest of the body.

So far, however, there are quite different theories. Two are dominant:

-The first concerns all animals and believes that following the major radiative explosion, a strong anatomical diversification was first made, and then faded;
-The second (actually, a set of models leading to similar conclusions), which applies only to vertebrates , assumes that changes have taken place initially at the bodies of animals, characteristics of the head has changed at a second time. According to this theory, it is the habitat conditions that have guided the evolution of characters.
The diet as a selection pressure

At the end of the Devonian , there are about 370 million years, held one of the largest mass extinctions, which mainly affected the aquatic life. Following this extinction, including actinopterygians fish, from the survivors, have greatly diversified to occupy ecological niches released by the organisms have been unsuccessful. The same applies to the first acanthomorph (teleost fish) as a result of the extinction at the end of the Cretaceous (which was also wiped out the dinosaurs there 65 million years).

By studying the evolution of characters after these extinctions,  the scientific community had led to the paradigm “Housing First” which considers that it is adapting to new living environments that guided the evolution in these periods, resulting in morphological changes of the body.

Changes in the anatomy of the head and body

But by analyzing the fossil of actinopterygians fish at their disposal, scientists have reached a different conclusion, even opposite. They believe it is the morphology of the head which was a strong subject variation: the evolution of the jaw , the appearance of sharp teeth or teeth that can chew blunt, indicating a carnivorous diet.

Then in a second step, the rest of the body has taken major changes: change of body size, its shape (elongation), etc.. This is not the environment that would have played the role of selective pressure, but more likely the diet.

Impossible to generalize, however, said Lauren Sallan. If the model “head first” seems to apply to fish from the Devonian and Cretaceous, the events may be held later inversely other radiation. In addition, the remaining agencies probably follow a different pattern. One can also expect some differences according to geographical area. Analysis in each case will learn more.

Abdullah Al Abbadi

Source: Dec 21, 2011


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