December 20, 2011
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Two radiations of ray-finned fishes (Actinopterygii), Carboniferous forms (facing left) and acanthomorph teleosts (facing right) underwent distinct cranial (feeding) and later postcranial (habitat) stages in trait diversification. Credit: (Photographs by Lauren Sallan and Matt Friedman).
The history of evolution is periodically marked by explosions in biodiversity, as groups of species try out a wide range of shapes and sizes. With a new analysis of two such adaptive radiations in the fossil record, researchers have discovered that these diversifications proceeded head-first.
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By analyzing the physical features of fossil fish that diversified around the time of two separate extinction events, scientists from the University of Chicago and the University of Oxford found that head features diversified before body shapes and types. The discovery disputes previous models of adaptive radiations and suggests that feeding-related evolutionary pressures are the initial drivers of diversification.
“It seems like resources, feeding and diet are the most important factors at the initial stage,” said lead author Lauren Sallan, graduate student in the Department of Organismal Biology and Anatomy at the University of Chicago. “Strange heads show up first – crushing jaws, animals with big teeth, with long jaws – but they’re all pretty much attached to the same body.”
Adaptive radiations underlie the evolution of dominant and diverse groups. After a major disruption, such as an extinction event, surviving species diversify into a myriad variety of forms. Modern examples of this diversity are the fish family of cichlids, with more than 1,000 documented species, or “Darwin’s finches” of the Galapagos Islands, which exhibit many different beak types.
Evolutionary biologists have used these living species to propose at least two models of how adaptive radiations work. One model proposes a single “burst” of divergence followed by a long period of relative stability. Another, sometimes known as the “general vertebrate model,” introduced the idea of staged divergences, with habitat-driven changes in body type preceding diversification of head types.
However, these models had not yet been tested with the rich data sets available in the fossil record.
“There hadn’t been any tests of these things using fossils,” said Sallan, a graduate student in the laboratory of University of Chicago Professor Michael Coates. “You have all these analyses of diversification, yet not one of them goes back to the fossil record and says what’s happening at this time period, and the next time period, and the one after that.”