Vertebrate Evolution: Heads or Tails?

sallan-fossilsIn the aftermath of a mass extinction, nature tends to get creative. Those lucky species that survive often explode with Seussian abandon into a diverse array of shapes, sizes, and behaviors, capitalizing upon the ecological opportunities left available by their less fortunate peers. Usually, the oddities produced by these “adaptive radiations” are whittled down by natural selection to only a few surviving forms. But evolutionary biologists are interested in the course these radiations take — the dynamics that result when nature hits the “randomize” button.

Scientists have tried to understand the order underlying this chaos by studying modern animals that have established broad diversity, such as the immense cichlid family of fishes (which encompasses over 1,000 documented species) or Darwin’s finches of the Galapagos islands. But these studies can only work backwards from the species that exist today. To watch an adaptive radiation unfold, a better source is the fossil record, as the University of Chicago’s Lauren Sallan and the University of Oxford’s Matt Friedman discovered in a recent journal article for Proceedings of the Royal Society B.

Sallan and Friedman used fossil databases from two prehistoric mass extinction events: the Hangenberg event, of roughly 359 million years ago, and the end-Cretaceous extinction, which ended the age of dinosaurs. By measuring how surviving fish species changed body shape and size after these ecological disturbances, the researchers could test two common theories of adaptive radiation inspired by studying surviving species. One model proposed a free-for-all “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.

“There hadn’t been any tests of these things using fossils,” said Sallan, a graduate student in the Department of Organismal Biology and Anatomy. “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.”

When Sallan and Friedman looked carefully at their data, they didn’t find evidence for either of the pre-existing theories. Instead, they saw a staged radiation that started not tail-first, but head-first, with surviving species initially trying out a wide range of head shapes attached to similar bodies. The driver of this diversity may have been a simple factor: food. Faced with far less competition, the surviving fish evolved new types of teeth, jaws, and heads to take advantage of the expanded menu suddenly available. Later, once head shapes stabilized, different body types from broad and flat to thin and eel-like appeared as new species adapted to their surroundings.

“It seems like resources, feeding and diet are the most important factors at the initial stage,” said Sallan, who works in the laboratory of University of Chicago Professor Michael Coates.

“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. Ecological limits are taken away, and there’s more opportunity out there, more available resources, and they’re taking advantage of that. Later, they’re taking advantage of specializing to new habitats. So it’s not something within the animals themselves; it’s more opportunity that matters.”

While the new study refutes previous theories and offers two distinct examples of head-first diversification separated by hundreds of millions of years, the universality of the model remains to be conclusively proven, the authors said.

“Evolution is really complex, and it’s not really clear that there should be only one model,” Sallan said. “It might be that this model might apply to fishes in certain time periods, or might apply to vertebrates, but a lot more investigation is needed to see whether that is actually true.”

[Read more about the study at Scientific American and Discovery News.]

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Sallan LC, & Friedman M (2011). Heads or tails: staged diversification in vertebrate evolutionary radiations. Proceedings. Biological sciences / The Royal Society PMID: 22189401

About Rob Mitchum (526 Articles)

Rob Mitchum is communications manager at the Computation Institute, a joint initiative between The University of Chicago and Argonne National Laboratory.

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