Scientists Study Emergence Of Ray-Finned Fish
July 22, 2013

Scientists Study Emergence Of Ray-Finned Fish

Brett Smith for – Your Universe Online

Just like the rapper Drake, ray-finned fish started from the bottom – now they’re here.

The vertebrate class of fish, which comprises 99 percent of all fish species, arose from a small, shell-crushing predatory fish called Fouldenia that survived a massive extinction event 359 million years ago, according to a new report in Zoological Journal of the Linnean Society.

"This event 359 million years ago is called the Hangenberg extinction, and it nearly wiped out vertebrate life, which at the time was limited to the water," said report co-author Lauren Sallan, an ecology and evolutionary biology professor at the University of Michigan. "The ray-finned fish come to the fore after that event. They not only recover from this extinction, but they go from being a few minor lineages to dominating all the oceans."

In the report, Sallan and her colleague Michael Coates, through the identification of a few juveniles, found that the misclassified Fouldenia was the first recorded ray-finned fish. An analysis of the fossils, which were discovered at a site in Scotland, showed that the fish dramatically changed their body shape as they developed.  This developmental idiosyncrasy allowed the fish to evolve into wide range of vertebrate fish, include tuna, cod and catfish.

"These early, post-Devonian ray-finned fish provide the first glimpse of what was to come: an evolutionary profusion of body forms, fin shapes, and extraordinary jaws and teeth. The ray-finned fish really do exemplify Darwin's comment about 'endless forms most beautiful and wonderful,†said Coates, an evolutionary biologist from the University of Chicago.

The team’s analysis also showed that Fouldenia and its relative Styracopterus, which researchers had previously classified as the same, are actually separate species. While Fouldenia had massive tooth plates on its upper and lower jaws, like Japanese amberjack or yellowtail, Styracopterus was more deep-bodied, like the angelfish.

"Those Scottish fossil beds have four or five known genera of ray-finned fish in them. They all look completely different, and they all do completely different things," Sallan said.

The extinction event that allowed the now-extinct Fouldenia to flourish also set the stage for an explosive diversification called an adaptive radiation. A similar event would later kill off the dinosaurs and make way for the rise of mammals.

Sea urchins, sea lilies and shelled invertebrates called brachiopods also survived the Hangenberg extinction. With previously dominant predators wiped out, early sharks and ray-finned fish used their crushing jaws to prey on these spiny, stalked and hard-shelled creatures.

"Because the ecosystem's been decimated, the only thing left to prey on are shelly animals," Sallan said. "So in this vacuum left by the mass extinction event, a bunch of different animals are going into these vacated niches and taking over those jobs."

The Hangenberg event ended the Devonian, often referred to as the Age of Fish. The Hangenberg extinction is associated with a period of extreme climate change, when sea levels rose and the amount of oxygen in the water dropped. This was followed by a cooling period that caused glaciers to reach as far as the tropics.