New research reported in the Zoological Journal of the Linnean Society shows that the previously known but misclassified small predatory fish Fouldenia was the first recorded shell-crushing ray-finned fish.

Fouldenia first appears in the fossil record a mere 11 million years after an extinction event that ended the Devonian Era and wiped out more than 90 percent of the planet’s vertebrate species.

“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 lead author Dr Lauren Sallan from the University of Michigan and the University of Chicago.

Fouldenia and a handful of its relatives demonstrate that in the immediate aftermath of the end-Devonian extinction, ray-finned fish had already acquired a diversity of forms that gave them an evolutionary edge, enabling them to fill the ecological vacuum left by the demise of most major fish groups.

“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.”

Dr Sallan and his colleague Dr Michael Coates of the University of Chicago were able to identify juveniles of Fouldenia, a rare find that allowed them to show that the body shape of these fish changed dramatically as they developed. The relatives of Fouldenia, shell-crushers all, apparently took advantage of this developmental quirk to produce new forms. This diverse band of survivors spread worldwide and persisted for nearly 100 million years.

There are around 30 thousand species of ray-finned fish today, comprising nearly 99 percent of all fish species. Think of the word ‘fish’ and the image that pops into your mind will likely be a ray-finned fish, members of a ubiquitous class that includes everything from tuna to trout, catfish to cod, swordfish to sunfish, perch to piranha, goldfish to goby.

After re-evaluating fossils from sites in Scotland dating to 348 million years ago, the scientists concluded that Fouldenia and its relative Styracopterus, which previous researchers had classified as the same species, are in fact separate genera. A genus is the category of biological classification between the family and the species.

They determined that Fouldenia had massive tooth plates on its upper and lower jaws, suitable for preying on hard-shelled animals. It resembled modern-day jacks, which include the Japanese amberjack, or yellowtail, familiar to sushi lovers. Styracopterus was an early mimic of modern deep-bodied fish such as the angelfish. Both of these primeval fish were less than 10 inches long.

“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,” Dr Sallan said.

Before the Hangenberg extinction, fish were dominated by two groups: the armor-plated, predatory placoderms and the lobe-finned fish, whose fins are borne on a fleshy, scaly stalk extending from the body. Placoderms were eliminated by the extinction, and most of the lobe-finned fish perished as well, though survivors live on today in the lungfish and the coelacanth. In addition to a few ray-finned fish, some sharks and tetrapods survived the event. Other survivors of the event included sea urchins, sea lilies and shelled invertebrates called brachiopods. With most other predators now out of the picture, early sharks and ray-finned fish like Fouldenia used their crushing jaws to dine on these spiny, stalked and hard-shelled creatures.

“Because the ecosystem’s been decimated, the only thing left to prey on are shelly animals. 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,” Dr Sallan concluded.

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Bibliographic information: Lauren Cole Sallan and Michael I. Coates FLS. Styracopterid (Actinopterygii) ontogeny and the multiple origins of post-Hangenberg deep-bodied fishes. Zoological Journal of the Linnean Society, published online July 23, 2013; doi: 10.1111/zoj.12054