Ancient fish may have set stage for jaws

New fossils reveal gills possibly on their way to chomping on prey

FRESH CATCH Fossils of Metaspriggina walcotti suggest that the primitive fish, which was roughly 6 centimeters long, had front gill bars that may have served as a precursor to the jaw. The gill bars, structures that supported the fish’s gills, appear as dark stitches.

Jean-Bernard Caron © ROM

By Ashley Yeager

June 11, 2014 at 1:46 pm

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One of the earliest outlines of jaws may have been written in the gills of a fish called Metaspriggina walcotti.

The fish, which lived roughly 500 million years ago, had been described previously. But 100 new specimens collected in the Canadian Rockies provide a detailed look at the creature’s gill bars, the structures that ran vertically down the body and supported the gills. In M. walcotti, each gill bar had an upper and lower arch, possibly made of cartilage-like tissue. The front set of gill bars was slightly thicker than the others and did not actually have gills beside them. These gill bars might have served as a precursor to the jaws that evolved around 420 million years ago in younger fish, researchers argue June 11 in Nature.

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To form a jaw from M. walcotti’s front grill bars and achieve a chomping motion, one of its descendants would have needed a hinge where the gill bars’ upper and lower arches meet and some new muscles, the scientists suggest.

FAST FISH Metaspriggina walcotti had a pair of bulging, camera-like eyes, which probably gave it good vision, and a stiff but flexible spinal cord–like rod. It had up to 40 W-shaped muscles running the length of its body, making it a good swimmer that maneuvered easily in the oceans half a billion years ago.