Astonishingly Preserved Fossil Reveals Origins Of Most Common Modern Fishes

Artist's reconstruction of Pickeringius acanthophorus, an early example of the ray-finned fishes that have since come to dominate the oceans. Dr Brian Choo

The ray-finned fish class includes 98 percent of living fishes. However, 400 million years ago, ray-finned fishes were rare, making the few Devonian Era fossils we have more interesting than those from fishes that became evolutionary dead-ends. In this context, an exceptionally well-preserved example of a previously unknown 380-million-year-old ray-finned fish is a precious find.

Dr Brian Choo of Flinders University and colleagues named the newly identified fish Pickeringius acanthophorus. It's only the third Devonian ray-finned fish where we have a three-dimensional relic of its braincase. Choo told IFLScience that intact brain cases help build species family trees. ”Brain cases change much more slowly than other body parts,” he said.

Pickeringius also has a highly distinctive feature, Choo reports in Papers in Palaeontology. Many Devonian Era fishes had openings called spiracles on the top of their heads, but Pickeringius' are much larger than any we have found on its contemporaries – up to a fifth the skull's length.

Today, African bichirs use similar openings to breathe air when the swamps in which they live become low on oxygen. Modern rays use them when their gills are pressed against the seabed. Choo told IFLScience neither function appears useful for what looks like a mid-depth fish, so their purpose remains a mystery, although Choo speculated they might have been handy if trapped in a rockpool waiting for high tide.

The fish also carried spikes all over its body, some of which had lateral barbs for extra protection against the era's fearsome predators.

The fossil itself, partly still encased in limestone. Dr Brian Choo.

The Gogo Formation is among the most remarkable fossil sites in the world. Most fossils, particularly early ones, from other sites are incomplete and have been squashed flat. Gogo, however, is one of a very small number of places where we find such ancient intact three-dimensional specimens. Many Gogo fossils are so well preserved, they are referred to as “swimming in stone”. 

This preservation occurred, Choo explained, because during the Devonian there was a thriving reef off the coast of Western Australia next to a steep drop-off into deep water. “There were no polar ice caps at the time to drive ocean circulation,” Choo said.

As a result, the deep oceans had much less oxygen than they do today, which prevented the scavenging of fish that sank into deep waters. Anaerobic bacteria consumed the fishes' flesh, but before the bones could decay, they became protected by a shell of calcium carbonate, which eventually became a nodule of limestone. When paleontologists split these nodules open, they are often rewarded with beautifully preserved species, in a way we don't see from eras when deeper waters had enough oxygen to support more life. Pickeringius is the fifth ray-finned species we've found at Gogo.

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