This ancient fish gave the entire ocean a solid lower lip

About 375 million years ago, armored fish ruled an aquatic world. These primitive jawed vertebrates, known as placoderms, came in all shapes and sizes, from tiny bottom-dwellers to giant filter-feeders. Some, like the wrecking-ball-shaped Dunkleosteus, were among the ocean's oldest predators.

Few of these ancient oddities were stranger than the name Alienacanthus. This Devonian fish was discovered in Poland in 1957, and was initially known for its set of large bony spines. But the recent discovery of the fossilized skull of Alienacanthus was described in a research paper published Wednesday in the journal Royal Society Open ScienceHe reveals that these spines were actually the fish's elongated lower jaw. Twice as long as the rest of the fish's skull, this lower jaw gave the lower jaw the maximum bite normal to Alienacanthus, and perhaps a sclerotic lower lip.

“It still has a very strange appearance, so the name is very appropriate,” said Melina Gubbins, a paleontologist who studies placoderms at the University of Zurich and is an author on the paper.

Since its discovery in the 1950s, Alienacanthus has only been known from a few fossils unearthed in the mountains of central Poland and Morocco. During the Late Devonian, these areas were submerged coasts on opposite sides of a vast sea separating the northern and southern supercontinents. But many of these fossils are fragmentary and provide few details about what this strange fish looked like.

Over the past two decades, researchers have discovered additional well-preserved Alienacanthus fossils in European museum collections. Dr. Gubbins collaborated with researchers from several of these museums to piece together the fossil pieces and more accurately describe the ancient fish.

The key to solving this suspicious mystery was a nearly complete Alienacanthus skull measuring more than two and a half feet long that originated in Morocco and is currently in the collection of the Institute of Paleontology at the University of Zurich. As elements of the skull continued to emerge, the team realized that the strangely shaped spines of Alienacanthus were actually lower jaw bones. This made the fish even stranger: when its mouth was closed, the placoderms resembled an upside-down billfish with a long, beak-like lower jaw.

While fish such as swordfish and sawshark have dramatic ridges on the upper jaw, very few species have long ridges on the lower jaw. Today, this feature is only seen in a group of small fish called hemibeaks. But the relative length of the lower jaw of Alencanthus was 20 percent greater than the length of the mandible. Alienacanthus' jaw was also proportionally longer than similar structures seen in prehistoric sharks and porpoises, making the fossil fish the undisputed champion of the underbite.

An extended jaw may have helped Alienacanthus sift through sediment, and this is how modern hemibeaks use their shovel-like jaw. Another hypothesis is that the prehistoric fish used its lower jaw to stun or injure prey.

Dr. Gubbins believes the long jaw, which was studded with curved teeth that extended far beyond where the upper jaw ended, was likely a trap. “Basically, it can invite prey in and then they can't get out because there's only one way to go,” she said. The shorter upper jaw of Alienacanthus can move independently of the lower jaw and close once the fish or squid is at a great depth.

This jagged-toothed fish is an interesting and curious evolutionary being. As a placoderm, Alienacanthus belongs to the oldest vertebrate groups to have evolved a complex jaw. The fish provides a glimpse into just how extreme the jaws are now that this widespread feature appears.

Alienacanthus also represents one of the final chapters of the evolutionary ingenuity of placoderms. Within 15 million years of the appearance of the cup-toothed Alienacanthus, these armored fish were wiped out and replaced by sharks.