About 375 million years ago, armored fish ruled an aquatic world. Known as placoderms, these primitive jawed vertebrates came in all shapes and sizes, from small ground dwellers to giant filter feeders. Some, like that Wrecking ball-shaped Dunkelosteuswere among the ocean’s earliest apex predators.

Few of these ancient curiosities were stranger than the aptly named Alienacanthus. Discovered in Poland in 1957, this Devonian-era fish was initially known for its large, bony spines. But the recent discovery of a fossilized Alienacanthus skull is described in an article published Wednesday in the journal Royal Society Open Scienceshows that these spines were actually the extended lower jaw of the fish. This lower jaw was twice as long as the rest of the fish’s skull, giving Alienacanthus nature’s most extreme underbite and possibly a stiff lower lip.

“It still looks very alien, so the name is very appropriate,” said Melina Jobbins, a paleontologist who studies placoderms at the University of Zurich and is an author of the paper.

Since its discovery in the 1950s, Alienacanthus has only been known from a few fossils discovered in the mountains of central Poland and Morocco. During the Late Devonian period, these areas were flooded coastlines at opposite ends of a vast sea that separated 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. Jobbins teamed up with researchers from several of these museums to bring together the fossil pieces and describe the ancient fish in more detail.

The key to solving this fishy mystery was a nearly complete Alienacanthus skull measuring more than two and a half feet, which came from Morocco and is currently in the collection of the University of Zurich’s Institute of Paleontology. While the elements of the skull were still mobile, the team realized that the strangely shaped spines of Alienacanthus were actually its lower jaw bones. This made the fish even stranger: When it closed its mouth, the placoderm resembled an upside-down swordfish with a long, beak-like lower jaw.

While fish such as swordfish and sawsharks have pronounced projections in the upper jaw, very few species have elongated projections in the lower jaw. Nowadays, this feature is only seen in a group of small fish called halfbeak fishes. But the relative length of the lower jaw of Alienacanthus was 20 percent greater than that of a half-beak. The jaw of Alienacanthus was also relatively longer than similar structures found in prehistoric sharks and porpoises, making the fossil fish the undisputed champion of the underbite.

The elongated jaws may have helped Alienacanthus sift through sediment, which is why modern halfbeaks use their shovel-like jaws. Another hypothesis suggests that the prehistoric fish used its lower jaw to stun or injure prey.

Dr. Jobbins believes that the elongated jaw, which was lined with recurved teeth that extended well beyond the end of the upper jaw, most likely served as a trap. “Essentially it could invite prey in and then it can’t get out because there’s only one way to go,” she said. The shorter upper jaw of Alienacanthus could move independently of the lower jaw and snap shut when a fish or squid was too deep.

This hooktooth fish is a fascinating evolutionary oddball. As a placoderm, Alienacanthus was among the first vertebrate groups to develop complex jaws. The fish gives a glimpse of how extreme the jaws could be after the now widespread feature emerged.

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