This old fish gave the entire ocean a stiff lower lip
About 375 million years ago, armored fish ruled a watery 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 first apex predators.
Few of these ancient oddities were stranger than the aptly named Alienacanthus. This Devonian period fish was discovered in Poland in 1957 and was initially known for its large, bony spines. But the recent discovery of a fossilized Alienacanthus skull, described in an article published Wednesday in the journal Royal Society Open Science, reveals that these spines were actually the fish's elongated lower jaw. This lower jaw was twice as long as the rest of the fish's skull and gave Alienacanthus nature's most extreme underbite, and perhaps a stiff lower lip as well.
“It still looks very alien, so the name is very appropriate,” says Melina Jobbins, a paleontologist who studies placoderms at the University of Zurich and authored 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 submerged coastlines on either side 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 twenty years, researchers have discovered even more well-preserved Alienacanthus fossils in European museum collections. Dr. Jobbins worked with researchers from several of these museums to put the fossil pieces together and more accurately describe the ancient fish.
The key to unraveling this fishy enigma was a nearly complete Alienacanthus skull measuring more than 6.5 meters in length, originating from Morocco and currently in the collection of the Paleontological Institute of the University of Zurich. While the elements of the skull were still articulated, the team realized that Alienacanthus' oddly shaped spines were actually the lower jaw bones. This made the fish even stranger: when it closed its mouth, the placoderm resembled an upside-down sailfish with a long, beak-like lower jaw.
While fish such as swordfish and sawsharks employ dramatic upper jaw protrusions, very few species have elongated lower jaw protrusions. Today, this feature is only seen in a group of small fish called half-beaks. But the relative length of Alienacanthus' lower jaw was 20 percent greater than that of half a beak. Alienacanthus' jaw was also proportionately longer than similar structures seen in prehistoric sharks and porpoises, making the fossil fish the undisputed champion of the underbite.
The extended jaw may have helped Alienacanthus sift through sediment, which is how modern half-beaks use their shovel-like jaws. Another hypothesis is that the prehistoric fish used its lower jaw to stun or injure prey.
Dr. Jobbins thinks the elongated jaw, which was studded with recurved teeth that extended well beyond the end of the upper jaw, most likely served as a trap. “Basically it could lure 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 could move independently of the lower jaw and close when a fish or squid was too deep.
This snaggletoothed fish is an intriguing evolutionary oddball. As a placoderm, Alienacanthus was among the first groups of vertebrates to develop complex jaws. The fish provides a glimpse of how extreme the jaws might have been just 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 Alienacanthus' tooth-shaped mug, these armored fish were wiped out and replaced by sharks.