Friday, May 24

This ancient fish gave the entire ocean a stiff lower lip

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 tiny bottom dwellers to giant filter feeders. Some, like the wrecking ball Dunkleosteus, were among the ocean’s earliest predators.

Few of these ancient oddities were stranger than the aptly named Alienacanthus. Discovered in Poland in 1957, this fish from the Devonian period was initially known for a series of large bony spines. But the recent discovery of a fossilized Alienacanthus skull, described in a paper published Wednesday in the journal Royal Society Open Science, reveals that these spines were actually the fish’s elongated lower jaw. Measuring twice as long as the rest of the fish’s skull, this lower jaw gave Alienacanthus nature’s most extreme bite and, perhaps, a stiff lower lip.

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

Since its discovery in the 1950s, Alienacanthus has only been known from a few fossils found in the mountains of central Poland and Morocco. During the Late Devonian Period, these areas were submerged shores at opposite ends of a vast sea that separated the northern and southern supercontinents. But many of these fossils are fragmentary and offer few details about what this strange fish looked like.

Over the past two decades, researchers have discovered more well-preserved Alienacanthus fossils in European museum collections. Dr. Jobbins collaborated with researchers at many of these museums to piece together the fossil fragments and more accurately describe the ancient fish.

The key to solving this mysterious puzzle was a nearly complete Alienacanthus skull, more than two and a half feet long, native to Morocco and currently in the collection of the Paleontological Institute of the University of Zurich. With the skull elements still articulated, the team realized that the strangely shaped spines of the Alienacanthus were actually the bones of the lower jaw. This made the fish even stranger: When it closed its mouth, the placoderm resembled an upside-down billfish with a long, beak-shaped lower jaw.

While fish such as swordfish and sawsharks possess spectacular upper jaw protrusions, very few species possess elongated lower jaw protrusions. Today this characteristic is found only in a group of small fish called halfbeaks. But the relative length of the lower jaw of Alienacanthus was 20% greater than that of a half-beak. The jaw of Alienacanthus was also proportionately longer than similar structures observed 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 halfbeaks use their shovel-shaped jaws. Another hypothesis is that the prehistoric fish brandished its lower jaw to stun or injure prey.

Dr. Jobbins believes that the elongated jaw, studded with curved teeth that extended well beyond where the upper jaw ended, most likely served as a trap. “It could basically invite prey in and then they can’t get out because there’s only one way to go,” he said. The shorter upper jaw of Alienacanthus could move independently of the lower jaw and snap shut once a fish or squid was too deep.

This crooked-toothed fish is an intriguing evolutionary oddity. Being a placoderm, Alienacanthus belonged to the first vertebrate groups to develop complex jaws. The fish offers an idea of ​​how extreme the jaws could have been soon after the now-widespread feature originated.

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