About 375 million years ago, multi-layered fish ruled the aquatic world. Known as placoderms, these primitive jawed vertebrates come in a variety of shapes and sizes, from tiny bottom dwellers to giant filter feeders. Some of them, such as the wrecking ball-shaped Dunkleosteus, were among the earliest apex predators in the ocean.
Few of these ancient oddities are stranger than the namesake Alienacanthus. Discovered in Poland in 1957, this Devonian age fish was originally known for its set of large bony spines. But the recent discovery of a fossilized Alienacanthus skull, described in a paper published last Wednesday in the journal Royal Society Open Science, reveals that these spines were actually the fish’s elongated lower jaw. Twice as long as the fish’s skull, this mandible gave Alienacanthus the most extreme reverse bite in nature, and, perhaps, a stiff lower lip.
“It still looks so strange that the name is very appropriate,” said Melina Jobbins, a paleontologist who studies placoderms at the University of Zurich and is one of the authors 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 coastlines at opposite ends of a vast ocean separating the northern and southern supercontinents. But many of these fossils are just fragments and provide few details about the appearance of these strange fish.
Over the past two decades, researchers have discovered more well-preserved Alienacanthus fossils in museum collections in Europe. Dr. Jobbins partnered with researchers from several of these museums to collect fossil fragments and describe these ancient fish more accurately.
The key to solving this fish puzzle is a nearly complete Alienacanthus skull more than two and a half feet long that comes from Morocco and is currently in the collection of the Institute of Paleontology of the University of Zurich. With elements of the skull still connected, the team realized that Alienacanthus’ strange spines were actually its lower jaw bones. This makes this fish even more bizarre: when its mouth is closed, this placoderm resembles an upside-down billfish with a long, beak-like lower jaw.
Although fish such as swordfish and saw sharks have dramatic upper jaw protrusions, only a few species have elongated lower jaw protrusions. Currently, this feature is only found in a group of small fish called halfbeaks. However, the relative length of the lower jaw of Alienacanthus is 20 percent greater than that of the halfbeak. The lower jaw of Alienacanthus was also proportionally longer than similar structures seen in ancient sharks and dolphins, making this fossil fish the undisputed champion of the reverse bite.
The elongated jaws may have helped Alienacanthus filter sediment, much as modern halfbeaks use their shovel-shaped jaws. Another hypothesis states that this ancient fish used its lower jaw to hit or injure its prey.
Dr. Jobbins thinks that the elongated jaw, filled with curved teeth that extend far beyond where the upper jaw ends, likely served as a trap. “Basically, it can invite prey in and then they can’t get out because there’s only one way out,” he said. Alienacanthus’ shorter upper jaw could move independently of its lower jaw and close once the fish or squid got too deep.
This sharp-mouthed fish is an interesting evolutionary phenomenon. As a placoderm, Alienacanthus belongs to the earliest group of vertebrates to develop complex jaws. This fish provides an idea of how extreme jaws could become once this now widespread feature appeared.
Alienacanthus also represents one of the final chapters in the genius of placoderm evolution. Within 15 million years after the appearance of the Alienacanthus hooked face, these plated fish disappeared and were replaced by sharks.
2024-01-31 01:57:38
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