The recent discovery of an ancient fish fossil in Poland, first unearthed in 1957, has revolutionized our understanding of the early jawed vertebrates. Named
Alienacanthus for its unique appearance, this prehistoric fish lived during the Devonian period, between 419 and 358.9 million years ago, at a time when Earth was divided into two supercontinents.
The reinterpretation of this specimen revealed an astonishing feature: an extension initially considered as dorsal spines is actually an extremely elongated lower jaw, equipped with teeth, thus constituting one of the most extreme examples of prognathism (misalignment of the two jaws) ever recorded.
The ancient giant fish Alienacanthus had pronounced lower jaw prognathism.
Credit: Artist's illustration courtesy of Beat Scheffold and Christian Klug is among the placoderms, a group of armored fish that are among the earliest known jawed vertebrates. This discovery highlights
Alienacanthus's ability to migrate across oceans despite sea-level fluctuations, with specimens found both in the mountains of central Poland and Morocco. This testifies to its presence on both sides of the supercontinent.
The analysis of nearly complete skulls discovered in the Anti-Atlas mountain range in Morocco revealed that the long protruding appendage from the head of
Alienacanthus was in fact its lower jaw, twice as long as the skull itself. This unique feature implied a different jaw mechanics from its placoderm counterparts, allowing some mobility of the upper jaws to accommodate the pronounced lower jaw prognathism.
Researchers, by comparing
Alienacanthus to modern species with unequal jaws such as the swordfish, have developed three main hypotheses on the usefulness of this lower jaw prognathism: for trapping live prey, for disorienting or injuring prey, or for filtering sediments at the ocean's bottom. The first hypothesis, based on the arrangement of teeth that prevent the prey from escaping once captured, seems the most plausible.
This discovery illustrates the astonishing diversity of jaw shapes and proportions during the late Devonian, a period marked by a remarkable evolution in vertebrate morphology. Researchers continue to study
Alienacanthus to better understand the mechanics of its jaw and the general aspect of its body, thereby clarifying an interpretation error several decades old.