Why the long face?

What do you think of when we say “vertebrates”? You could think about us humans, or other living animals like dogs or zebras. You could also think of the features they share, like a vertebral column. But have you stopped to think about how vertebrates came to be, or how defining features like jaws, teeth, or arms and legs came to evolve? 

New fossils of a fish with an underbite, called Alienacanthus, give us a hint that jaws diversified rapidly soon after their origin. Alienacanthus is a member of a group of fish called placoderms, which include some of the first jawed vertebrates. They look like armoured fish of various shapes and sizes and are essential to understanding the evolution of vertebrates and their features, particularly jaws and teeth. Together, placoderm jaws and teeth hold evidence of feeding strategies and diets, giving us insights into what and how some of our fishy ancestors ate. 

The extended lower jaw of Alienacanthus, twice as long as the skull, is unique among placoderms and extremely rare in other living and fossil groups.  In most animals, jaw protrusion is seen in the upper jaw, like in the swordfish, or both upper and lower jaws like most ichthyosaurs or gharials. Among living species, only a tiny fish called halfbeak shows an elongated lower jaw. The halfbeak measures just 5-10 cm in length, while Alienacanthus’ head and jaws alone are 80 cm. 

In order to accommodate for such a long jaw, the anatomy of the skull, particularly the other mouth elements, had to undergo notable changes. As such, the upper jaws were modified so they could better adjust to the movement with the lower jaw. The shape of the mouth is different than other placoderm. The front of their mouth was rounded while that of Alienacanthus was sharp. 

Alienacanthus and relatives lived during a period in Earth’s history called the Devonian, which is also called the Age of fishes. During this time, a variety of fish groups ruled the oceans, including sharks, bony fishes, jawless fishes, and placoderms, which illustrated a wide range of body, head, and jaw shapes. Alienacanthus not only highlights but also stretches that diversity to the limit with such a unique look. During this age of fishes, the evolution of more complex jaws allowed for a diversification in feeding and hunting methods. The oldest placoderms favoured a fast-closing mouth for fish eating. Later in time, some placoderms started feeding on hard prey, and others may have been filter-feeders. Alienacanthus used its sharp teeth to catch and trap live prey, possibly using its elongated jaw to confuse or injure its future meal, as seen in swordfish or some ichthyosaurs. 

The further back in time we go, the more our planet’s continents looked different. During the Late Devonian period, when Alienacanthus lived, Poland was situated on the Northeastern coast and Morocco on the Southern coast of a vast ocean. The presence of the same species on both sides shows that migration occurred in that Ocean at that time, despite sea level fluctuation.