Long before the appearance of dinosaurs, predatory fish developed jaws capable of crushing and tooth-like structures to capture and process prey in ancient oceans. A study from Flinders University now shows that these first jawed vertebrates found different evolutionary solutions to feeding on other animals protected by armor.
The investigation focused on placoderms, a group of armored fish that were among the first vertebrates to develop jaws and teeth, more than 400 million years ago. The researchers analyzed eight species that lived about 385 million years ago in a marine reef that existed where the northern part of Western Australia is now located.
The team combined three-dimensional models of the fossils with computer simulations of biting, using a finite element analysis technique. The aim was to understand how the different jaws withstood the forces exerted during feeding and how these features related to the animals’ diets.
“Placoderms underwent an extraordinary variety of jaw shapes and bite structures during the early evolution of vertebrates. They provide a rare opportunity to understand how some of the first jaws specialized for different diets,” explains Alice Clement, a researcher at Flinders University and the lead author of the study.
“The results revealed two distinct strategies. The smaller species used broad, relatively smooth bite plates to crush prey whole. The larger species, however, developed tooth-like structures capable of piercing and fragmenting animals shielded by shells or armor and too large to be swallowed in a single bite.”
Two solutions to the same problem
Unlike many modern animals, including humans, placoderms did not have a single lower jaw. Their jaws were made up of paired bony plates supported by cartilage, with surfaces ranging from broad plates for crushing to sharp tooth-like edges for cutting, with structures resembling teeth.
“Placoderms did not possess a single bone in the lower jaw. Instead, their jaws were composed of paired bony plates, supported by cartilage, with surfaces that varied from broad plates for crushing to sharp edges for cutting, with tooth-like structures,” explains Rex Mitchell, the study’s first author.
The researchers carried out digital bite simulations on three-dimensional models of the fossilized jaw bones, measuring the capacity of each structure to withstand the forces of biting. They then compared the results with the complexity of the surfaces used for biting.
Rather than finding a simple relationship between the size of the animals and the strength of the jaws, they found that both the smaller placoderms and the larger ones had jaws particularly strong for handling hard foods. The difference lay in how they used that force.
The smaller species had broad crushing plates with almost no differentiated structures. The larger ones displayed more complex and elevated tooth-like surfaces.
“It was an interesting surprise. Both the smaller and larger animals had developed strong jaws, but they solved the problem of processing harder foods in completely different ways,” says Austin Fitzpatrick, a doctoral student and co-author of the study.
According to the researchers, the explanation lies in the relationship between the predator’s size and the prey’s size.
Smaller prey could be swallowed whole and crushed between broad, flattened plates. Larger prey, on the other hand, had to be broken into smaller pieces first, requiring tooth-like structures capable of piercing the prey’s armor before crushing it.
The largest species examined had teeth arranged along a raised bony crest, forming a structure reminiscent of the heads of some medieval weapons designed to pierce armor, such as war hammers and halberds. The researchers suggest that these fish could use their jaws to pierce the prey’s carapace or armor before fragmenting them.
The results show that feeding on organisms protected by hard structures did not have a single evolutionary solution among the earliest jawed vertebrates. Instead, evolution produced different jaw configurations, allowing placoderms to explore distinct prey within the same reef ecosystem.
The study was based on fossils from the famous Gogo Formation in Western Australia, one of the world’s most important sites for the study of placoderms. John Long, an emeritus professor at Flinders University and a co-author of the investigation, has been working in the region for about 40 years and discovered some of the specimens analyzed.
The study of the Gogo Formation also includes collaborations with the Gooniyandi and Gogo communities.
“Since 2013, placoderms have been directly linked to our evolution, as the origin of the lineage that leads from fish to humans. Understanding placoderms is now essential to reveal the origins of the human body plan,” says John Long.
“Our work contributes to an increasingly clear picture of niche separation and ecological specialization in the Devonian reef, during the so-called ‘Age of Fishes,’ which is now recognized as the long lineage leading from fish to humans,” he adds.
