Bite simulations split eight ancient armoured fish into crushers and piercers

The simulations measure how well each jaw shape supported biting force. The team built 3D models of the fossil jaw bones, loaded them digitally, then compared the results against a second measurement, the complexity of each species' biting surface ^[11]. Diet itself is an inference drawn from those two numbers together with body size ^[11][12].

The usual expectation runs in one direction. Dr Alice Clement, an ARC Future Fellow at Flinders, said: "We know that animals feeding on hard foods usually evolve stronger jaws and broader, flatter crushing surfaces, but that's not exactly what we found in this analysis." ^[7]

In the largest species, the teeth sat along a raised bony crest, a shape the authors compare to the heads of medieval armour-piercing weapons such as war hammers and poleaxes ^[13]. Their explanation for the split turns on the size of the predator against the size of its prey. Small armoured prey could be engulfed whole and crushed between broad flat plates, while larger prey had to be broken into pieces first, and piercing armour needs a point ^[12]. Austin Fitzpatrick, a PhD student and co-author, said the two ends of the size range "solved the problem of processing harder foods in completely different ways" ^[10].

The jaws themselves are unfamiliar. "Unlike many animals around today, including humans, placoderms did not possess a single lower jaw bone. Instead, their jaws consisted of paired bony plates supported by cartilage, with surfaces ranging from broad crushing plates to sharp slicing edges with tooth-like structures," Mitchell said ^[9].

The sample is eight species, all from the Gogo Formation in northern Western Australia and all of one age ^[2][3]. Clement said placoderms "experimented with an extraordinary range of jaw shapes and biting parts during the early evolution of vertebrates" ^[8]. Eight species support the conclusion the authors draw for this reef, which is that hard-object feeding among the earliest jawed vertebrates had more than one solution ^[14]. They are a weaker basis for dating that diversity to the origin of jaws. Placoderm jaws and teeth appeared more than 400 million years ago, at least 15 million years before these fossils ^[1][18].

John Long, an emeritus professor at Flinders and a co-author, has worked at the Western Australian site for 40 years and found some of the specimens used in the study ^[15]. Research at the Gogo Formation has included collaborations with the local Gooniyandi and Gogo community ^[16]. "Since 2013, placoderms have been directly linked to our evolution, as the start of the line leading from fishes to humans," Long said ^[17].