Our jaws can be traced back to the extinct placoderms, armoured prehistoric fish that lived over 400 million years ago, palaeontologists have found.
Jaws first appear in the developing embryo as a cartilage bar similar to a gill arch. In a shark, this develops directly into the adult jaws, but in an embryo of a bony fish or a human being new bones appear on the outside of the cartilage.
In our own skull, these bones - the dentary, maxilla and premaxilla - make up the entire jaws and carry our teeth, said researchers, including those from Uppsala University in Sweden.
It is universally accepted that the dentary, maxilla and premaxilla are a shared heritage of bony fishes and tetrapods: you will find these same bones in a crocodile or a cod, scientists said. Only one other group of fishes, the extinct placoderms, have a similar set of jaw bones.
However these bones, known as 'gnathal plates' and shown to spectacular effect in the giant placoderm Dunkleosteus where they are developed into blades like sheet-metal cutters, have always been regarded as unrelated to the dentary, maxilla and premaxilla, researchers said.
They are located slightly further inside the mouth, and in any case the general opinion has been that placoderms and bony fishes are only very distantly related.
The picture began to change fundamentally in 2013 with the description of Entelognathus, a Silurian (423 million year old) fossil fish from Yunnan in China which combines a classic placoderm skeleton with presence of a dentary, maxilla and premaxilla, said researchers, including palaeontologists from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in Beijing.
Together with the discovery of placoderm-like characteristics in some of the earliest bony fishes, this began to build a strong case for a close relationship between placoderms and bony fishes, accompanied by a substantial carry-over of placoderm characteristics into bony fishes (and hence ultimately to us). This is where the new fossil, Qilinyu, comes in.
Qilinyu comes from the same place and time period as Entelognathus, and also combines a placoderm skeleton with dentary, maxilla and premaxilla, though the two fishes otherwise look quite different and must have had different lifestyles.
Both fishes combine characters of the bony fish jaw bones (they contribute to the outer surface of the face and lower jaw) and placoderm gnathal plates (they have broad biting surfaces inside the mouth), researchers said.
It has been argued that placoderm gnathal plates represent an inner jaw arcade, similar in position to the 'coronoid bones' of bony fishes, and if that were true we would expect to find gnathal plates just inside of the dentary, maxilla and premaxilla of Entelognathus and Qilinyu; but there is nothing there, researchers said.
The study was published in the journal Science.
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